THE  PRINCIPLES 

OF 

ECONOMIC  GEOLOGY 


Trie,  Qra&JfillBock  (a  1m 

PUBLISHERS     OF     BOOKS      F  O  R_/ 

Coal  Age  ^  Electric  Railway  Journal 
Electrical  World  v  Engineering  News-Record 
American  Machinist  v  The  Contractor 
Engineering  8  Mining  Journal  v  Power 
Metallurgical  6  Chemical  Engineering 
Electrical  Merchandising 


THE  PEINCIPLES 


OF 


ECONOMIC  GEOLOGY 


BY 
WILLIAM  HARVEY  EMMONS,  PH.  D. 

PROFESSOR  AND  HEAD  OF  DEPARTMENT  OP  GEOLOGY  AND  MINERALOGY, 
UNIVERSITY  OF  MINNESOTA;  DIRECTOR  MINNESOTA  GEOLOGICAL 
SURVEY;  FORMERLY,   GEOLOGIST,  SECTION  OP  METALLIF- 
EROUS DEPOSITS,   UNITED  STATES  GEOLOGICAL 
SURVEY. 


FIRST  EDITION 
SECOND  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 
239  WEST  39TH  STREET.    NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD, 

6  &  8  BOUVERIE  ST.,  K.  C, 
1918 


COPYRIGHT,  1918,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


PREFACE 

This  volume  is  an  attempt  to  present  as  briefly  as  practicable 
a  perspective  of  the  science  of  metalliferous  and  nonmetalliferous 
deposits  to  advanced  students  of  geology.  It  includes  a  series 
of  lectures  on  economic  geology  which  for  the  past  ten  years  I 
have  offered  at  the  University  of  Chicago  and  at  the  University 
of  Minnesota.  These  have  been  expanded  and  descriptions  of 
certain  mining  districts  have  been  added. 

This  work  does  not  include  a  treatment  of  mineral  fuels,  an 
omission  which  perhaps  is  justified  by  the  development  of  the 
geologic  branches  in  our  colleges.  Coal  deposits  are  treated  at 
some  length  in  text-books  of  general  geology.  Petroleum  geology 
is  treated  in  several  texts  on  oil  which  are  no  more  comprehensive 
than  is  desirable  in  a  thorough  course  in  economic  geology.  An 
equally  comprehensive  treatment  would  make  this  book  so 
voluminous  as  to  defeat  some  of  its  purposes. 

The  order  of  treatment  is  indicated  in  the  table  of  contents. 
The  first  part  of  the  book  is  a  general  treatment  of  mineral  de- 
posits. The  second  part  is  a  treatment  of  each  of  the  metals  and 
of  the  more  valuable  nonmetallic  minerals.  Numerous  mining 
districts  and  their  deposits  are  described.  There  is  no  attempt 
to  include  discussions  of  all  or  even  all  the  more  valuable  de- 
posits of  every  metal.  Examples  are  chosen  to  illustrate  classes 
and  as  far  as  practicable  they  are  chosen  from  North  America. 

The  descriptions  of  mining  districts  are  arranged  so  that  some 
of  them  may  be  omitted  in  class-room  work  when  that  seems 
desirable.  I  am  convinced  that  a  few  districts  studied  thoroughly 
are  more  helpful  for  instruction  than  many  districts  discussed 
in  brief.  In  advanced  classes,  as  a  rule,  many  or  all  of  the  stu- 
dents are  familiar  with  one  or  more  districts  as  a  result  of  field 
work.  It  is  advantageous  in  lecturing  to  use  such  districts,  as 
far  as  practicable,  for  purposes  of  illustration  even  if  that  results 
in  giving  inadequate  treatment  to  more  important  districts. 
It  is  difficult  in  one  or  two  lectures  to  give  a  student  a  clear 
picture  of  a  district  which  he  will  retain  and  when  fifty  or  a 
hundred  districts  have  been  described  the  difficulties  are  in- 


vi  PREFACE 

creased.  Maps,  photographs,  and  sets  of  rocks  and  ores  from 
the  larger  districts  are  helpful,  especially  if  these  are  before  the 
student  during  the  discussion.  Even  with  such  assistance 
probably  better  results  may  be  obtained  by  omitting  discussions 
of  some  districts  treated  herein. 

Some  readers  doubtless  will  disagree  with  certain  features  of 
the  classification  of  ore  deposits  (Fig.  6),  also  with  the  weights 
I  have  set  down  for  certain  processes  in  the  formation  of  various 
ores  and  illustrated  in  Figs.  40  and  74.  These  weights  will 
be  changed  as  our  knowledge  of  ore  deposits  increases.  I  have 
introduced  these  figures  because  I  believe  that  they  will  help  to 
give  the  student  a  perspective. 

I  acknowledge  my  indebtedness  to  Professors  F.  F.  Grout, 
T.  T.  Quirke,  and  T.  M.  Broderick  of  the  Department  of  Geology 
and  Mineralogy  of  the  University  of  Minnesota,  who  have  critic- 
ally read  certain  sections  of  this  volume,  and  to  Dr.  E.  C.  Harder 
of  the  United  States  Geological  Survey,  who  has  read  several 
sections,  among  them  the  chapters  treating  deposits  of  iron  and 
manganese.  Many  of  the  drawings  have  been  made  by  Mr. 
A.  I.  Levorsen  and  Mr.  G.  S.  Nishihara.  I  have  endeavored 
suitably  to  acknowledge  sources  of  information  by  footnote 
references. 

W.  H.  E. 

UNIVERSITY  OF  MINNESOTA,  MINNEAPOLIS, 
December,  1917. 


CONTENTS 

PAGE 

PREFACE v 

CHAPTER  I 
INTRODUCTION 1 

CHAPTER  II 

CLASSIFICATION  OF  ORE  DEPOSITS 4 

CHAPTER  III 


DEPOSITS  FORMED  BY  MAGMATIC  SEGREGATION 
General  Features .    .    . 


Occurrence 12 

Composition 13 

Shape 13 

Size 14 

Texture 14 

References 16 

CHAPTER  IV 

PEGMATITE  DEPOSITS 18 

Eutectics 18 

Agents  of  Mineralization 20 

Occurrence    . 22 

Composition 23 

Shape ' 24 

Size 24 

Texture 25 

Gradations 26 

Temperature  of  Formation  of  Pegmatites 27 

References 28 

CHAPTER  V 

CONTACT  METAMORPHIC  DEPOSITS 29 

Occurrence ' 29 

Composition 32 

Shape  and  Relation  to  Fissuring 33 

Size 35 

Texture 36 

Material  Added  to  Intruded  Rock  by  Contact  Metamorphism  .    .  37 

Silication  with  Relatively  Small  Addition  of  Other  Materials ...  38 
vii 


viii  CONTENTS 

PAGE 

Development  of  Magnetite  Bodies ;    .    .    .  38 

Development  of  Zones  of  Garnet  and  Other  Heavy  Silicates  ...  40 
Age  of  Contact  Metamorphic  Deposits  in  the  United  States  ...  42 
Depths  at  Which  Contact  Metamorphic  Ore  Deposits  'are  Formed.  43 
Function  of  Mineralizing  Agents  and  Evidence  of  Their  Activity.  44 
Significance  of  Mineral  Associations  and  Synthetic  Experiments  .  44 
Temperatures  and  Conditions  of  Solutions  Forming  Contact  Meta- 
morphic deposits 45 

Fissuring  during  Contact  Metamorphism 46 

Endomorphic  Changes' 47 

References 47 

CHAPTER  VI 

DEPOSITS  OP  THE  DEEP  VEIN  ZONE 49 

General  Features ' 49 

Occurrence 50 

Composition 50 

Shape 51 

Size 52 

Texture 52 

Depth  of  Formation •  .  53 

Origin  of  Openings ' 54 

Connection  with  Surface  at  Time  of  Deposition 56 

Function  of  Mineralizers 57 

Age  of  Deposits  of  the  Deep  Vein  Zone 57 

Gradation  into  Pegmatite  Veins 58 

Gradation  into  Deposits  Formed  by  Hot  Solution?  at  Moderate 

Depth '. 59 

Gradation  into  Contact  Metamorphic  Deposits 61 

References.    . 61 

CHAPTER  VII 

DEPOSITS  FORMED  AT  MODERATE  DEPTHS  BY  HOT  SOLUTIONS    ...  62 

General  Features 62 

Occurrence 63 

Composition 65 

Shape 66 

Size 66 

Texture 66 

Age     .    .    '. 67 

References 68 

CHAPTER  VIII 

DEPOSITS  FORMED  AT  SHALLOW  DEPTHS  BY  HOT  SOLUTIONS   ....  69 

General  Features 69 

Occurrence.    .                                                                                           .  69 


CONTENTS  ix 

PAGE 

Composition 70 

Shape 71 

Size .  71 

Texture.    .    .    .  v  .    . 72 

References 72 

CHAPTER  IX 

DEPOSITS  FORMED  AT  MODERATE  AND  SHALLOW  DEPTHS  BY  COLD 

METEORIC  SOLUTIONS 74 

General  Features 74 

Occurrence 76 

Composition 79 

Shape 80 

Size 81 

Texture      81 

References 82 

CHAPTER  X 

SEDIMENTARY  DEPOSITS 84 

General  Features 84 

Occurrence 86 

Composition 87 

Shape 89 

Size 89 

Texture 90 

References ' .    -  91 

CHAPTER  XI 
PRIMARY  ORE  SHOOTS 93 

CHAPTER  XII 
DEFORMATION  OF  ORE  DEPOSITS 100 

CHAPTER  XIII 

FAULTING  AND  FOLDING  OF  ORE  DEPOSITS 102 

Faulting  of  Ore  Deposits 102 

General  Features. 1°2 

Searching  for  Faulted  Segments .  107 

Folding  of  Ore  Deposits .... 

CHAPTER  XIV 

DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS 114 

General  Character I14 

Orientation  of  Ore  Bodies 117 


x  CONTENTS 

PAGE 

Chemical  Changes  during  Metamorphism 118 

Mineral  Composition 119 

Texture  and  Paragenesis 120 

Deposits  Inclosed  hi  Schists  not  Dynamically  Metamorphosed.    .  122 

Age  Relations  of  Dynamically  Metamorphosed  Deposits    ....  123 

CHAPTER  XV 

SUPERFICIAL  ALTERATION  AND  ENRICHMENT  OF  ORE  DEPOSITS  ....  124 

General  Features ' 124 

Weathering  of  Rocks 125 

Hydrometamorphism 129 

Superficial  Alteration  and  Enrichment  of  Sulphide  Deposits  .    .    .  130 

Level  of  Ground  Water 131 

Vadose  Circulation 132 

Deeper  Circulation 132 

Region  of  Nearly  Stagnant  Water .133 

Pulsating  Movements  of  Underground  Waters 133 

Physical  Conditions  that  Influence  Secondary  Enrichment.    .    .    .  134 

Glaciation 137 

General  Character  of  Outcrops  above  Sulphide  Deposits     ....  138 

Conditions  in  the  Oxidized  Zone 139 

Depth  of  the  Oxidized  Zone .   ,'   .    .140 

Position  and  Extent  of  Secondary  Sulphide  Zone 140 

Relation  of  Secondary  Sulphide  Zones  to  Water  Level 142 

Precipitation  of  Sulphides  above  Water  Level .  143 

Textures  of  Secondary  Ores 143 

Primary  and  Secondary  Minerals 148 

Summary  of  Criteria  for  Determination  of  Secondary  Ores     .    .    .  150 

Estimates  of  Portions  of  Lodes  Eroded 153 

Experimental  Data  on  Solution  and  Precipitation  of  Metals  .    .    .  154 

Composition  of  Mine  Waters 158 

General  Character  of  Mine  Waters 160 

Changes  of  Mine  Waters  in  Depth 162 

Precipitates  from  Mine  Waters  under  Superficial  Conditions  .    .    .  162 

Oxidation  and  Solution  of  Metallic  Sulphides 163 

Chemical  Changes  during  Oxidation  of  Certain  Ores 163 

Metasomatic  Replacement  of  Primary  Sulphides 165 

Influence  of  Primary  Ores  on  Extent  of  Secondary  Sulphide  Zones .  167 

CHAPTER  XVI 

OPENINGS  IN  ROCKS 170 

Size  of  Openings 170 

Origin  of  Openings 171 

Primary  Openings 171 

Intergranular  Spaces  hi  Sedimentary  Rocks 171 

Bedding  Planes 173 

Vesicular  Spaces 173 


CONTENTS  xi 

PAGE 

Openings  in  Pumice 174 

Miarolitic  Cavities 174 

Submicroscopic  Spaces 174 

Secondary  Openings .  175 

Openings  Formed  by  Solutions 175 

Openings  Due  to  Shrinkage 175 

Openings  Due  to  Force  of  Crystallization 176 

Openings  Due  to  Pressure  of  Solutions 177 

Openings  Due  to  Greater  Stresses 177 

Compressional  Fractures 178 

Tensional  Fractures 180 

Torsional  Fractures 182 

CHAPTER  XVII 

STRUCTURAL  FEATURES  OF  OPENINGS  IN  ROCKS  AND  OF  EPIGENETJC 

DEPOSITS ' 184 

Fissure 184 

Vein 186 

Fault  Fissure  Vein 186 

Lode 186 

Reef 186 

Ledge 186 

Ladder  Vein 187 

Fractured  Zone 187 

Reticulated  Vein 188 

Disseminated  Deposit 188 

Stockwork 188 

Breccia  Vein 188 

Sheeted  Zone 188 

Shear  Zone 189 

Gash  Vein 189 

Run 190 

Flats  and  Pitches • 190 

Lens .190 

Pod 190 

Fahlband 190 

Reopened  Veins - 190 

Fault  Fissures  as  Seats  of  Deposition 192 

Influences  of  Rock  Structure  on  Fissuring  .' 196 

Bedding  Plane  Deposits 197 

Long  Slender  Deposits  and  Equidimensional  Deposits 199 

Anticlinal  Deposits — Saddle  Reefs 201 

Synclinal  Deposits — Inverted  Saddles — Troughs 207 

Fracture  Systems  in  Mining  Districts 208 

Conjugated  Systems   •. 209 

Horsetail  Structure 210 

Parallel  Systems 211 

Radial  Patterns 212 


xii  CONTENTS 

PAGE 

Irregular  Patterns 212 

Topographic  Expression  of  Deposits 215 

CHAPTER  XVIII 

METASOMATIC  PROCESSES 218 

Mechanism  of  Replacement 218 

Criteria  for  Recognition  of  Replacement  Deposits 223 

General  Features 223 

Pseudomorphs 224 

Banding  and  Crustification 225 

Cavities 225 

Crystal  Boundaries 225 

Boundaries  of  Deposits 226 

Contacts 226 

Fragments 227 

Orientation  of  Fragments 227 

Variations  in  Width  Depending  on  Country  Rock 227 

Residual  Minerals 229 

CHAPTER  XIX 

MINERAL  ASSOCIATIONS  IN  VEINS  AND  WALL  ROCK  ALTERATIONS  .    .  230 

Basis  of  Classification -.    .    .    .  230 

Garnetiferous  Gold  Veins 232 

Garnetiferous  Silver-copper  Veins 232 

Garnetiferous  Lead-silver  Veins 233 

Cassiterte  Veins 233 

Tourmaline  Veins 236 

Sericitic  Calcitic  Gold  Veins 237 

Sideritic  Lead  Veins 240 

Sericitic  Copper-silver  and  Sericitic  Zinc-silver  Veins 242 

Sericitic  Copper  Veins  and  Disseminated  Copper  Ores 244 

Sericitic  Silver -gold  Veins 248 

Gold-silver-adularia  Veins 248 

Chloritic  Alteration  in  Granitic  Rocks 248 

Chloritic  Alteration  in  Lavas  (Propylitic  Alteration) 249 

Sericitic  and  Propylitic  Alteration  Compared 250 

Fluoritic  Tellurium-adularia  Gold  Veins 254 

Alunitic  Kaolinic  Gold  Veins 255 

Zeolitic  Native  Copper  Veins 260 

Chalcedonic  and  Calcitic  Cinnabar  Veins 261 

Baritic  Fluorite  Veins.    .    . 263 

Deposits  Formed  at  Orifices  of  Hot  Springs 263 

CHAPTER  XX 

METALLOGENIC  PROVINCES  AND  METALLOGENIC  EPOCHS 269 

Metallogenic  Provinces 269 

Metallogenic  Epochs '    .    .   270 


CONTENTS  xiii 
CHAPTER  XXI 

PAGE 

COMPOSITION  AND  SOURCES  OF  ASCENDING  THERMAL  METALLIFEROUS 

WATERS 271 

Chemical  Composition 271 

Sources  of  Ascending  Thermal  Waters 274 

Summary 283 

CHAPTER  XXII 

IRON 290 

Ore  Minerals  of  Iron 290 

Genesis  of  Iron-ore  Deposits 291 

Age  of  Iron-ore  Deposits  of  the  United  States 295 

Production 296 

Lake  Superior  Iron-ore  Deposits 297 

Mesabi  Range,  Minnesota 301 

Cuyuna  Range,  Minnesota .  309 

Penokee-Gogebic  Range,  Wisconsin  and  Michigan 311 

Menominee  District,  Michigan. 315 

Marquette  District,  Michigan 317 

Vermilion  Range,  Minnesota 321 

Clinton  Hematite  Deposits 323 

Birmingham  Region,  Alabama 328 

Eastern  Tennessee  Iron  Deposits .  328 

Clinton  Region,  New  York 331 

Eastern  Wisconsin 332 

Tertiary  Ores  of  Northeastern  Texas 332 

Iron  Carbonate  Ores  of  Eastern  United  States 333 

Brown  Ores  of  Eastern  United  States 334 

Magnetite  Ores  of  Pennsylvania 337 

Magnetite  Ores  of  New  York  and  New  Jersey 337 

Hematites  and  Magnetites  of  Western  United  States 338 

Hartville,  Wyoming 338 

Iron  Springs,  Utah 338 

Eagle  Mountain,  California 339 

Hanover  (Fierro)  District,  New  Mexico 339 

Iron  Age  Deposit,  Dale,  California 341 

Iron  Mountain  and  Pilot  Knob,  Missouri 341 

Titaniferous  Iron  Ores 343 

Iron  Mountain,  Wyoming 343 

Adirondack  Region,  New  York 344 

CHAPTER  XXIII 

COPPER 345 

Mineral  Composition  of  Copper  Deposits 345 

Copper  Deposits  in  the  United  States 348 

Genesis  of  Copper  Deposits 348 

Age  of  Copper  Deposits  of  the  United  States 351 


xiv  CONTENTS 

PAGE 

Outcrops  of  Copper  Deposits    .    .    . ' '.  351 

Sulphide  Enrichment  of  Copper  Deposits 352 

Copper  Minerals 354 

Copper-bearing  Districts 357 

Butte,  Montana 357 

Bingham,  Utah 365 

Bisbee,  Arizona 367 

Globe  and  Miami,  Arizona 371 

Ray,  Arizona 375 

Morenci,  Arizona 376 

Ely,  Nevada • 379 

Santa  Rita,  New  Mexico 381 

Burro  Mountain  District,  New  Mexico 383 

Ajo,  Arizona 384 

Jerome,  Arizona 385 

Shasta  County,  California 386 

Foothill  Copper  Belt,  California 388 

Encampment,  Wyoming 389 

Ducktown,  Tennessee 390 

Chitina  Copper  Belt,  Alaska 394 

.    Lake  Superior  Copper  Deposits 395 

CHAPTER  XXIV 

GOLD ...  401 

Mineral  Associations 401 

Genesis  of  Gold  Deposits 402 

Age  and  Associations  of  Gold  Deposits 403 

Pre-Cambrian  Deposits 403 

Cretaceous  Veins  of  the  Pacific  Coast 403 

Late  Cretaceous  or  Early  Tertiary  Deposits 404 

Middle  and  Late  Tertiary  Deposits .  404 

Superficial  Enrichment 405 

General  Features 405 

Placers  and  Outcrops 408 

Concentration  in  the  Oxidized  Zone 408 

Relation  of  Gold  Enrichment  to  Chalcocitization 410 

Tellurides 410 

Summary 410 

Gold  Placers 411 

General  Features 411 

Scour  and  Fill '.    .  413 

Minerals  Associated  with  Gold  in  Placers 414 

Solution  of  Gold  in  Placer  Deposits 414 

Relation  of  Gold  Placers  to  Gold  Lodes 415 

Eolian  and  Glacial  Deposits  Containing  Gold 416 

Buried  Placers 416 

References 418 

Witwatersrand  Auriferous  Conglomerates 419 


CONTENTS 


xv 


FACIE 

Gold  Lodes 422 

Porcupine,  Ontario 422 

Southern  Appalachian  Region 424 

Black  Hills,  South  Dakota 426 

California  Gold  Belt 430 

Nevada  City  and  Grass  Valley,  California 432 

Juneau,  Alaska 432 

Cripple  Creek,  Colorado 434 

Goldfield,  Nevada 437 

CHAPTER  XXV 

SILVER 439 

Mineral  Composition  of  Silver  Deposits 440 

Age  of  Silver  Deposits  in  North  America 441 

Enrichment 441 

Silver-bearing  Districts 444 

Cobalt,  Ontario 444 

Eureka,  Nevada 446 

Boulder-Leadville  Belt,  Colorado 447 

Breckenridge  ' 449 

Silver  Plume 450 

Idaho  Springs 450 

Gilpin  County 450 

Leadville 451 

Aspen,  Colorado 453 

San  Juan  Region,  Colorado 454 

Silverton 455 

Telluride .'    .    .  456 

Ouray 456 

Rico 457 

La  Plata  Mountains 457 

Lake  City 457 

Creede 458 

Park  City,  Utah 460 

Tintic,  Utah 462 

Philipsburg,  Montana 464 

Comstock  Lode,  Nevada 467 

Tonopah,  Nevada 470 

CHAPTER  XXVI 

ZINC  AND  LEAD 474 

ZINC 474 

Zinc  Minerals 474 

Genesis  of  Zinc  Deposits 475 

Joplin  Region 477 

Wisconsin  Region 481 

Eastern  Tennessee 484 


xvi  CONTENTS 

PAGE 

Butte,  Montana  .    .    . 485 

Coeur  d'Alene  District,  Idaho 486 

Franklin  Furnace,  N.  J 487 

LEAD .<  ... 489 

Lead  Minerals 489 

Southeastern  Missouri 491 

Coeur  d'Alene  District,  Idaho 493 

San  Francisco  Region,  Utah 496 

CHAPTER  XXVII 

MISCELLANEOUS  METALLIFEROUS  DEPOSITS 499 

MERCURY • 499 

Mercury  Minerals  and  Deposits 499 

Recovery  and  Uses 501 

Production 501 

References 502 

ALUMINUM  AND  BAUXITE 502 

Aluminum  Ores 502 

Uses  and  Production .. 503 

Arkansas 503 

Appalachian  Districts 506 

CHROMIUM 507 

MANGANESE 508 

Origin  of  Deposits 509 

Uses  and  Production 509 

Deposits 510 

NICKEL 511 

Nickel  Minerals  and  Ores 511 

Uses  and  Production 512 

Sudbury,  Ontario 513 

Alexo,  Ontario 514 

Lancaster  Gap,  Pennsylvania 515 

New  Caledonia 515 

Riddle,  Oregon 515 

COBALT 515 

PLATINUM 516 

ANTIMONY i   v'. 517 

ARSENIC 519 

BISMUTH 520 

TIN 521 

Occurrence 521 

Uses  and  Production 522 

Tin  Deposits  of  the  United  States 523 

TUNGSTEN 524 

URANIUM  AND  RADIUM 526 

VANADIUM 528 

CADMIUM 529 

MOLYBDENUM  .                                                                                    .  530 


CONTENTS  xvii 

PAGE 

SELENIUM 531 

TELLURIUM 531 

TITANIUM 531 

TANTALUM 532 

CHAPTER  XXVIII 

DEPOSITS  OF  THE  NONMETALS 533 

Building  Stones 533 

Slate 535 

Clay 536 

Fuller's  Earth 539 

Feldspar 540 

Mica 540 

Lithium  Minerals 542 

Cements  and  Limes 542 

Cement 542 

Mortar 544 

Concrete .    .  544 

Lime 545 

Puzzolan 545 

Distribution  of  Materials 546 

Natural  Abrasives 548 

Infusorial  Earth 550 

Quartz 551 

Glass  Sand 551 

Gems  and  Precious  Stones 552 

Graphite 556 

Barite : 557 

Celestite 558 

Witherite 559 

Fluorite  and  Cryolite 559 

Mineral  Paints 560 

Salt 561 

Origin  of  Thick  Salt  Beds 562 

Salt  Deposits  of  the  United  States 563 

Potash  Salts 565 

Bromine 566 

Sodium  Sulphate 566 

Gypsum 567 

Sulphur 569 

Pyrite  and  Sulphuric  Acid 571 

Boron  Compounds •  572 

Nitrates 574 

Mineral  Fertilizers 575 

Magnesium  Minerals 579 

Olivine  and  Serpentine 579 

Magnesite 580 

Talc    .  581 


xviii  CONTENTS 

PAGE 

Meerschaum 581 

Asbestos 581 

Mqnazite  and  Xenotime 584 

Zircon 584 

Water 585 

Occurrence 585 

Artesian  Water 585 

Mineral  Waters 587 


THE  PRINCIPLES  OF 
ECONOMIC  GEOLOGY 

CHAPTER  I 
INTRODUCTION 

An  ore  deposit  is  a  geologic  body  that  may  be  worked  com- 
mercially for  one  or  more  metals.  Ore  deposits  do  not  differ 
essentially  in  genesis  from  other  geologic  bodies.  Many  of 
them  are  simply  igneous,  sedimentary,  or  metamorphic  rocks 
that  contain  materials  which  are  valuable  in  the  arts.  Such 
rocks  are  treated  in  studies  of  general  geology  and  petrology, 
which  the  student  of  economic  geology  is  supposed  to  have 
mastered,  and  they  are  discussed  but  briefly  here. 

In  general  the  geologist  who  is  consulted  on  problems  con- 
nected with  ore  deposits  is  expected  to  answer  the  questions, 
Where  is  the  ore?  How  shall  one  find  the  deposit  that  is  lost? 
What  changes  are  likely  to  take  place  in  depth?  To  what  depth 
will  the  deposit  continue  to  be  profitable?  These  questions  can 
be  answered  most  intelligently  by  one  who  has  an  adequate 
knowledge  of  the  structural  and  historical  geology  of  the  region 
containing  the  deposits,  such  as  is  gained  by  detailed  mapping, 
together  with  an  understanding  of  the  genesis  of  the  deposits, 
their  relation  to  the  structure,  their  deformation,  and  their 
superficial  alteration  and  enrichment. 

Deposits  that  have  been  precipitated  from  aqueous  solutions 
in  and  along  fractures  in  rocks  receive  but  scant  attention  in  the 
other  branches  of  geologic  science  as  now  developed,  and  the 
study  of  such  deposits  is  especially  appropriate  in  the  economic 
branch.  Certain  definitions,  most  of  which  refer  to  veins  or  to 
vein  formation,  are  given  below.  The  list  is  not  exhaustive;  the 
student  is  advised  to  refer  to  the  index  for  other  definitions  that 
are  introduced  at  appropriate  places  in  the  text. 

An  ore  is  a  mineral  or  association  of  minerals  that  may,  under 
favorable  conditions,  be  worked  commercially  for  the  extraction 
of  one  or  more  metals. 

Protore  is  low-grade  metalliferous  material  which  is  not  itself 
valuable  but  from  which  valuable  ore  may  be  formed  by  super- 
ficial alteration  and  enrichment. 

1 


2          THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

An  ore  mineral  is  one  that  contains  a  valuable  metal. 

A  gangue  mineral  is  an  earthy  or  nonmetallic  mineral  associated 
with  the  ore  minerals  of  a  deposit. 

A  tabular  body  is  shaped  like  a  tablet,  short  in  one  and  long  in 
two  dimensions. 

A  vein  is  a  mineral  mass,  more  or  less  tabular,  deposited  by 
solutions  in  or  along  a  fracture  or  group  of  fractures. 

Country  rock  is  the  rock  that  incloses  a  metalliferous  deposit. 

Vein  walls  are  the  rock  surfaces  on  the  borders  of  veins.  If  there 
is  much  replacement  of  the  country  rock  along  the  fissure  the  ore 
may  grade  into  the  wall  rock  and  its  walls  may  be  indistinct. 

A  druse  or  vug  is  an  unfilled  portion  of  a  vein. 

Banded  ore  is  ore  composed  of  bands  or  layers  (see  Fig.  1).  The 
layers  may  be  composed  of  the  same  minerals  differing  in  color  or 


FIG.    1. — Section   of   banded 
vein. 


FIG.  2. — Section  of  vein  with 
crustified  banding. 


texture  or  proportions,  or  they  may  be  composed  of  different 
minerals. 

Crustification  or  crustified  banding  (Fig.  2)  is  produced  when 
mineral  layers  of  different  character  are  deposited  successively 
one  upon  another  on  the  borders  of  openings.  In  comb  structure 
elongated  prisms  project  approximately  at  right  angles  to  a 
surface,  like  teeth  of  a  comb. 

Symmetrical  banding  results  where  solutions  deposit  similar  ma- 
terial on  both  sides  of  an  opening,  layer  on  layer,  as  shown  in  Fig.  3. 

Vein  material  is  the  matter  that  constitutes  veins,  whether  ore 
or  gangue,  workable  or  not  workable. 

Gouge  is  soft  clay-like  material  that  occurs  at  some  places  as  a 
selvage  between  a  vein  and  country  rock.  It  is  usually  formed 
by  the  crushing  of  ore  or  country  rock,  or  both, 


INTRODUCTION 


Replacement  is  a  process  in  the  operation  of  which  rocks  and 
ores  are  slowly  dissolved  and  material  of  different  composition 
is  deposited  in  the  spaces  which  they  occupied.  Deposition  fol- 


Fia.  3. — Section  of  vein  with  symmetrical  crystified  banding,  natural  size 
(Creede,  Colo.).  1,  Chlorite,  quartz,  sphalerite  and  galena;  2,  finely  banded 
quartz;  3,  sphalerite  and  a  little  quartz;  4,  amethystine  quartz;  5,  union  of  quartz 
combs;  6,  vug. 

lows  solution  closely  so  that  forms  and  textures  of  earlier  sub- 
stances are  often  preserved.     (See  Figs.  4  and  5.) 


FIG.  4. — Vein  filling  a  fissure. 


FIG.  5. — Replacement  vein 
filling  a  fissure  and  replac- 
ing wall  rock  along  the  fissure. 


The  paragenesis  of  an  ore  expresses  the  relations  of  its  minerals, 
especially  the  relations  that  bear  upon  its  origin. 

Hydrothermal  alteration  is  a  process  by  which  rocks  and  ores 
are  changed  by  hot  waters. 


CHAPTER  II 
CLASSIFICATION  OF  ORE  DEPOSITS 

A  classification  of  ore  deposits  should  recognize  three  groups 
of  processes — those  concerned  in  (1)  deposition,  (2)  deformation, 
and  (3)  superficial  alteration  and  enrichment  of  the  deposits. 

Ores  deposited  by  any  process  may  be  deformed  by  faulting, 
folding,  and  deep-seated  metamorphism.  Through  processes  of 
superficial  alteration  ore  deposits  may  be  enriched  or  impover- 
ished. Deposits  that  exist  essentially  as  they  were  originally 
formed  are  termed  primary  deposits ;  those  that  have  been  altered 
by  dynamic  processes  may  be  termed  deformed  deposits;  and 
those  that  have  been  altered  by  superficial  agencies  are  fre- 
quently referred  to  as  secondary  deposits.  Part  of  a  deposit  may 
be  primary  and  another  part  secondary.  When  metals  are 
transferred  either  in  solution  or  mechanically  and  deposited 
where  there  was  no  ore  or  protore  before,  the  deposit  is  primary, 
whether  it  is  a  placer,  a  chemical  sediment,  or  a  vein. 

CLASSIFICATION  OF  PRIMARY  DEPOSITS 

1.  Deposits  formed  by  magmatic  segregation;  consolidated  from  molten 
magmas. 

2.  Pegmatite  veins;  deposited  by   " aqueo-igneous "  magmatic  solutions. 

3.  Contact-metamorphic  deposits;  deposited  in  intruded  rocks  by  fluids 
passing  from  consolidating  intruding  rocks. 

4.  Deposits  of  the  deep  vein  zone;  formed  at  high  temperature  and  under 
great  pressure,  generally  in  and  along  fissures. 

5.  Deposits  formed  at  moderate  depths  by  ascending  hot  solutions. 

6.  Deposits  formed  at  shallow  depths  by  ascending  hot  solutions. 

7.  Deposits  formed  at  moderate  and  shallow  depths  by  cold  meteoric 
solutions. 

8.  Sedimentary  deposits;  chemical,  mechanical,  organic,  etc. 

Syngenetic  deposits  are  those  formed  contemporaneously 
with  the  inclosing  rocks.  They  include  deposits  formed  by  mag- 
matic segregation  and  sedimentary  deposits. 

Epigenetic  deposits  are  formed  later  than  the  rocks  that  in- 
close them.  They  are  deposited  in  openings  in  rocks,  or  by 
replacement. 

4 


CLASSIFICATION  OF  ORE  DEPOSITS  5 

Deposits  formed  by  magmatic  segregation  are  products  of  the 
differentiation  of  igneous  magmas.  Genetically  considered  they 
are  in  the  strict  sense  igneous  rocks.  These  deposits  include  ore 
bodies  of  considerable  value,  among  them  the  great  magnetite 
deposits  of  the  Kiruna  region,  Sweden,  and  some  of  the  magnetic 
iron  ores  of  the  Adirondack  Mountains,  in  New  York.  No  large 
sulphide  deposits  of  this  class  are  known  in  the  United  States. 
The  nickel-copper  deposits  of  Sudbury,  Ontario,  are  the  best- 
known  examples  of  this  group  in  North  America. 

In  many  places  where  igneous  rocks  rich  in  iron,  nickel,  or 
chromium' minerals  are  weathered  at  the  surface,  especially  under 
temperate  or  tropical  conditions,  the  metals  are  concentrated, 
owing  to  the  removal  of  other  material.  So  prominent  are  the 
secondary  processes  in  the  genesis  of  such  ores  that  they  are 
classed  by  some  as  a  distinct  group,  although  the  protores,  or  un- 
workable material  from  which  the  ores  are  derived  by  weathering, 
are  igneous  rocks. 

Pegmatite  veins  are  nearly  related  to  deposits  formed  by 
magmatic  segregation.  They  are  end  products  of  crystalliza- 
tion that  have  been  thrust,  like  igneous  dikes,  into  openings 
in  rocks  already  consolidated.  Pegmatites  that  have  not  moved 
from  their  parent  magma  and  are  not  related  to  openings  in 
rocks  could  properly  be  classed  with  syngenetic  deposits,  as 
deposits  formed  by.  magmatic  segregation,  but  some  authorities 
reserve  the  latter  term  for  the  more  basic  differentiation  products. 
Pegmatites,  although  they  supply  many  valuable  non-metallic 
substances  and  many  gems,  are  comparatively  unimportant  as 
sources  of  metals. 

Contact-metamorphic  deposits  are  formed  in  intruded  rocks 
by  fluids  (liquids  and  gases)  given  off  by  intruding  igneous 
magmas  near  by.  They  may  generally  be  distinguished  from 
lode  deposits  by  their  irregular  shape  and  by  their  apparent 
independence  of  nssuring,  together  with  the  fairly  constant 
association  of  the  minerals  they  contain. 

The  deposits  of  the  deep  vein  zone  are  mineralogically  related 
more  or  less  closely  to  contact-metamorphic  deposits.  They 
have  formed  in  and  along  openings  in  rocks,  however,  and  in  the 
main  they  are  more  nearly  tabular  in  form  than  the  contact- 
metamorphic  deposits.  As  pointed  out  by  Lindgren,  who  first 
defined  the  group,  the  deposits  of  the  deep  zone  have  formed 
under  conditions  of  high  temperature  and  pressure,  which  prevail 


6          THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

also  in  contact  metamorphism.  Because  high  temperature  and 
pressure  are  necessary  for  their  genesis,  these  deposits  do  not 
form  at  moderate  and  shallow  depths,  at  least  not  in  open  fis- 
sures that  extend  to  the  surface,  and  for  that  reason  they  are 
seldom  found  in  the  more  recent  rocks.  The  deposits  of  the 
deep  vein  zone  are  affiliated  on  one  hand  with  contact-meta- 
morphic  deposits  and  on  the  other  with  veins  formed  at  moderate 
depths,  from  which  they  can  not  be  sharply  separated. 
.  The  deposits  formed  at  moderate  depths  by  ascending  hot 
solutions  constitute  an  important  group.  They  differ  from  ores 
formed  in  the  deep  veins  and  from  those  formed  at  shallow  depths 
in  their  mineral  composition  and  in  the  character  of  the  altera- 
tion of  the  wall  rock  accompanying  their  formation. 

Deposits  formed  at  shallow  depths  by  ascending  hot  solutions 
are  generally  related  to  well-defined  openings  in  rocks,  such  as 
fissures  or  to  the  intergranular  spaces  in  conglomerates  or  sand- 
stones or  openings  in  vesicular  lavas.  These  deposits  may  be  dis- 
tinguished from  veins  formed  at  moderate  or  greater  depths  by 
the  minerals  they  contain  and  by  the  character  of  the  alteration 
of  their  wall  rocks. 

The  deposits  formed  at  moderate  and  shallow  depths  by  cold 
solutions  include  a  large  number  of  valuable  deposits  of  lead 
and  zinc  in  the  Mississippi  Valley  and  many  small  copper  de- 
posits in  the  Southwest.  Much  evidence  has  been  cited  to  show 
that  these  deposits  were  formed  by  ground  water  that  gathered 
its  metallic  contents  from  great  masses  of  rocks  in  which  the 
metals  were  sparingly  disseminated.  The  metallic  salts,  chiefly 
sulphates,  chlorides,  and  carbonates,  were  gathered  in  water 
channels,  and  the  metals  were  deposited  as  sulphides  where 
conditions  were  favorable.  In  many  localities  some  form  of 
organic  material  supplied  the  precipitating  agent.  If  deposition 
had  taken  place  on  an  older  sulphide  these  deposits  would  be 
classed  as  secondary  sulphide  ores,  but  in  general  there  is  no 
evidence  that  bodies  of  older  sulphide  ore  occupied  the  places 
of  the  deposits.  These  ores  are  therefore  considered  primary, 
although  they  have  been  leached  by  ground  water  from  older 
metalliferous  rocks. 

Sedimentary  beds  of  mechanical,  organic,  or  chemical  origin 
are  the  sources  of  many  economic  products,  such  as  coal,  clay, 
gypsum,  salt,  potash,  lime,  phosphate  rock,  iron,  manganese, 
and  placer  gold.  Workable  sulphide  deposits  of  sedimentary 


CLASSIFICATION  OF  ORE  DEPOSITS  7 

origin  are  exceedingly  rare.  Sedimentary  deposits,  like  sedi- 
mentary rocks,  are  derived  mainly  from  the  decay  of  older  rocks 
and  older  deposits. 

Certain  classes  of  ore  deposits  that  have  been  recognized  by 
some  investigators  seem  not  to  be  included  in  the  foregoing 
classification.  Among  these  are  the  "segregated  veins"  and 
"overlapping  lenses"  that  are  generally  assumed  to  have  been 
concentrated  during  dynamic  or  regional  metamorphism,  from 
substances  contained  in  the  country  rock.  Some  of  the  ore 
bodies  assumed  to  have  been  formed  in  such  a  manner  are  the 
dynamically  metamorphosed  deposits  of  the  primary  classes 
mentioned  above.  If  such  deposits  are  formed  during  dynamic 


1  Masmatic  sejrejalions 

2  Pegmatite  veins 

3  Contad-metamorphic 

•4  Deposits  of  deep  vein  zone 
----5  Deposits  formed  at  moderate 

depth  (hot  solutions) 
'-—6  Deposits  formed  ai  shallow 

depth  (hot  solutions) 
7  Deposits  formed  by  cold 

solutions 

v> — »fl  Sedimentary  deposits 
I — , Fe'L-;  _•_"?  jtesL  *y. 


DEPOSITION 


-M4TCIHAL3      BCim-tO     flKV     OUX*     O£fV3JT3 

DEFORMATION 


ENRICHMENT 


FIG.  6. — Diagram  illustrating  mode  of  deposition,  deformation,  superficial 
alteration  and  enrichment  of  ore  deposits  and  protores.  Diagram  shows  also 
how  deposits  may  be  broken  down  mechanically  or  dissolved  and  their  pro- 
ducts may  enter  new  deposits.  It  is  helpful  to  trace  out  on  the  diagram  the 
genesis  of  several  typical  deposits. 

metamorphism  they  should  be  placed  with  the  deposits  of  the 
deep  vein  zone.  They  are  uniformly  associated  with  rocks  that 
have  been  deformed  beneath  the  surface  by  great  pressures  under 
heavy  load. 

Placers  are  deposits  formed  by  mechanical  processes  from 
materials  that  resist  weathering — such  as  gold,  platinum,  tin, 
and  iron  oxides.  They  are  classed  as  sedimentary  deposits,  for 
they  are  formed  in  running  or  standing  water. 

"Land  sediments,"  such  as  those  formed  in  basins  and  in 
troughs  that  are  dry  part  of  the  time  and  those  formed  on  the 
slopes  of  hills,  are  likewise  considered  sedimentary,  although  less 
water  has  taken  part  in  their  transportation.  A  few  of  them 


8          THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

contain  gold  placers.  Deposits  at  the  orifices  of  hot  springs 
may  also  properly  be  classed  with  sedimentary  beds.1  Residual 
deposits  may  be  classed  according  to  the  origin  of  the  protore  or 
material  from  which  they  were  derived. 

A  diagram  illustrating  the  classification  of  ore  deposits  is 
shown  by  Fig.  6. 

1  In  the  outline  above  "aggradational  deposits"  could  be  substituted 
for  "sedimentary  deposits"  and  the  group  subdivided  into  sedimentary, 
glacial,  and  eolian  deposits.  This  would  give  too  much  emphasis  to  glacial 
and  eolian  deposits,  whose  importance  as  sources  of  the  metals  is  compara- 
tively small.  These  deposits,  however,  yield  nonmetallic  products  of  value, 
including  clays  and  sands.  A  place  is  provided  for  them  in  Fig.  6. 


CHAPTER  III 

DEPOSITS  FORMED  BY  MAGMATIC  SEGREGATION 

Occurrence. — In  igneous  rocks,  principally  in  rocks  that  have  crystallized 
slowly.  Some  but  not  all  are  at  the  bases  or  edges  of  rock  units.  Some 
dikes  and  stocks  are  all  ore  or  protore. 

Composition. — The  minerals  are  the  minerals  of  igneous  rocks.  The 
gangue  minerals  include  quartz,  feldspars,  pyroxene,  olivine,  mica,  and  other 
rock-making  minerals.  In  general  the  minerals  of  the  parent  country  rock 
and  the  minerals  of  the  deposit  are  similar,  but  the  proportions  are  different. 
Metals  won  include  iron,  nickel,  titanium,  chromium,  platinum,  and  subordi- 
nate copper  and  gold. 

Shape. — Some  are  irregular  in  outline,  others  are  rudely  ellipsoidal,  and 
still  others  are  tabular. 

Size. — Some  are  small  and  others  are  very  large.  The  value  of  the  ore 
per  ton  is  generally  low,  and  as  a  rule  the  deposit  must  be  large  to  be  of  com- 
mercial value. 

Texture. — The  minerals  are  intergrown  like  the  minerals  of  igneous  rocks. 
Banding,  though  not  uncommon,  is  not  crustified  as  in  veins.  Miarolitic 
cavities  are  sometimes  found  in  deposits  formed  by  magmatic  segregation, 
but  these  are  not  nearly  so  common  as  the  vugs  in  veins,  and  they  are  not 
symmetrically  lined,  as  is  common  in  the  unfilled  portions  of  veins.  The 
ore  is  contemporaneous  with  the  parent  country  rock  and  grades  into  it, 
although  in  some  deposits  the  gradational  zones  are  narrow.  Contacts 
with  rocks  that  are  not  contemporaneous  are  not  gradational.  The  rocks 
are  not  hydrothermally  altered  at  the  time  of  deposition  of  the  ore.  Idio- 
morphic  rock-making  crystals  may  be  inclosed  in  the  ore  minerals.  Frag- 
ments of  intruded  rock  may  be  included  in  the  ore. 

General  Features. — Some  investigators  use  the  term  "mag- 
matic segregation"  for  all  ore  deposits  formed  by  waters  of  mag- 
matic origin.  As  used  here  this  term  is  applied  only  to  those 
deposits  which  solidify  from  magmas.  The  deposits  are  there- 
fore igneous  rocks  in  a  strict  sense,  but  on  account  of  their 
unusual  character  many  of  them  are  not  always  so  regarded. 
Magmatic  differentiation  is  the  process  by  which  a  magma  or 
molten  rock  stuff  of  supposed  uniform  composition  splits  up  into 
bodies  of  different  composition.1  The  subject  properly  belongs 
to  petrology,  but  certain  aspects  should  be  considered  here. 

1  VOGT,  J.  H.  L. :  Bildung  von  Erzlagerstatten  durch  Differentiations- 
processe  in  basischen  Eruptivemagmata.  Zeitschr.  prakt.  Geologic,  vol.  1, 
pp.  4-11,  125-143,  257-284,  1893. 

9 


10        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Under  some  conditions  in  a  magma  the  heavy  material  settles 
to  the  bottom  and  the  lighter  material  rises  to  the  top,  perhaps 
after  the  manner  of  separation  of  metals  from  slag  in  a  blast 
furnace.  Segregation  may  thus  take  place  before  crystalliza- 
tion begins.1  Magmas  are  solutions  and  obey  the  laws  of  solu- 
tions. Thus  the  minerals  that  solidify  or  separate  out  from  a 
cooling  magma  do  so  in  the  order  of  their  saturation  points  under 
the  conditions  that  prevail.  As  a  general  rule  the  more  basic 
materials,  such  as  iron,  magnesium,  and  titanium  minerals,  will 
crystallize  first.  These  in  the  main  are  the  oxides  and  sulphides. 
As  crystallization  goes  on  the  liquid  portion  generally  becomes 
more  and  more  acidic  or  siliceous,  although  some  silica  also  may 
crystallize  out  early  in  the  process.  If  viscosity  is  not  too  high, 
the  heavy  minerals  fall.  Later,  if  pressure  is  relieved  by  extra- 
vasation of  the  upper  lighter  molten  liquid,  the  heavier  material 
remaining  at  the  bottom,  owing  to  relief  of  pressure,  may  be 
remelted.2 

Soret,  in  1879,  established  the  principle  that  when  two  parts 
of  the  same  solution  are  at  different  temperatures  there  may  be 
a  concentration  of  the  dissolved  substance  in  the  cooler  portion 
of  the  solution.  Differences  in  pressure  also  may  cause  differ- 
ences in  concentration.3 

Becker4  attacked  the  application  of  Soret's  principle  to  mag- 
mas, urging  that  magmas  are  so  viscous  that  diffusion  would  be 
too  slow  to  permit  the  segregation  of  heterogeneous  masses  of 
large  size.  Becker8  has  shown  also  that  fractional  crystalliza- 
tion on  cooling,  in  a  dike  or  laccolith,  may  cause  differentiation. 
Along  the  cooler  walls  the  less  soluble  material  will  crystallize 
first,  and  convection  currents  will  tend  to  carry  the  more  soluble 
material  into  the  still  liquid  portion  of  the  mass,  where  it  will 
later  solidify.  Thus  the  less  soluble  material  will  predominate 

1  MOROZEWICZ,  J.:  Min.  pet.  Mitt.,  vol.  18,  p.  233,  1898. 

DALY,  R.  A. :  Differentiation  of  a  Secondary  Magma  through  Gravitation 
Adjustment.  Rosenbusch  Festschr.,  pp.  203-233,  1906. 

"ScHWEio,  MARTIN:  Differentiation  der  Magma.  Neues  Jahrb.,  Beilage 
Band  17,  p.  516,  1903. 

3  IDDINGS,  J.  P. :  The  Origin  of  Igneous  Rocks.     Philos.  Soc.  Washington 
Bull.  12,  pp.  89-130,  1892. 

IDDINGS,  J.  P. :  Igneous  Rocks,  pp.  1-464,  1909. 
PIRSSON,  L.  V.:  Rocks  and  Rock  Minerals,  414  pp.,  1909. 

4  BECKER,  G.  F.:  Some  Queries  on  Rock  Differentiation.     Am.  Jour.  Sci., 
4th  ser.,  vol.  3,  pp.  121,  257,  1897. 

6  Idem,  p.  257. 


DEPOSITS  FORMED  BY  MAGMATIC  SEGREGATION  11 

on  the  outer  walls.  This  process  may  be  compared  to  the  Pattin- 
son  process,  for  desilverizing  lead-silver  ore.  The  argentiferous 
lead  is  melted,  and  as  lead  is  precipitated  at  a  lower  tempera- 
ture than  a-  mixture  of  lead  and  silver,  the  lead  will  solidify  first. 
It  is  ladled  out  from  the  rich  lead-silver  solution  while  the  latter 
is  still  liquid.  By  this  process  a  considerable  concentration  of 
silver  results. 

Although  the  mechanism  of  magmatic  differentiation  is  not 
fully  understood,  there  is  much  evidence  that  it  has  operated, 
for  commonly  an  igneous  rock  grades  almost  imperceptibly 
into  another  igneous  rock  of  different  composition,  although  the 
two  have  formed  from  the  same  molten  body.  In  comparatively 
rare  examples  one  rock  is  peripheral  with  respect  to  the  dif- 
ferentiated body.1 

In  many  large  bodies  of  igneous  rocks,  however,  if  not  in  most 
of  them,  the  chemical  composition  is  nearly  uniform  over  large 
areas.  It  would  be  natural  to  suppose  that  tjie  processes  above 
outlined  would  operate  to  segregate  the  elements  much  more 
generally  than  they  do.  These  processes  are  opposed  by  con- 
vection currents  and  other  movements  of  the  magmas,  which 
tend  to  keep  the  solutions  uniform,  and  by  viscosity,  which  tends 
to  prevent  diffusion  or  movement  of  molecules  through  the  mass. 
Whatever  the  cause,  it  is  generally  found  that  magmatic  differ- 
entiation, if  operative,  is  incomplete,  and  that  more  commonly 
than  otherwise  noteworthy  proportions  of  magnetite,  pyrite,  or 
other  heavy  materials  remain  distributed  throughout  the  great 
rock  masses. 

One  other  aspect  of  the  subject  of  ores  associated  with  igneous 
rocks  should  be  mentioned.  Practically  all  investigators  agree 
that  the  magmas,  or  the  igneous  rocks,  are  the  sources  of  all 
the  metals.  The  study  of  the  genesis  of  ore  deposits  is  therefore 
a  study  of  the  methods  of  concentration  of  the  ores  from  igneous 
rocks.  Processes  operating  in  the  magma  may  cause  differ- 
entiation and  segregation  of  workable  bodies.  Deposits  so 
formed,  including  both  oxides  and  sulphides,  are  well  authenti- 
cated, although  deposits  of  sulphides  are  rare.  Low-grade' 
materials  of  magmatic  origin  may  on  weathering  become  rich 

1  HARKEK,  ALFRED:  "The  Natural  History  of  Igneous  Rocks,"  pp. 
311-332,  1909. 

One  of  the  clearest  discussions  of  differentiation  of  magmas  is  that  by 
L.  V.  PIRSSON  in  U.  S.  Geol.  Survey  Bull.  237,  pp.  181-190,  1905. 


12        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

residual  masses,  or  they  may  weather  and  be  subsequently  re- 
concentrated  by  mechanical  processes,  forming  sedimentary 
beds.  Meteoric  waters  may  dissolve  the  valuable  constituents 
from  the  solid  igneous  rock  and  concentrate  them  by  precipita- 
tion in  openings.  But  magmas  carry  water  and  other  fluids 
that  solidify  at  low  temperatures,  and  during  crystallization 
these  fluids  may  escape.  Coursing  through  fissures  and  reacting 
with  rocks,  they  precipitate  metals  with  which  they  are  charged, 
and  in  the  higher  regions  they  mingle  with  ground  water,  which 
generally  causes  further  precipitation  of  mineral  matter.  Ulti- 
mately they  may  escape  as  hot  springs.  The  processes  by  which 


FIG.  7. — Cross-section    and    plan  of  part  of  Sudbury  nickel  district,  Ontario. 
(After  Coleman,  Ontario  Bureau  of  Mines,  Canada.} 

ore   deposits  are  formed  from  these  igneous  emanations   are 
discussed  elsewhere  in  this  book. 

Occurrence. — From  the  foregoing  discussion  it  would  be  sup- 
posed that  magmatic  segregation  would  not  occur  in  ordinary 
surface  lavas.  These  bodies  cool  quickly,  and  appreciable  segre- 
gation rarely  takes  place  in  them.  Owing  to  the  relief  of  pres- 
sure when  they  are  poured  out,  steam  and  other  gases  which  aid 
diffusion  readily  escape.  At  a  few  places  gems  and  metals  are 
found  in  surface  lavas,  such  are  probably  due  to  magmatic  segre- 
gation before  eruption,  but  they  are  of  relatively  small  value. 
The  deep-seated  rocks,  especially  the  basic  rocks,  such  as  norite, 
gabbro,  and  peridotite  appear  to  be  especially  favorable  to  mag- 


DEPOSITS  FORMED  B  Y  MAGMA  TIC  SEGREGA  TION  13 

matic  segregation.  The  more  acidic  rocks,  on  the  other  hand, 
are  more  fruitful  as  sources  of  epigenetic  ores.  Lithium,  tung- 
sten, tin,  and  some  other  elements  are  characteristically  asso- 
ciated with  the  acidic  magmas;  nickel,  platinum,  and  chromium 
are  associated  with  basic  or  ferromagnesian  magmas.1  Some 
iron  deposits  appear  to  have  segregated  from  moderately  acidic 
rocks. 

Gravity  and  fractional  crystallization  tend  to  segregate  por- 
tions of  the  magmas  at  the  bottom  or  on  the  sides  of  rock  units. 
Among  the  deposits  found  in  such  positions  a  conspicuous  example 
is  that  of  the  nickel  ores  of  the  Sudbury  nickeliferous  eruptive 
mass  (see  Fig.  7).  On  the  other  hand,  many  deposits  formed  by 
magmatic  segregation  are  entirely  surrounded  by  the  parent  rock. 
Some  dikes  and  flows  are  essentially  all  ore  or  protore.  Such 
bodies  are  assumed  to  be  the  products  of  magmas  as  differentiated 
before  extravasation.  All  the  laws  governing  these  localizations 
have  not  yet  been  elucidated. 

Composition. — As  deposits  due  to  magmatic  segregation  are 
igneous  rocks,  their  minerals  are  exclusively  the  igneous  rock- 
making  minerals  and  their  alteration  products.  A  list  of  the 
more  important  minerals  is  given  below. 

acmite  diallage  magnetite  riebeckite 

aegirite  diamond  melilite  rutile 

albite  diopside  molybdenite  sapphire 

allanite  elseolite  monazite  silver 

amphiboles  emery  muscovite  sodalite 

analcite  feldspars  nepheline  specularite 

anorthite  fluorite  noselite  spinel 

apatite  garnet  olivine  spodumene 

arfvedsonite  gold  orthoclase  titanite 

augite  graphite  pentlandite  topaz 

biotite  hauynite  perofskite  tourmaline 

cancrinite  hematite  picotite  tridymite 

cassiterite  hornblende  platinum  xenotime 

chalcopyrite  hypersthene  pyrite  zircon 

chromite  ilmenite  pyroxenes 

corundum  iron  pyrrhotite 

cordierite  leucite  quartz 

Shape. — Some  valuable  minerals,  like  diamonds,  sapphires, 
and  rubies,  are  sparingly  disseminated  in  igneous  rocks.  In 
some  deposits  the  rock  mass  exposed  may  be  essentially  homo- 

1  WASHINGTON,  H.  S. :  The  Distribution  of  Elements  in  Igneous  Rocks. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  39,  p.  735,  1908. 


14        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


geneous,  and  any  magmatic  differentiation  must  have  taken 
place  before  the  body  now  exposed  came  to  rest.  The  entire 
mass  is  then  the  mineral  deposit.  Such  masses  obviously  may 
be  tabular  dikes  (like  the  sapphire  deposits  at  Yogo,  Mont.)  or 
irregular  stocks  (like  the  Kimberly  diamond  deposit),  or  they 
may  be  irregularly  shaped  like  any  irregular  intrusive  igneous 
body.  Some  deposits  of  titaniferous  magnetite  are  simply 
igneous  dikes.  A  deposit  that  forms  only  part  of  the  parent 

rock  mass  is  generally  irregu- 
lar, but  some  of  these  also  are 
broadly  tabular.  In  general, 
however,  deposits  segregated 
in  the  parent  magma  ap- 
proach the  tabular  form  less 
closely  than  fissure  veins  or 
sedimentary  beds. 

Size. — In  size  the  deposits 
due  to  magmatic  segregation 
are  exceedingly  diverse. 
Among  the  large  ore  bodies 
formed  by  this  process  are 
certain  deposits  of  magnetic 
iron  ores  that  aggregate 
millions  of  tons  and  the  enor- 
mous deposits  of  nickel-copper 
ores  in  Sudbury,  Ontario.  On 
the  other  hand,  small  segre- 
gated deposits  of  chalcopyrite 
or  galena  and  schlieren1  of 


FIG.  8. — Iron  ore  formed  by  magmatic 
segregation,  from  Iron  Lake,  Minnesota. 
Dark  is  intergrowth  of  magnetite  and 
ilmenite;  light  is  feldspar.  (After 
Singewald.) 

chromite  may  be  too  small  to  be  of  value. 

Texture. — The  constituent  minerals  of  these  deposits  are 
generally  mutually  interlocked,  like  the  minerals  of  granular 
igneous  rocks.  In  some  deposits  magnetite,  pyrite,  olivine,  and 
certain  other  minerals  have  crystallized  out  before  the  more 
siliceous  minerals,  such  as  feldspar  and  quartz.  If  the  deposit 
has  been  formed  by  the  separation  of  falling  metalliferous  crys- 
tals, the  ore  minerals,  which  are  the  heavier  constituents,  should 

1  Schlieren  is  a  German  term  without  an  English  equivalent,  used  to 
describe  poorly  denned  streaks  of  material  formed  in  or  near  the  border  of  an 
igneous  mass  before  the  magma  came  completely  to  rest.  They  are  even 
less  persistent  than  most  veinlets  or  gash  veins. 


DEPOSITS  FORMED  BY  MAGMATIC  SEGREGATION  15 

be  among  the  first  minerals  that  were  formed  in  the  overlying 
country  rock,  for  they  could  not  fall  through  a  mass  already 
solidified.  A  study  of  the  relative  age  of  the  mineral  constituents 
of  the  ore  and  rock,  as  shown  by  their  contact  relations  in  thin 
section,  is  therefore  a  check  on  a  hypothesis  that  postulates  a 
genesis  by  magmatic  segregation.  It  should  be  noted,  however, 
that  segregation  may  take  place  prior  to  crystallization,  and  that 
in  some  ore  bodies  of  this  class  the  ore  minerals  have  crystallized 
after  the  gangue  minerals  and  now  inclose  them  (Fig.  8). 


FIG.  9.— Section  of  ore  formed  by  magmatic  segregation.  Well-shaped 
crystals  of  feldspar  and  quartz,  partly  dissolved  are  floated  in  a  matrix  of 
galena. 

Although  banding  is  not  characteristic  of  deposits  formed  by 
magmatic  segregation,  some  of  them — for  example,  some  of  the 
magnetites  of  New  York — show  many  bands  of  magnetite  alter- 
nating with  feldspar.  The  banding  is  not  crustified,  however, 
and  this  feature,  together  with  the  mineral  composition,  serves 
to  distinguish  them  from  normal  fissure  veins.  The  same 
minerals  are  found  in  the  ore  and  in  the  parent  country  rock,  and 
as  a  rule  the  ore  passes  into  the  country  rock  by  a  gradual  dimi- 
nution of  ore  minerals  or  by  an  increase  in  gangue  minerals. 
The  zone  of  gradation  between  ore  and  country  rock  may  be 
very  narrow;  in  some  deposits  it  is  less  than  half  an  inch  wide. 
Vugs  are  lacking  in  this  group  of  deposits,  and  miarolitic  cavities 


16        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

are  much  less  common  than  in  pegmatite  veins.  The  original 
rock  minerals  are  not  hydrothermally  altered  in  these  deposits, 
as  they  are  in  the  wall  rock  along  veins  deposited  from  thermal 
waters.  As  the  deposits  formed  by  magmatic  segregation  are 
not  metasomatic,  pseudomorphous  replacements  are  unknown 
in  the  original  ore.  If  the  sulphide  ores  inclose  well-shaped 
(idiomorphic)  crystals  of  feldspar  (as  shown  in  Fig.  9),  there  is  a 
very  strong  probability  that  the  deposit  is  due  to  magmatic 
segregation.  If  it  had  been  formed  along  a  fissure,  the  borders 
of  the  fissure  should  cross  the  crystals  of  the  rock,  and  if  it  had 
been  formed  by  replacement,  the  boundaries  of  the  rock-making 
minerals  would  not  everywhere  coincide  so  nearly  with  the 
crystal  boundaries.1 

Deposits  formed  by  magmatic  segregation,  like  other  igneous 
rocks,  may  contain  fragments  of  the  rocks  invaded  by  the  magma. 
In  some  places  the  parent  magmas  have  doubtless  contributed 
solutions  that  have  deposited  veins  in  fissures.  Thus  ores 
formed  by  magmatic  segregation  and  ores  deposited  in  and 
along  fissures  may  be  closely  associated  and  closely  related 
genetically. 

References 
DEPOSITS  FORMED  BY  MAGMATIC  SEGREGATION 

BARLOW,  A.  E. :  Report  on  the  Nickel  and  Copper  Deposits  of  the  Sud" 
bury  Mining  District.  Canada  Geol.  Survey  Fourteenth  Ann.  Rept.,  part  H, 
1904.  On  the  Origin  and  Relations  of  the  Nickel  and  Copper  Deposits  of 
Sudbury,  Ontario,  Canada,  with  Bibliography.  m  Econ.  Geol.,  vol.  1,  pp. 
454-^166,  545-553,  1906. 

BECK,  R.:  "Lehre  von  den  Erzlagerstatten,"  Berlin,  1909. 

WEED,  W.  H.:  "The  Nature  of  Ore  Deposits"  (translation  of  BECK), 
New  York,  1909. 

BEYSCHLAG,  F.,  KRUSCH,  P.,  and  VOGT,  J.  H.  L. :  "Lehre  von  den  Erzlager- 
statten," translation  by  S.  J.  TRUSCOTT,  pp.  242-347,  London,  1914. 

COLEMAN,  A.  P.:  The  Sudbury  Nickel  Field.  Ontario  Bur.  of  Mines 
Rept.,  vol.  14,  part  3,  1905. 

DALY,  R.  A. :  Differentiation  of  a  Secondary  Magma  through  Gravitative 
Adjustment.  Rosenbusch  Festschr.,  pp.  203-233,  1906. 

KEMP,  J.  F. :  The  Geology  of  the  Magnetites  near  Port  Henry,  N.  Y.,  and 
Especially  those  of  Mineville.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  27,  pp. 
146-203,  1897.  A  Brief  Review  of  the  Titaniferous  Magnetites.  School  of 
Mines  Quart.,  vol.  20,  pp.  323-356,  July,  1899;  vol.  21,  pp.  56-65,  November, 
1899. 

1  EMMONS,  W.  H. :  Some  Ore  Deposits  of  Maine  and  the  Milan  Mine,  New 
Hampshire.  U.  S.  Geol.  Survey  Bull.  432,  p.  20,  1910. 


DEPOSITS  FORMED  BY  MAGMATIC  SEGREGATION  17 

LINDGREN,  WALDEMAR:  "Mineral  Deposits,"  pp.  735-772,  New  York, 
1913. 

PRATT,  J.  H.:  The  Occurrence,  Origin,  and  Chemical  Composition  of 
Chromite,  with  Especial  Reference  to  the  North  Carolina  Deposits.  Am. 
Inst.  Min.  Eng.  Trans.,  vol.  29,  pp.  17-39,  1899.  Separation  of  Alumina 
from  Molten  Magmas,  and  the  Formation  of  Corundum.  Am.  Jour.  Sci.,  4th 
ser.,  vol.  8,  pp.  227-231,  1899.  On  the  Origin  of  Corundum  Associated  with 
the  Peridotites  in  North  Carolina.  Am.  Jour.  Sci.,  4th  ser.,  vol.  6,  pp.  49- 
65,  1898.  Corundum  and  Its  Occurrence  and  Distribution  in  the  United 
States.  U.  8.  Geol.  Survey  Bull.  269,  1906. 

VOGT,  J.  H.  L. :  The  Formation  of  Eruptive  Ore  Deposits.  Min.  Indus- 
try, vol.  4,  pp.  743-754,  1895.  Beitrage  zur  genetischen  Classification  der 
durch  magmatische  Differentiationsprocesse  und  der  durch  Pneumatolyse 
entstandenen  Erzvorkommen.  Zeitschr.  fur  Prakt.  Geologic,  Chrom- 
eisenerz,  pp.  384-393,  1894. 

WASHINGTON,  H.  S. :  The  Distribution  of  the  Elements  in  Igneous  Rocks. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  39,  pp.  735-764,  1908. 


CHAPTER  IV 

PEGMATITES 

Occurrence. — In  or  near  deep-seated  igneous  rocks;  many  are  near  the 
outer  margins  of  intrusives.  Where  pegmatites  intrude  sedimentary  rocks 
or  schists,  they  generally  follow  bedding  planes  or  the  planes  of  schistosity, 
but  locally  they  cut  across  such  planes. 

Composition. — The  minerals  are  essentially  the  minerals  found  in  igneous 
rocks,  but  the  range  in  composition  of  pegmatites  is  less  than  that  of  the 
igneous  rocks.  Feldspar,  quartz,  and  mica  are  very  common  constituents. 
Many  rare  minerals  and  gem  minerals  are  found  in  pegmatites.  Metals 
include  tin,  tungsten,  bismuth,  yttrium,  thorium,  tantalum,  and  others. 

Shape. — Many  pegmatites  have  very  irregular  outlines,  especially  those 
which  lie  within  the  parent  igneous  rock.  A  large  number  of  pegmatites  are 
rudely  tabular.  Pipe-like  and  dendritic  bodies  are  represented. 

Size. — Pegmatites  range  in  size  from  minute  bodies  to  masses  that  extend 
over  many  acres.  Where  the  material  has  been  injected  in  highly  foliated 
schist,  the  individual  pegmatite  sheets  may  be  paper  thin.  Metalliferous 
concentrations  in  pegmatites  are  generally  small. 

Texture. — The  crystals  are  usually  large  and  commonly  are  intergrown 
as  in  igneous  rocks.  Some  pegmatites  grade  into  the  containing  parent 
rock.  Miarolitic  cavities  are  common.  Banded  or  comb  structure  and 
openings  with  crustified  bands  are  developed  in  some  pegmatites,  but  such 
features  are  much  less  common  than  in  normal  ore  veins.  Fluid  inclusions 
are  locally  abundant. 

Pegmatites  are  the  principal  sources  of  marketable  feldspars, 
quartz,  and  mica,  and  they  contain  also  several  varieties  of 
gems,  among  them  tourmaline,  beryl,  and  kunzite.  They  are 
not  important  sources  of  the  metals,  although  they  have  sup- 
plied a  little  tin,  bismuth,  and  tungsten,  as  well  as  the  rare  metals 
yttrium,  thorium,  tantalum,  etc.  Pegmatites  are  of  great  scien- 
tific interest,  because  they  resemble  in  some  respects  igneous 
rocks  and  in  other  respects  certain  ore  veins. 

Eutectics. — Igneous  rocks  may  be  regarded  as  frozen  magmas. 
Rock  magmas  are  not  physical  mixtures  but  solutions  of  sub- 
stances mutually  dissolved.  Difficultly  fusible  materials  may 
be  readily  fused  if  mixed  together.  A  familiar  example  is  the 
formation  of  a  common  slag  with  iron,  lime,  and  silica.  Lime 
(CaO)  has  a  melting  point  so  high  that  it  withstands  the  great 

18 


PEGMATITES 


19 


+20° 


heat  of  the  oxyhydrogen  flame  when  it  is  used  to  give  a  calcium 
light.  Silica  (SiO2)  likewise  has  a  high  melting  point,  and  for 
that  reason  it  is  used  by  chemists  for  "quartz  ware"  receptacles 
whose  contents  must  be  heated  to  temperatures  above  the  melt- 
ing point  of  common  glass.  Iron  also  when  pure  has  a  high  melt- 
ing point.  These  three  substances,  lime,  silica,  and  iron,  if 
mixed  in  a  blast  furnace,  form  a  liquid  slag  at  a  moderately 
low  temperature;  indeed,  one  of  the  common  problems  of  the 
metallurgist  is  to  bring  iron,  lime,  and  silica  together  cheaply  and 
in  the  proper  proportions  so  that  the  valuable  metals  which  they 
may  contain  will  be  separated  at  a  minimum  cost  for  fuel.  To- 
gether they  "melt  easily"  because  they  dissolve  one  another. 

The  freezing  temperature  of  a  solution  is  generally  lower  than 
that  of  the  pure  solvent.     A  solution  of  sodium  chloride  in 
water  solidifies  at  —23°.     Pure 
water  solidifies   at  0°,  and  salt 
solidifies  or  is  precipitated  out 
of  solution  at  a  temperature  de- 
pending on  the  amount  in  the 
solution.     The  relations  are  ex- 
pressed diagrammatically  by  Fig. 
10.     As  shown  by  this  figure1  ice 
will  begin  to  separate  from  a  5       0 
per  cent,  salt  solution  when  the 

solution     is     COoled     to      —3.4°.         FIG.  10.— Diagram  illustrating  tem- 
With  a  10  per  cent.  Salt  Solution     Peratures  of  crystallization  of  salt  and 
.  .       .  ,      ice  from  salt-water  solutions. 

ice  separates  at  —  6  ,  and  with 

23.6  per  cent,  of  salt  ice  solidifies  at  —23°.  The  sequence  is 
analogous  when  solutions  containing  more  than  23.6  per  cent, 
of  salt  are  gradually  cooled,  but,  instead  of  pure  ice,  pure  salt 
separates  until  the  residual  liquid  contains  23.6  per  cent,  of 
salt.  Ice  and  salt  solidify  together  at  —23°.  If  the  cooling 
solution  has  23.6  per  cent,  of  salt  neither  ice  nor  salt  separates 
until  the  temperature  has  fallen  to  —  23°,  when  the  whole  freezes 
to  a  solid  mass.  No  other  mixture  of  water  and  salt  freezes 
at  a  lower  temperature  than  this.  Hence  a  solution  containing 
23.6  per  cent,  of  salt  is  called  a  eutectic2  mixture,  and  —23°  the 

1  From  MELLOR,  J.  W. :   "Modern  Inorganic  Chemistry,"  p.  161,  1914. 
Based  on  determinations  made  by  F.  GUTHRIE  in  1875. 

2  Eutectic  is  derived  from  the  Greek  words  that  mean  "  melt  easily."     A 
eutectic  mixture  or  eutectic  is  not  a  mineral. 


20        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

eutectic  point.  The  ice  and  salt  are  mutually  intergrown,  and 
no  matter  what  the  original  solution  may  have  been  the  last  frac- 
tion to  solidify  has  the  same  composition  and  a  constant  melting 
point.  The  crystals  of  ice  may  be  seen  lying  in  a  matrix  of  salt, 
more  readily  if  a  colored  salt  like  copper  sulphate  is  used.1 

It  has  been  suggested  by  Teall2  and  by  Vogt3  that  the  micro- 
pegmatites  or  intergrowths  of  orthoclase  and  quartz  that  form 
last  in  many  igneous  rocks  and  fill  the  interstices  between  the 
older  crystals  are  eutectic  mixtures,  and  more  recently  Day  and 
his  associates  have  investigated  many  igneous  rocks  with  these 
premises  in  view.  Teall,  from  microscopic  analyses  of  such  a 
micropegmatite,  regards  a  mixture  of  62.05  per  cent,  orthoclase 
and  37.95  per  cent,  quartz  as  a  possible  mixture  of  maximum 
solubility;  Vogt  gives  a  mixture  of  74.25  per  cent,  orthoclase  and 
25.75  per  cent,  quartz.  Many  pegmatite  veins  show  proportions 
of  quartz  and  feldspar  approximating  those  suggested  by  Teall 
and  by  Vogt,  but  the  proportions  are  in  general  less  constant  in 
pegmatite  veins  than  in  the  micropegmatite  that  forms  the 
groundmass  of  some  granites  and  similar  rocks.  Although  there 
are  noteworthy  differences  in  the  composition  of  pegmatites, 
nevertheless  they  tend  to  approach  a  uniform  composition  more 
closely  than  the  normal  igneous  rocks.  The  presence  of  the 
mineralizers  discussed  below  doubtless  influences  the  chemical 
systems  from  which  pegmatites  are  formed;  consequently  appre- 
ciable variations  in  the  composition  of  the  pegmatites  should  be 
expected,  even  in  pegmatites  that  are  genetically  related  to  the 
same  rock  magma,  and  there  may  be  considerable  variations 
in  a  single  pegmatite  vein. 

Agents  of  Mineralization. — As  a  rule  the  crystals  of  the  peg- 
matites are  larger  than  the  crystals  of  igneous  rocks  and  of  ore 
veins.  In  ore  veins  the  great  masses  of  solid  quartz,  when  viewed 
under  the  microscope,  are  seen  generally  to  be  composed  of  a 
large  number  of  small  crystals  closely  interlocking,  each  having 
a  different  optical  orientation.  On  the  other  hand,  a  mass  of 
pegmatitic  quartz  or  feldspar  commonly  shows  a  uniform  optical 
orientation  for  each  of  these  two  minerals.  Elie  de  Beaumont, 
Deville,  Daubre"e,  and  others,  experimenting  upon  the  synthesis 
of  minerals,  found  that  small  quantities  of  H20,  CO2,  H3BO3, 

1  MELLOR,  J.  W.:  "Modern  Inorganic  Chemistry,"  pp.  161-162,  1914. 

2  TEALL,  J.  J.  H.:   "British  Petrography,"  p.  395,  1888. 

3  VOGT,  J.  H.  L.:  "Die  Silikatschmelzlosungen,"  part  2,  p.  113,  1904. 


PEGMATITES  21 

HC1,  HF,  and  certain  other  compounds  greatly  facilitated  the 
crystallization  of  melts,  and  the  term  "mineralizing  agents"  or 
"  mineralizers "  has  been  applied  to  these  compounds.  It  is 
believed  that  their  presence,  by  decreasing  viscosity,  aids  diffu- 
sion and  thus  favors  the  development  of  large  crystals.  Small 
amounts  of  the  elements  of  these  agents  of  crystallization  occur 
in  many  pegmatites,  and  they  are  supposed  to  have  been  effective 
in  lowering  the  point  of  solidification  or  precipitation  and  in  pro- 
moting diffusion  in  the  magmas. 

Some  of  the  minerals  that  contain  the  elements  of  the  mineral- 
izers are  common  in  pegmatite  veins  and  in  certain  deep-seated 
ore  veins.  Among  the  minerals  of  pegmatites  which  carry 


CH    Sa    E3 

Mostlyquartz    Mostly  biotite.       Feldspar  Muscov.te  Monaz.te 

little  quartz 
and  graphite 

FIG.  11.— Monazite-bearing  pegmatite.     Shelby,  N.  C.     (After  Sterrett,   U.  S. 
Geol.  Survey.) 

fluorine  are  topaz,  fluorite,  fluor-apatite,  and  some  micas. 
Tourmaline  contains  boron,  and  chlorine  is  contained  in  chlor- 
apatite  and  in  some  scapolite.  Compounds  of  the  rarer  elements 
lithium,  beryllium,  tungsten  and  cerium  (Fig.  11),  seem  also  to 
act  as  agents  of  mineralization.  Minerals  containing  lithium  are 
lithium  mica,  rubellite,  amblygonite,  and  spodumene.  Beryl, 
or  aquamarine,  contains  beryllium.  Minerals  in  pegmatites 
containing  tungsten  are  wolframite,  scheelite,  and  hiibnerite. 
Columbite  and  tantalite  contain  columbium  and  tantalum. 
Water  is  contained  in  biotite,  muscovite,  lepidoite,  etc. 

These  minerals,  except  those  containing  water,  are  lacking 
in  some  and  are  found  only  in  relatively  small  quantities  in  most 


22        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


pegmatites.  Many  of  them  when  present  were  the  last  minerals 
formed  in  the  pegmatites,  for  they  line  the  miarolitic  cavities. 
Fluid  inclusions  in  quartz  are  common.  Larger  quantities  of 
water  and  of  these  other  mobile  compounds  may  have  escaped 
from  the  cavities  after  the  pegmatite  solidified.  If  not,  it  must 
be  assumed  that  the  amounts  of  the  mineralizers  necessary  to 
accomplish  coarse  crystallization  were  in  most  places  small. 

Occurrence. — Pegmatites1  are  associated  with  deep-seated 
igneous  rocks,  generally  with  the  siliceous  rocks,  less  commonly 
also  with  basic  rocks.  Rhyolites,  basalts,  and  other  surface 
lavas  and  intrusive  rocks  formed  near  the  surface  are  not 
accompanied  by  pegmatites;  consequently  it  is  assumed  that 
great  pressure  is  necessary  for  their  genesis.  As  the  deep-seated 
rocks  cool,  the  end  products  of  crystallization  may  segregate, 
and  the  segregated  materials  include  the  more  soluble  compounds 


Portions  Excavated 
Previous  to  1908 


S.E. 


Schist  and  Gneisa 
Pocket-Bearing  Zone 

Normal    Unproductive 
Pegmatite 


FIG.  12. — Sketch  of  pegmatite  with  zone  of  tourmaline  pockets  near  the  top 
of  Mount   Mica,  Paris,    Maine.     (After  Bastin,    U.   S.  Geol.  Survey.) 

of  the  magma.  Certain  investigators  have  applied  to  such  end 
products  of  crystallization  the  term  "granite  juice" — a  term 
that  implies  mobility,  which  is  a  striking  characteristic  of  the 
pegmatitic  solutions.  The  end  products  are  more  mobile  than 
the  parent  magma  because  of  their  excess  of  gases,  and  if  fissures 
are  formed  in  the  solidified  portion  of  the  magma  or  in  the 
country  rock  near  by,  the  pegmatitic  material  may  be  injected 
into  them.  Pegmatites  may  therefore  follow  cracks,  planes 
of  schistosity,  or  any  openings  or  planes  of  weakness  that  were 
accessible.  The  pegmatitic  solution  may  even  force  itself  into 
the  surrounding  rock,  making  the  opening  as  well  as  filling  it. 
To  such  processes  are  doubtless  due  "leaf  injections,"  in  which 
paper-thin  sheets  of  pegmatite  alternate  with  thin  sheets  of 
schists.  Pegmatites  of  larger  size  fill  openings  and  may  have 
xFor  a  good  digest  of  the  literature  relating  to  the  origin  of  pegmatites, 
see  HASTINGS,  J.  B. :  Origin  of  Pegmatite.  Am.  Inst.  Min.  Eng.  Trans., 
vol.  39,  pp.  104-128,  1908. 


PEGMATITES 


23 


sharp,  clean-cut  boundaries;  still  others  are  segregated  in  place, 
without  movement  of  the  pegmatitic  magma  as  a  mass,  and  are 
surrounded  by  the  parent  igneous  rock  and  locally  grade  into  it. 
At  many  places,  however,  the  pegmatite  stuff  is  forced  out  of  the 
magma,  and  crystallizes  in  fissures  and  in  small  cracks  in  rocks 
around  the  parent  body.  Except  in  layered  rocks  pegmatites  do 
not  generally  occur  in  parallel  or  other  regular  systems  but 
appear  to  be  haphazard. 

Composition. — Of  the  minerals  that  make  up  the  pegmatites 
orthoclase  and  quartz  are  generally  the  most  abundant,  and 
many  pegmatites  are  composed  essentially  of  these  minerals. 
In  others  mica  appears  also,  and  in  still  others  mica  and  albite. 
Though  albite  is  in  the  main  much  less'  abundant  in  pegmatites 
than  orthoclase,  in  some  of  the  pegmatites  it  is  the  principal 
feldspar.  A  pegmatite  in  Nelson  County,  Virginia,  is  composed 
essentially  of  apatite  and  ilmenite.1  Most  pegmatites,  however, 
are  composed  principally  of  feldspars  and  quartz,  with  very 
small  amounts  of  one  or  more  of  the  other  minerals  listed  below. 
In  general  the  pegmatites  are  richerin  silica  than  the  parent  rocks. 


acmite 

cassiterite 

ilmenite 

rutile 

albite 

chalcopyrite 

kyanite 

sapphire 

allanite 

chromite(?) 

lepidolite 

scapolite 

amphiboles 

columbite 

magnetite 

scheelite 

andalusite 

corundum 

microcline 

specularite 

anorthite 

diamond 

molybdenite 

spinel 

apatite 

diopside 

monazite 

spodumene 

aquamarine 

emerald 

muscovite 

tantalite 

arsenopyrite 

emery 

orthoclase 

titanite 

augite(?) 

fluorite 

pyrite 

topaz 

beryl 

galena 

pyroxenes 

tourmaline 

bismuth 

garnet 

pyrrhotite 

wolframite 

bismuthinite 

gold 

quartz 

xenotime 

biotite 

graphite 

rhodochrosite 

zinc  blende 

bornite 

hematite 

rhodonite 

calcite 

hornblende 

ruby 

Nearly  all  the  minerals  named  above  have  been  found  in 
igneous  rocks  also,  but  many  minerals  that  occur  in  igneous 
rocks  have  not  been  reported  from  pegmatites.  Although  pegma- 
tites commonly  contain  many  sulphides  they  are  but  rarely 
commercial  sources  of  the  metals. 

1  WATSON,  T.  L. :  Occurrence  of  Rutile  in  Virginia.  Econ.  Geol,  vol.  2, 
pp.  492-504,  1907. 


24        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Shape. — Many  pegmatites  are  very  irregular  in  shape,  espe- 
cially those  remaining  in  the  parent  rock,  and  of  these  some  have 
weird  shapes  that  are  very  puzzling.  The  dendritic  mass  or 
branching  pipe  figured  by  Butler1  (see  Fig.  13)  is  believed  to 
have  been  formed  by  movement  of  the  pegmatitic  solution  in  a 
cooling  but  still  viscous  mass  of  the  parent  magma.  Some  peg- 
matites are  sheet-like,  and  some  are  pipes,  long  in  one  and  short 
in  two  dimensions.  The  thin,  rudely  tabular  mass  is  perhaps 
the  most  common  form. 


FIG.  13". — Generalized  stereogram  showing  the  relation  of  pegmatitic  quartz 
and  altered  and  mineralized  quartz  monzonite  in  the  O.  K.  mine.  San  Francis- 
co region,  Utah.  1.  Pipe  of  quartz;  2,  altered  monzonite;  3,  monzonite;  4, 
high-grade  ore.  (After  Butler,  U.  S.  Geol.  Survey.) 

Size. — Although  pegmatites  vary  greatly  in  size,  most  of 
those  which  are  commercially  valuable  are  small.  Some  quarries 
producing  graphic  granite,  quartz,  feldspar,  and  mica  exploit 
pegmatite  masses  of  considerable  size.  On  the  other  hand, 
many  of  the  pegmatite  deposits  of  gems  or  of  rare  metals  are  too 
small  to  justify  more  than  crude  installations  for  mining.  Some 
of  them  are  mere  pockets;  others  consist  of  a  few  large  crystals. 
A  few  are  large  and  persistent. 

1  BUTLER,  B.  S. :  Geology  and  Ore  Deposits  of  the  San  Francisco  and 
Adjacent  Districts,  Utah.  U.  S.  Geol.  Survey  Prof.  Paper  80,  p.  125,  1913. 


PEGMATITES 


25 


Texture. — The  crystals  of  pegmatites  are  generally  large, 
and  some  of  them  are  enormous.  One  crystal  of  spodumene  at 
the  Etta  mine,  in  the  Black  Hills,  is  more  than  40  feet  long,  and 
crystals  several  inches  long  are  not  uncommon.  The  constituent 
minerals,  as  a  rule,  are  intergrown  as  in  granular  rocks.  Por- 
phyritic  texture  is  rarely  developed. 

The  concentration  of  elements  to  form  the  large  crystals  in 
pegmatites  is  due  to  diffusion  in  the  liquid  magma.  A  crystal 
while  forming  will  tend  to  draw  material  of  the  same  composition 
to  it.  Such  attraction  is  opposed  by  the  viscosity  of  the  magma, 
which  depends  on  the  amount  and  condition  of  the  included 
gases. 


FIG.  14. — Section  of  pegmatite  vein,   New  York  mine,  near  Custer,   S.    Dak. 
(After  Sterrett,  U.  S.  Geol.  Survey.) 

In  the  main  the  fissures  into  which  the  pegmatites  were  thrust 
were  probably  not  connected  freely  with  the  surface.  If  there 
had  been  such  open  connection  the  steam  and  other  mineral- 
izers  that  facilitated  diffusion  and  the  formation  of  large  crys- 
tals should  have  readily  escaped,  except  where  the  connection 
was  crooked,  the  depth  great,  and  the  pressure  correspondingly 
high.  Moreover,  if  there  had  been  open  connection  deposition 
would  probably  have  been  effected  by  circulating  solutions  mov- 
ing to  the  surface,  which  tend  to  form  the  banded  structure  that 
is  characteristic  of  fissure  veins.  Banded  structure  is  relatively 
rare  in  pegmatites.  In  the  greater  number  of  pegmatites  the 
structure  is  hypidiomorphic-granular,  like  that  of  most  deep- 
seated  igneous  rocks.  An  example  of  a  layered  pegmatite  may  be 
seen  in  the  New  York  mica  mine,  near  Custer,  S.  Dak.  (Fig.  14), 


26        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

where  a  pegmatite  vein  30  feet  wide  intrudes  schist,  and  several 
feet  of  mica  occurs  here  and  there  at  the  margin  of  the  vein  on 
both  the  hanging  wall  and  the  foot  wall1  (see  also  Fig.  15).  It 
is  probable  that  in  some  places  the  magmas  which  formed  peg- 
matites also  gave  off  solutions  which  deposited  ore  veins. 

Gradations. — Van  Hise2  has  described  a  series  of  pegmatites 
in  the  Black  Hills,  where  a  granite  batholith  sends  off  quartz- 
feldspar  dikes  into  the  surrounding  crystalline  schists;  farther 


Feldspar 
Scale  of  inches 


Fio.  15. — Sketch  of  layered  pegmatite  dike.     (After  Graton,  U.  S.  Geol.  Survey.) 

out  in  the  schists  these  dikes  are  small  and  become  pegmatites. 
Still  farther  out  they  show  a  rough  concentration  of  minerals 
into  layers,  and  farther  still  true  comb  structure  and  ribbon  struc- 
ture. At  a  yet  greater  distance  feldspar  entirely  disappears  and 
the  openings  are  filled  with  ordinary  vein  quartz.  A  quartz 
offshoot  of  pegmatite  is  shown  in  Fig.  16. 

In  the  Encampment  district,  Wyoming,  Spencer3  found  that 
coarser  pegmatites  and  also  aplite  dikes  grade  into  quartz  veins. 

1  STERRETT,  D.  B. :  Mica  Deposits  of  South  Dakota.     U.  S.  Geol.  Survey 
Bull.  380,  p.  382,  1909. 

2  VAN  HISE,  C.  R. :  A  Treatise  on  Metamorphism.     U.  S.  Geol.  Survey 
Mon.  47,  p.  724,  1904. 

3  SPENCER,  A.  C.:  The  Copper  Deposits  of  the  Encampment  District, 
Wyoming.     U.  S.  Geol.  Survey  Prof.  Paper  25,  p.  41,  1904. 


PEGMATITES 


27 


In  Shasta  County,  Cal.,  according  to  Graton,1  pegmatite  veins 
grade  into  quartz  veins  that  carry  sulphides. 

Pegmatites  that  grade  into  gold-bearing  quartz  veins,  however, 
are  surprisingly  rare.  Spurr2  has  shown  that  some  gold-bearing 
veins  of  Silver  Peak,  Nev.,  and  in  Alaska  have  pegmatitic  char- 
acteristics, but  such  examples  are  not  common.  In  general, 
normal  vein  deposits  were  formed  by  solutions  which  were  highly 
aqueous  and  which  normally  circulated  in  openings  that  connected 
either  directly  or  by  devious  routes  with  the  surface  of  the  earth, 
whereas  the  pegmatites  were  deposited  by  solutions  which  were 
much  richer  in  the  rock-making  constituents  other  than  water, 
and  these  solutions  in  general  were 
not  moving  freely  to  the  surface. 

Temperature  of  Formation  of 
Pegmatites. — From  their  asso- 
ciations it  is  believed  that  peg- 
matites are  normally  formed  at 
high  temperatures.  This  infer- 
ence is  supported  by  the  work  of 
Wright  and  Larsen,3  who  utilized 
the  discovery  of  Miigge,  namely, 
that  when  quartz  is  heated  it 
suffers  at  575°  an  enantiotropic  FlG.  16._Quartz  offshoot  from 

change  to  a  Second  phase,    called    pegmatite,    Paris,     Maine.      (After 
,          TV,.,  j      ,,      ,     Bastin,  U.  S.  Geol.  Survey.) 

/3-quartz    by    Mugge,    and    that 

above  800°  it  is  no  longer  stable  at  ordinary  pressures  but  passes 
into  tridymite.  The  change  from  the  stable  form,  called 
a-quartz  by  Miigge  to  /3-quartz  is  attended  by  an  abrupt 
change  in  the  birefringence,  circular  polarization,  and  expansion 
coefficient  at  that  temperature.  Cold  quartz  that  was  formed 
above  575°  will  show  certain  changes,  such  as  fracturing  and 
irregular  twining.  Examinations  by  Wright  and  Larsen  showed 
that  many  pegmatites  have  formed  below  575°  and  some  above 
that  temperature. 

1  GRATON,  L.  C. :  The  Occurrence  of  Copper  in  Shasta  County,  California. 
U.  S.  Geol.  Survey  Bull  430,  p.  86,  1910. 

2  SPURR,  J.  E. :  Ore  Deposits  of  the  Silver  Peak  Quadrangle,  Nevada. 
U.  S.  Geol.  Survey  Prof.  Paper  55,  pp.  130-156,  1906. 

3  WRIGHT,  F.  E.,  and  LARSEN,  E.  S. :  Quartz  as  a  Geologic  Thermometer. 
Am.  Jour.  Sci.,  4th  ser.,  vol.  27,  pp.  421-447,  1909. 


Moss 
covering 


28        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

References 

PEGMATITES 

BASTIN,  E.  S.:  Origin  of  the  Pegmatites  of  Maine.  U.  S.  Geol.  Survey 
Bull.  445,  1911. 

CATHERINET,  JULES:  Copper  Mountain,  British  Columbia.  Eng.  and 
Min.  Jour.,  vol.  79,  pp.  125-127,  1905. 

CROSBY,  W.  O.,  and  FULLER,  M.  L. :  Origin  of  Pegmatites.  Am.  Geologist, 
vol.  19,  pp.  147-180,  1897. 

DERBY,  O.  A.:  Notes  on  Brazilian  Gold  Ores.  Am.  Inst.  Min.  Eng.  Trans., 
vol.  33,  pp.  282-287,  1902. 

EMMONS,  W.  H. :  Some  Ore  Deposits  of  Maine  and  the  Milan  Mine,  New 
Hampshire.  U.  S.  Geol.  Survey  Bull.  432,  pp.  34-35,  1910. 

HASTINGS,  J.  B.:  Origin  of  Pegmatite.  Am.  Inst.  Min.  Eng.  Trans.,  vol. 
39,  pp.  104-128,  1908.  Subclassification  of  Zenogenous  Ore  Deposits. 
Eng.  and  Min.  Jour.,  vol.  59,  p.  268,  March  23,  1895. 

HESS,  F.  L.:  Tin,  Tungsten,  and  Tantalum  Deposits  of  South  Dakota. 
U.  S.  Geol.  Survey  Bull.  380,  p.  149,  1909. 

SPURR,  J.  E. :  A  Consideration  of  Igneous  Rocks  and  Their  Segregation  or 
Differentiation  as  Related  to  the  Occurrence  of  Ores.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  33,  pp.  288-340,  1902.  Relation  of  Rock  Segregation  to  Ore 
Deposition.  Eng.  and  Min.  Jour.,  vol.  76,  pp.  54-55,  1903.  Genetic  Rela- 
tions of  the  Western  Nevada  Ores.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  36,  pp. 
372-402,  1905.  Ore  Deposits  of  the  Silver  Peak  Quadrangle,  Nevada.  U.  S. 
Geol.  Survey  Prof.  Paper  55,  1906;  also  Eng.  and  Min.  Jour.,  vol.  77,  pp. 
759-760,  1904.  The  Southern  Klondike  District,  Esmeralda  County,  Nev- 
ada— A  Study  in  Metalliferous  Quartz  Veins  of  Magmatic  Origin.  Econ. 
Geol,  vol.  1,  pp.  369-382,  1906. 

SPURR,  J.  E.,  GARREY,  G.  H.,  and  BALL,  S.  H.:  Geology  and  Ore  Deposits 
of  the  Georgetown  Quadrangle,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper 
63,  pp.  157-158,  169,  171,  1908. 

VOGT,  J.  H.  L. :  The  Formation  of  Eruptive  Ore  Deposits.  M  in.  Indus- 
try, vol.  4,  pp.  743-754,  1895.  Beitrage  zur  genetischen  Classification  der 
durch  magmatische  Differentiationsprocesse  und  der  durch  Pneumatolyse 
entstandenen  Erzvorkommen.  Zeitschr.  prakt.  Geologic,  vol.  2,  pp.  381-399, 
1894;  vol.  3,  pp.  145-156,  367-370,  444-459,  465-484,  1895. 

WILLIAMS,  G.  H.:  The  General  Relations  of  the  Granitic  Rocks  of  the 
Middle  Atlantic  Piedmont  Plateau.  U.  S.  Geol.  Survey  Fifteenth  Ann. 
Rept.,  pp.  675-679,  1895. 


CHAPTER  V 
CONTACT-METAMORPHIC  DEPOSITS 

Occurrence. — 1.  In  soluble  or  replaceable  rocks — limestones  or  calcareous 
shales, — more  rarely  in  quartzites  and  in  igneous  rocks. 

2.  Near  intruding  igneous  rocks  of  intermediate  or  acidic  composition, 
such  as  diorites,  granodiorites,  monzonites,  granites,  or  their  porphyries; 
more  rarely  at  contacts  of  basic  rocks,  such  as  gabbros  and  diabases.     Not 
genetically  related  to  surface  lavas  or  glassy  rocks. 

3.  Most  of  them  touch  or  lie  within  a  few  rods  of  the  outcrops  of  igneous 
rocks,  but  they  may  be  as  much  as  100  rods  away,  or  rarely  farther.     Some 
form  broken  or  disconnected  belts  around  the  igneous  masses.     The  ores  are 
generally  segregated  in  irregular  bunches  or  large  masses  in  the  contact- 
metamorphic  zones. 

Composition. — The  minerals  are  characteristic.  The  ore  is  commonly  a 
mixture  of  silicates  intergrown  with  oxides  and  sulphides  of  metals.  The 
metals  include  copper,  iron,  zinc,  tungsten;  more  rarely  gold,  silver  and  lead. 

Shape. — Generally  irregular  in  detail;  deposits  nearly  equidimensional 
are  common ;  some  are  rudely  tabular;  many  show  gradational  boundaries  with 
rocks  introduced. 

Size. — From  bodies  yielding  a  few  tons  to  large  masses. 

Texture. — The  ore  minerals  are  intergrown  with  the  contact-metamorphic 
silicates.  Where  shale  or  other  banded  rocks  are  replaced  the  ore  may  be 
banded,  but  there  is  no  crustified  banding.  Vugs  are  rare,  if  not  lacking. 

Occurrence. — Contact-metamorphic  deposits  are  formed  in  in- 
truded rocks  by  fluids  given  off  by  intruding  igneous  magmas. 
In  general  the  intruded  rock  is  changed  near  the  contact.  Such 
changes  may  be  small,  consisting  merely  of  baking,  induration, 
or  vitrification  of  the  intruded  rocks,  or  of  recrystallization  of 
their  constituent  minerals.  At  many  places,  however,  the  changes 
are  extensive,  and  the  zone  of  altered  rock  may  extend  many  rods 
from  the  intruding  mass.  The  agents1  that  cause  changes  so 
intense  are  believed  to  be  the  fluids,  probably  gases,  contained  in 
the  intruding  magma.  If  the  magmas  are  intruded  near  the 
surface,  and  especially  if  the  intrusive  reaches  the  surface,  the 
gases  more  readily  escape,  either  through  the  molten  mass  itself 
or  through  the  fissures  in  the  intruded  rock,  which  normally  are 

1  LINDGREN,  WALDEMAR:  The  Character  and  Genesis  of  Certain  Contact 
Deposits.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  31,  pp.  226-244,  1901. 

29 


30        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

more  open  or  more  numerous  at  relatively  shallow  depths,  where 
the  overlying  load  is  lighter.  Consequently  the  effects  of  meta- 
morphism  along  the  contacts  of  basalts,  glassy  andesites,  and 
rhyolites  are  slight,  whatever  rock  they  intrude.  More  intense 
contact  metamorphism,  producing  the  contact-metamorphic  de- 
posits of  the  metals,  is  associated  with  the  deep-seated  granular 
rocks,  such  as  granites,  monzonites,  and  diorites,  and  with  the 
deep-seated  porphyries,  which  have  a  finely  crystalline  ground- 
mass,  rather  than  with  those  that  have  a  glassy  groundmass. 

In  general  contact  metamorphism  is  more  profound  along  the 
contacts  with  intermediate  or  acidic  rocks,  such  as  diorites, 
monzonites,  and  granites,  than  along  the  contacts  of  gabbros, 
diabases,  and  pyroxenites.  But  not  all  granites  are  bordered 
by  contact-metamorphic  zones,  even  where  they  intrude  rocks 
favorable  for  the  development  of  such  zones.  Some  magmas, 
both  acidic  and  basic,  seem  poor  in  fluids,  or. else  they  give  them 
up  reluctantly. 

Of  the  rocks  that  may  be  intruded,  limestone  is  most  readily 
changed  and  is  usually  converted  into  a  rock  composed  of  cal- 
cite,  garnet,  diopside,  actinolite,  tremolite,  epidote,  magnetite, 
specularite,  and  other  minerals.  Shale  likewise  changes  readily, 
especially  calcareous  shale;  the  same  minerals  may  be  developed 
in  shale  as  in  limestone,  but  the  aluminum-rich  minerals,  such 
as  andalusite,  scapolite,  and  sillimanite,  •  are  formed  also.  All 
these  minerals  and  many  others  are  formed  by  replacement  of 
the  country  rock;  the  replaced  rock  may  preserve  the  bedding, 
jointing,  fossils,  and  some  other  features  of  the  original  rock. 
If  a  limestone  is  metamorphosed  all  the  constituents  of  certain 
minerals,  such  as  chalcopyrite,  may  be  added  to  the  rock.  For 
others,  such  as  andradite,  certain  elements  are  added  from  the 
magma  to  the  elements  of  the  intruded  rock.  Still  others,  like 
calcite,  are  formed  simply  by  recrystallization  of  the  elements  of 
the  limestone. 

The  changes  in  a  quartzite  that  is  invaded  are  ordinarily  slight 
compared  to  those  in  limestones  and  in  shales.  Garnet,  usually 
microscopic,  may  be  developed  near  the  contact.  In  many 
places  tourmaline  is  deposited,  but  this  mineral  is  not  confined 
closely  to  the  contact,  being  formed  to  a  considerable  distance 
from  the  intrusive.  Tourmaline  is  not  usually  abundant  in 
contact-metamorphic  deposits  but  is  singularly  widespread  in 
certain  areas  of  quartzite  intruded  bymonzonite,  as  at  Philips- 


CON 'T 'ACT-MET 'AMORPHIC  DEPOSITS  31 

burg,  Mont.,1  and  in  the  Coeur  d'Alene  district,  Idaho.2  Siliceous 
schists  generally  are  less  affected  by  contact  metamorphism 
than  calcareous  rock. 

Where  igneous  rocks  are  intruded  extensive  garnet  zones  are 
rarely  developed,  but  along  the  fracture  planes  of  the  older 
rock  some  garnet  with  tremolite,  actinolite,  epidote,  biotite, 
and  other  minerals  may  be  deposited,  and  locally  these  minerals 
may  replace  the  older  rock.  Contact-metamorphic  silicates  are 
developed  in  igneous  rocks  at  Cananea  and  Velardena,  Mexico.3 
At  Velardena  the  metamorphism  of  igneous  rocks  is  unusually 
extensive. 

Igneous  intrusives  solidify  progressively  downward.  If  suit- 
able channels  are  provided,  solutions  from  the  deeper  liquid  por- 


FIG.  17. — Diagram  showing  relations  of  contact-metamorphic  deposits  (black) 
to   contact  metamorphic  zone  (stippled)   and  to  intrusive  mass. 

tions  may  rise  to  the  solidified  portions  and  profoundly  alter 
them 

Contact-metamorphic  ore  deposits  are  portions  of  the  meta- 
morphic zones  that  contain  the  metallic  minerals  in  valuable 
concentrations  (see  Fig.  17).  As  a  rule  they  occur  only  here  and 
there  in  such  zones,  and  the  features  that  control  their  localiza- 
tion are  often  obscure.  The  more  valuable  contact-meta- 
morphic ore  deposits  are  generally,  though  not  always,  confined 
to  the  metamorphic  zones  in  limestones  and  in  calcareous  shales, 
but  valuable  deposits  are  not  developed  in  all  such  zones.  Gar- 
netization  is  erratic,  and  at  many  places  where  sulphides  are  pres- 
ent they  are  very  irregularly  distributed.  As  a  rule  contact- 

1  EAIMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.     U.  S.  Geol.  Survey  Prof.  Paper  78,  p. 
160,  1913. 

2  RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Coeur  d'Alene  District,  Idaho.     U.  S.  Geol.  Survey  Prof.  Paper,  62,  p.  101, 
1908. 

3  SPURR,  J.  E.,  and  GARREY,  G.  H. :  Ore  Deposits  of  the  Velardena  Dis- 
trict, Mexico.     Econ.  Geol.,  vol.  3,  p.  698,  1908. 


32        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

metamorphic  ores  are  not  of  very  high  grade,  but  some,  even  of 
those  that  have  not  been  enriched  by  secondary  processes,  are 
extensive  and  are  worked  at  a  profit. 

The  intruding  rocks  that  cause  contact  metamorphism  in 
which  garnet  sulphide  ores  are  developed  are  in  the  main  of 
acidic  or  intermediate  composition,  and  many  of  them  are 
rather  rich  in  potash.  At  Cable,  Mont.,  the  intruding  rocks  are 
quartz  monzonite  and  quartz  monzonite  porphyry;  at  Ely, 
Nev.,  the  agent  of  metamorphism  is  a  rather  acidic,  potash-rich 
monzonite  porphyry;  at  Bullion,  Nev.,  it  is  granodiorite  por- 
phyry; at  Bisbee,  Ariz.,  the  intruding  rocks  are  granite  and 
granite  porphyry;  at  Cananea,  Mexico,  quartz  porphyry  and 
quartz  diorite  porphyry. 

In  the  Morenci  district,  Arizona,  the  metamorphic  effects  of  the 
acidic  and  intermediate  rocks  may  be  compared  in  a  single  rock 
mass,  which  grades  from  diorite  porphyry  through  monzonite 
porphyry  into  granite  porphyry.  There,  as  shown  by  Lindgren 
(page  376)  the  monzonitic  porphyry  and  granite  porphyry  have 
caused  great  changes  at  the  contact,  but  the  diorite  has  affected 
the  intruded  rock  only  slightly.  However,  not  all  metamorphos- 
ing intrusives  are  acidic.  At  Hedley,  British  Columbia,  as  shown 
by  Camsell,1  garnet  ores  with  sulphides  and  gold  have  been  de- 
veloped in  limestone  near  gabbro  and  at  Cornwall,  Pa.,  mag- 
netite deposits  are  developed  near  diabase. 

Some  deposits  due  to  contact  metamorphism  are  as  much  as 
half  a  mile  from  the  exposed  contacts.  A  few  have  been 
found  where  no  igneous  rocks  are  exposed,  but  these  are  pre- 
sumably above  intrusive  masses  not  yet  uncovered  by  erosion. 
Because  not  all  these  deposits  are  at  the  contacts  they  are  termed 
by  some  investigators  "deposits  of  igneous  metamorphism," 
but  this  term  is  not  in  general  use. 

Composition. — The  following  minerals  are  among  those  that 
have  been  identified  in  contact-metamorphic  zones: 

actinolite  anorthite  bismuthinite  chromite 

albite  anthophyllite  biotite  corundum 

allanite  apatite  bornite  cordierite 

amphiboles  arsenopyrite  calcite  diopside 

andalusite  augite  cassiterite  dolomite 

andradite  axinite  chalcopyrite  emerald 

ankerite  beryl  chlorite  emery 

1  CAMSELL,  CHARLES:  The  Geology  and  Ore  Deposits  of  the  Hedley  Dis- 
trict, British  Columbia.  Canada  Geol.  Survey  Mem.  2,  pp.  164-174,  1910. 


CONTACT-MET 'AMORPHIC  DEPOSITS  33 

epidote  ilmenite  platinum  spinel 

fluorite  ilvaite  pyrite  staurolite 

forsterite  jadeite  pyroxenes  titanite 

franklinite  kyanite  pyrrhotite  topaz 

galena  ludwigite  quartz  tourmaline 

garnet  magnetite  rhodonite  tremolite 

glaucophane  microcline  ruby  vesuvianite 

gold  molybdenite  rutile  willemite 

graphite  monazite  sapphire  wollastonite 

grossularite  muscovite  scapolite  zinc  blende 

hematite  olivine  scheelite  zincite 

honblende  orthoclase  sericite  zoisite 

hulsite  paigeite  sillimanite 

humites  picotite  specularite 

It  is  noteworthy  that  although  the  associations  of  minerals 
that  make  up  contact-metamorphic  deposits  are  characteristic 
if  not  unique,  nearly  all  the  minerals  themselves  may  be  formed 
under  other  conditions  also.  Some  of  them  occur  in  igneous 
rocks.  Others  are  formed  as  a  result  of  regional  metamorphism, 
and  still  others  are  formed  in  metalliferous  veins.  Staurolite, 
vesuvianite,  and  wollastonite  are  probably  limited  to  contact- 
metamorphic  deposits  and  to  deposits  formed  by  dynamic  meta- 
morphism. '  The  tellurides  and  the  antimony  and  arsenic  sul- 
phosalts  of  silver,  on  the  other  hand,  are  rarely  developed  in 
contact-metamorphic  deposits. 

There  is  strong  ground  for  supposing  that  some  minerals  re- 
ported to  occur  in  igneous  rocks  are  in  the  intensely  metamor- 
phosed products  of  fragments  of  sedimentary  rocks  caught  up  in 
the  igneous  magmas.  This  is  probably  true  of  andalusite  and 
ilvaite;  possibly  of  actinolite  and  other  minerals.  It  is  note- 
worthy that  the  sulphates  barite  and  celestite,  which  are  formed 
under  many  conditions,  are  almost  unknown  in  contact-meta- 
morphic deposits.  The  veinlets  of  barite  that  cut  such  deposits 
in  some  districts  are  distinctly  of  later  age. 

The  ores  of  the  contact-metamorphic  deposits  include  (1) 
iron  ore,  carrying  magnetite  and  specularite;  (2)  copper  ore, 
carrying  chalcopyrite  and  bornite;  (3)  zinc  ore,  carrying  zinc 
blende;  (4)  lead  ore,  carrying  galena;  and  (5)  gold  ore,  carrying 
gold  with  quartz,  pyrite,  arsenopyrite,  calcite,  or  other  minerals. 
A  small  amount  of  silver  is  present  in  some  deposits  associated 
with  copper  or  with  gold  ores. 

Shape  and  Relation  to  Fissuring. — At  the  high  temperatures 
and  pressures  at  which  contact-metamorphic  deposits  are  formed, 


34.       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


the  solutions  are  capable  of  entering  minute  openings,  such  as 
cleavage  planes  of  minerals  and  intergranular  spaces.  Conse- 
quently these  deposits  are  not  necessarily  related  to  fissures  in 
the  intruded  rocks — in  fact,  most  of  them  seem  to  be  independent 
of  any  fissuring  that  can  be  recognized  after  metamorphism. 
Garnet  veins,  however,  are  formed  in  some  regions  of  contact 
metamorphism.  A  short  vein  of  garnet  rock  with  sulphides 


'  "  Granodlorite  T~  '/' 


->\  /-  7 '|V£  -Garnet.  Rock  :  •/' 
•OV/  ;  ;Witli  .A  iv;.. 


Gray  Marbleized  Limestone 
N 


0     ScaloofFeet   3QQ 


FIG.  18. — Sketch  showing  the  relation  of  the  zone  of  contact  metamorphism 
to  granodiorite  and  limestone.     Bullion  District,  Nevada. 

occurs  at  Bullion,  Nev.  (Fig.  18),  and  garnet  veins  in  limestone 
are  developed  at  Jarilla  and  Hachita,  N.  Mex.1 

The  solutions,  after  they  lost  their  power  to  replace  the 
country  rock  with  garnet  and  other  heavy  silicates,  must  have 
moved  to  points  of  less  pressure,  away  from  the  magma  and  gener- 
ally toward  the  surface,  for  the  magma  was  exhaling  its  compressed 
gases  or  liquids  wherever  they  could  escape.  The  solutions  that 

^INDGREN,  WALDEMAR,  GRATON,  L.  C.,  and  GORDON,  C.  H.:  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Pro/.  Paper  68,  p.  56,  1910, 


CONTACT-METAMORPHIC  DEPOSITS  35 

escaped  from  the  intrusive  mass  and  the  contact-metamorphic 
zone  must  have  carried  with  them  some  lime  and  large  quanti- 
ties of  carbon  dioxide  removed  from  the  calcerous  rocks.  Pre- 
sumably the  solutions  gathered  in  fissures  or  in  trunk  channels 
as  soon  as  such  openings  were  encountered,  and  if  they  were  still 
charged  with  the  metallic  sulphides  they  may  have  deposited 
ores  free  from  the  heavy  silicates  and  poor  in  minerals  contain- 
ing the  elements  of  the  "mineralizing"  gases.  Thus  a  contact- 
metamorphic  zone  may  be  surrounded  by  an  area  in  which  fissure 
veins  and  other  deposits  not  of  contact-metamorphic  origin  have 
formed  (see  Fig.  19).  Consequently  there  are  gradational  phases 
between  contact-metamorphic  deposits  and  veins,  some  of  which 
contain  heavy  silicates  or  other  minerals  characteristic  of  con- 
tact-metamorphic deposits.  Veins  of  the  deep  zone  that  contain 


FIG.  19. — Section  showing  sedimentary  rocks  intruded  by  igneous  rock,  with 
contact-metamorphic  deposits  (irregular  black  areas)  and  veins. 

heavy  silicates  and  oxides  and  were  formed  at  high  temperature 
and  pressure  are  discussed  on  pages  49-61. 

A  few  contact-metamorphic  deposits  are  approximately  tabu- 
lar in  shape.  Where  a  single  bed  is  replaced,  or  where,  along  an 
intrusive  rock,  replacement  follows  the  contact  closely,  the  de- 
posits that  are  formed  may  be  long  in  two  dimensions  and  fairly 
uniform  in  thickness. 

Size. — Contact-metamorphic  deposits  vary  greatly  in  size. 
Many  of  the  sulphide  deposits  are  merely  small  concentrations 
of  ore  minerals  in  a  gangue  of  heavy  silicates,  and  the  ore  body 
grades  into  the  country  rock  by  decrease  in  metallic  minerals. 
Some  deposits  of  sulphides  in  Montana,  Nevada,  and  Arizona 
have  apparently  been  worked  out  after  a  few  thousand  tons  of 
ore  were  mined.  But  other  contact-metamorphic  deposits  are 
large  and  have  supplied  ore  for  continuous  mining  operations 


36        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

extending  over  many  years.  The  irregular  character  and  erratic 
distribution  of  these  deposits,  however,  make  it  necessary  to 
observe  caution  in  estimating  ore  reserves,  and  generally  for 
blocking  out  ore  the  exposures  of  such  deposits  must  be  closer 
and  more  nearly  continuous  than  those  of  sedimentary  deposits 
or  persistent  veins.  Iron  oxide  deposits  of  this  class  likewise 
show  great  variation  in  size.  Several  bodies  of  magnetite  in 
the  Philipsburg  region,  Montana,1  contained  only  a  few  thou- 
sand tons.  On  the  other  hand,  the  magnetite  bodies  that  re- 
place limestone  at  Cornwall,  Pa.,2  are  extensive.  Perhaps  the 
most  extensive  deposits  of  contact-metamorphic  iron  ore  in  the 
United  States  are  those  of  Iron  Springs,  Utah,3  and  the  Eagle 
Mountains,  California.4 

Texture. — The  texture  of  contact-metamorphic  ore  is  gener- 
ally characteristic.  As  a  rule  the  minerals  of  the  ore  of  the 
sulphide  and  heavy  silicate  type  are  intimately  interlocked  and 
were  formed  approximately  at  the  same  time.  The  silicates, 
such  as  garnet,  vesuvianite,  tremolite,  diopside,  and  the  micas, 
and  the  oxides,  magnetite  and  hematite,  are  commonly  inter- 
grown  with  pyrite,  chalcopyrite,  zinc  blende,  and  other  sulphides. 
Residual  calcite  is  almost  invariably  present.  The  greasy  ap- 
pearance of  the  massive  garnet  and  vesuvianite  where  freshly 
broken,  the  feathery  texture  of  ore  composed  in  part  of  actinolite 
or  tremolite,  and  the  greenish,  flaky  appearance  of  ore  in  which 
the  micas  and  chlorite  predominate  are  more  or  less  character- 
istic of  the  sulphide-silicate  rock.  The  proportions  of  the  con- 
tact-metamorphic minerals  vary  greatly.  In  some  regions  there 
are  acres  of  nearly  pure  andradite;  in  others  the  several  minerals 
occur  as  small  bodies  mutually  intergrown.  Where  the  ore 
minerals  and  the  silicates  replace  shale  or  slate  a  banding  may  be 
shown,  but  such  texture  is  pseudomorphous  after  the  rock  re- 
placed and  is  not  crustified.  Some  contact-metamorphic  ores 
contain  open  spaces,  into  which  well-formed  crystals  of  the  heavy 

1  EMMONS,  W.  H.,  and  CALKINS,  F.  C. :  The  Geology  and  Ore  Deposits  of 
the   Philipsburg   Quadrangle,    Montana.     U.  S.  Geol.  Survey  Prof.   Paper 
78,  p.  186,  1912. 

2  SPENCER,  A.  C. :  Magnetite  Deposits  of  the  Cornwall  Type  in  Pennsyl- 
vania.    U.  S.  Geol.  Survey  Bull.  339,  p.  74,  1908. 

3L.EiTH,  C.  K.,  and  HARDER,  E.  C.:  The  Iron  Ores  of  the  Iron  Springs 
District,  Southern  Utah.  U.  S.  Geol.  Survey  Bull.  338,  1908. 

4  HARDER,  E.  C. :  Iron  Ore  Deposits  of  the  Eagle  Mountains,  California. 
U.  S.  Geol.  Survey  Bull.  503,  1913. 


CONT ACT-MET AMORPHIC  DEPOSITS  37 

silicates  project;  but  the  crystals  that  line  the  spaces  are  not 
deposited  symmetrically  in  sheets  or  crusts,  one  above  another, 
as  is  common  in  ore  veins.  Many  of  the  open  spaces  in  contact- 
metamorphic  ores  are  solution  cavities  from  which  calcite  has 
been  dissolved. 

The  statement  that  the  minerals  of  contact-metamorphic 
deposits  are  intergrown  and  contemporaneous,  though  broadly 
true,  needs  amplification.  In  some  deposits  the  heavy  silicate 
minerals  and  pyrite  appear  to  have  formed  before  the  sulphides 
pyrrhotite  and  chalcopyrite,  which  inclose  them  and  heal  frac- 
tures in  them. 

At  Dillsburg,  Pa.,  according  to  Harder,1  the  minerals  of  the 
magnetite  ore  have  formed  in  the  following  order :  garnet,  pyrite, 
pyroxene,  magnetite,  feldspar,  epidote.  At  many  places,  as  at 
Ducktown,  Tenh.,  the  heavy  silicate  minerals  are  intergrown 
with  the  sulphides  but  are  also  fractured  and  filled  by  them.  It 
does  not  follow  that  there  are  two  epochs  of  mineralization, 
sharply  set  off  one  from  the  other.  The  period  of  mineraliza- 
tion may  have  been  continuous,2  the  deposition  of  the  sulphides 
overlapping  to  some  extent  that  of  the  silicates. 

Material  Added  to  the  Intruded  Rock  by  Contact  Metamor- 
phism. — The  effects  of  contact  metamorphism  are  different  in 
different  places.  The  rocks  that  are  invaded  differ  in  composi- 
tion, and  doubtless  so  also  do  the  solutions  that  are  exhaled  by 
the  intruding  magma.  There  are  many  types  of  metamorphosed 
rocks,  and  these  are  connected  by  gradational  types.  At  some 
places  the  alteration  consists  chiefly  in  recrystallization  due  to 
heat  and  possibly  to  steam  escaping  from  the  intrusive. 

On  the  Mesabi  range,3  east  of  the  town  of  Mesaba,  Minn.,  the 
iron-bearing  formation  is  intruded  by  an  immense  body  of  gab- 
bro,  and  its  mineral  composition  has  been  profoundly  altered 
over  many  miles.  Rock  composed  of  quartz,  iron  silicate  (green- 
alite),  and  iron  carbonate  has  been  changed  to  rock  composed 
of  quartz,  magnetite,  griinerite,  actinolite,  olivene  and  augite. 
Except  for  decrease  of  combined  water  the  magnetite  rock  is 

1  HARDER,  E.  C. :  Structure  and  Origin  of  the  Magnetite  Deposits  near 
Dillsburg,  York  County,  Pennsylvania.  Econ.  Geol.,  vol.  5,  pp.  599-622, 
1910. 

SLINDGREN,  WALDEMAR;   "Mineral  Deposits,"  p.  666,  1913. 

3  LEITH,  C.  K. :  The  Mesabi  Iron-Bearing  District  of  Minnesota.  U.  S. 
Geol.  Survey  Mon.  43,  p.  159,  1903. 


38        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

essentially  similar  in  chemical  composition  to  the  parent  green- 
alite  rock.1 

At  a  great  many  places  however  where  sedimentary  or  other 
rocks  are  invaded  there  have  been  large  additions  of  material. 
Examples  of  several  types  are  discussed  below. 

Silication  with  Relatively  Small  Additions  of  Materials  Other 
than  Silica. — At  some  places  contact  metamorphism  consists 
principally  in  silication  of  the  invaded  rocks.  At  Marysville, 
Mont.,  quartz  diorite  breaks  through  siliceous  shales  and  im- 
pure limestones  of  Algonkian  age.  The  intruded  strata  have 
been  greatly  changed,  chiefly  by  recrystallization  and  induration. 
At  distances  of  1,000  feet  or  more  from  the  batholith,  according 
to  Barrell,2  the  composition  of  the  strata  indicates  little  or  no 
general  accession  of  material  from  the  magma  during  meta- 
morphism, but  within  a  variable  distance,  usually  within  600  to 
1,000  feet,  of  the  batholith,  emanations,  largely  siliceous,  havfe 
combined  with  the  lime  and  other  bases,  and  carbon  dioxide  has 
been  eliminated. 

Development  of  Magnetite  Bodies. — At  Cable  and  Philips- 
burg,  Mont.,  Paleozoic  limestones  are  intruded  by  great  batho- 
liths  of  quartz  monzom'te.  Here  and  there  along  the  borders  of 
the  intrusives  large  masses  of  magnetite  replace  the  marbleized 
limestone.  The  minerals  associated  with  the  magnetite  are 
tremolite,  green  mica,  diopside,  scapolite,  quartz,  calcite,  pyrite, 
and  pyrrhotite.  In  the  Redemption  iron  mine,  near  Philipsburg, 
ludwigite  is  intergrown  with  magnetite.3  Between  Anaconda 
and  Cable  there  are  deposits  of  magnetite  with  garnet  gangue. 
At  the  Cable  mine  large  bodies  of  relatively  pure  magnetite  are 
surrounded  by  coarsely  crystalline  marble.  Similar  bodies  of 
ore  occur  in  the  Clifton-Morenci  district,  Arizona,  and  in  many 
other  mining  districts  in  the  West.  Some  of  them  are  utilized 
by  smelters  for  flux,  and  some  are  smelted  f or •  iron.  Those  at 
Fierro,  N.  Mex.,  are  among  the  most  valuable  deposits  of  this 
class  that  are  now  utilized. 

1  VAN  HISE,  C.  R.,  and  LEITH,  C.  K. :  The  Geology  of  the  Lake  Superior 
Region.     U.  S.  Geol.  Survey  Mon.  52, 1911.     Compare  analysis  on  p.  167  with 
that  on  p.  185. 

2  BARRELL,  JOSEPH  :  Geology  of  the  Marysville  Mining  District,  Montana. 
U.  S.  Geol.  Survey  Prof.  Paper  57,  pp.  121-142,  1907. 

3  EMMONS,  W.  H.,  and  CALKINS,  F.  C. :  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.     U.  S.  Geol.  Survey  Prof.  Paper  78,  pp. 
186,  221-231,  1913. 


CONTACT-METAMORPHIC  DEPOSITS  39 

In  the  region  of  Cornwall,  Pa.,1  large  deposits  of  magnetite 
replace  limestones  and  other  calcareous  rocks.  These  deposits 
have  yielded  considerable  iron  ore.  The  Paleozoic  and  Mesozoic 
sedimentary  rocks  are  cut  by  intrusive  diabase,  which  has  prob- 
ably supplied  the  solutions  that  deposited  the  ores.  The  iron 
deposits  of  Iron  Springs,  Utah,  and  the  Eagle  Mountains,  Cali- 
fornia, have  been  mentioned. 

Development  of  Zones  of  Garnet  and  Other  Heavy  Silicates. — 
Garnetization,  a  relatively  intense  phase  of  contact  metamor- 
phism,  very  often  attends  the  intrusion  of  the  granular  rocks  and 
deep-seated  porphyries.  Sulphides  are  commonly  formed  here 
and  there  in  the  garnet  zones  (Fig.  20,  a  and  6).  In  the  garnetiza- 
tion  of  limestone  much  material  is  added  to  the  country  rock. 
Iron,  copper,  gold,  and  silver  minerals  may  be  introduced.  Silica 
is  often  added  in  large  quantities,  as  well  as  small  amounts  of 
boron,  fluorine,  and  chlorine  compounds. 

Lindgren2  has  shown  quantitatively  the  changes  that  have 
taken  place  in  the  limestone  of  the  Clifton-Morenci  district, 
Arizona,  where  large  additions  of  material  have  been  supplied 
to  the  intruded  rock  by  the  magma.  Over  large  areas  the  Modoc 
limestone  has  become  almost  completely  changed  to  lime-iron 
garnet,  with  some  epidote  and  magnetite.  The  limestone  con- 
tains about  94  per  cent,  of  lime  carbonate.  When  it  was  changed 
to  garnet  rock  all  the  C02  must  have  been  expelled  and  large 
quantities  of  SiO2  and  Fe2Oa  must  have  been  added.  The 
chemical  changes  were  metasomatic,  and  there  was  no  great 
increase  nor  great  reduction  of  volume  of  the  rock  intruded.  The 
weights,  in  grams,  of  the  constituents  in  1  cubic  centimeter  of 
limestone  and  of  andradite  are  shown  below. 

Limestone  Andradite 

CaO 1 . 52  1 . 08 

SiO2 1.33 

A12O2 0.03 

Fe2O3 1.18 

FeO , 0.01 

MgO 0.01 

H20 0.01 

CO2 1.19 

2.71  3.65 

1  SPENCER,  A.  C. :  Magnetite  Deposits  of  the  Cornwall  Type  in  Pennsyl- 
vania.    U.  S.  Geol.  Survey  Bull.  359,  p.  21,  1908. 

2  LINDGREN,  WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.     U.  S.  Geol.  Survey  Prof.  Paper  42,  pp.  71,  134,  1905. 


40        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

If  the  whole  of  the  CaO  in  1  cubic  centimeter  of  CaC03  has 
been  used  to  convert  the  rock  into  garnet,  then  this  volume  be- 
comes about  1.40  cubic  centimeters  of  garnet — that  is,  the  volume 
is  increased  nearly  one-half  during  metamorphism.  Such  an  in- 
crease, according  to  Lindgren,  has  almost  certainly  not  taken 
place.  On  the  other  hand,  if  there  has  been  no  change  of  volume 


FIG.  20a. — Limestone  metamorphosed  to  coarse  calcite,  garnet,  quartz,  zinc 
blende,  and  chalcopyrite.  Magnified  about  20  diameters.  Morenci,  Arizona. 
(After  Lindgren,  U.  S.  Geol.  Survey.) 

during  the  alteration,  0.44  gram  of  CaO  has  been  carried  away 
together  with  1.19  grams  of  CO2,  while  1.33  grams  of  Si02  and 
1.18  grams  of  Fe20s  have  been  added. 

Kemp1  has  compared  the  analyses  of  unmetamorphosed  lime- 
stones of  Tamaulipas,  Mexico,  with  those  of  their  garnet-rock 

1  KEMP,  J.  F.:  The  Copper  Deposits  of  Tamaulipas,  Mexico.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  36,  pp.  178,  203.  1905. 


CONTACT-METAMORPHIC  DEPOSITS 


41 


equivalents  and  shows  that  profound  changes  have  been 
brought  about  in  the  composition  of  the  limestones  through  their 
intrusion  by  igneous. bodies. 

At  Philipsburg  and  Cable,  Mont.;  Bullion,  Lone  Mountain, 
and  Golconda,  Nev.;  Bingham,  Utah;  Mackay,  Idaho;  and 
Cananea,  Mexico,  the  metamorphic  zones  have  received  large 
contributions  of  various  elements  from  the  magmas.  At  many 


FIG.  206. — Key  to  Fig.  20a.  Fc,  Fine-grained  calciteof  normal  limestone, 
with  dotted  line  indicating  approximate  transition  to  coarse  calcite;  C,  coarse- 
grained calcite;  Q,  quartz;  S,  sericite;  G,  garnet;  Z,  zinc  blende;  Cu,  chalcopyrite ; 
O,  open  field.  (After  Lindgren,  U .  S.  Geol.  Survey.) 

places  the  line  of  contact  between  the  relatively  pure  limestone 
and  the  garnet  rock  is  sharp  and  distinct,  and  the  transfer  of 
material  can  not  be  questioned.  The  rock  mass  is  continuous, 
yet  the  metamorphosed  rock  is  much  heavier  than  the  unaltered 
rock.  The  mass  of  evidence  indicates  that  garnetization  is 
usually  accompanied  by  the  addition  and  subtraction  of  much 
material. 


42        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Age  of  Contact-metamorphic  Deposits  in  the  United  States. — 
With  respect  to  age  or  time  of  deposition,  contact-metamorphic 
deposits  show  a  wide  range.  Some  of  the  contact  garnet-sulphide 
ores  of  Maine1  are  earlier  than  the  period  of  great  regional 
metamorphism  of  northern  New  England  and  are  therefore  pre- 
Silurian  and  probably  Cambrian  or  pre-Cambrian.  In  early 
Cretaceous  time  there  were  great  intrusions  of  granodiorites  in 
California,  attended  by  some  garnetization.  At  Bisbee,  Ariz.,2 
contact-metamorphic  ores  were  formed  in  early  Mesozoic  time, 
before  the  Lower  Cretaceous,  Bisbee,  beds  were  laid  down.  At 
Morenci,  Ariz.,3  intrusions  of  deep-seated  porphyries  attended  by 
the  development  of  contact-metamorphic  zones  containing  gar- 
net-sulphide ores  succeeded  the  deposition  of  the  Pinkard  forma- 
tion (Cretaceous). 

At  Philipsburg,  Mont.,  a  quartz  monzonite  batholith  has 
caused  contact  metamorphism  in  sedimentary  rocks  that  range 
from  Cambrian  to  late  Cretaceous,  but  it  does  not  intrude  middle 
Tertiary  beds.  The  intrusion  took  place  in  post-Cretaceous, 
doubtless  in  very  early  Tertiary  time.  At  Velardena,  Mexico, 
dioritic  and  alaskitic  rocks  that  are  believed  to  be  of  Miocene 
age  have  caused  contact  metamorphism,  including  the  forma- 
tion of  garnet  and  sulphides.  These  are  probably  the  youngest 
contact-metamorphic  deposits  of  sulphide  ores  on  the  continent. 
No  middle  or  late  Tertiary  sulphide  ores  of  this  type  are  known 
in  the  United  States. 

In  nearly  all  the  regions  cited  above,  the  silicate-sulphide 
contact-metamorphic  ores  occur  in  sedimentary  rocks  that  are 
at  least  as  old  as  very  early  Tertiary,  and  in  the  main  they  are 
associated  with  intrusives  that  are  not  younger  than  the  early 
Tertiary.  Although  intrusive  rocks  of  later  than  early  Tertiary 
age  occupy  extensive  areas  in  the  Western  States,  contact- 
metamorphic  deposits  of  sulphide  ores  are  rarely  found  in  genetic 
association  with  them,  probably  because  the  exposed  parts  of  the 
younger  intrusives  have  not  been  so  deeply  eroded  as  the  intrusives 
formed  earlier. 

1  EMMONS,  W.  H. :  Some  Regionally  Metamorphosed  Ore  Deposits  and 
the  So-called  Segregated  Veins.     Econ.  Geol.,  vol.  4,  pp.  755-781,  1909. 

2  RANSOME,  F.  L. :  Geology  and  Ore  Deposits  of  the  Bisbee  Quadrangle, 
Arizona.     U.  S.  Geol.  Survey  Prof.  Paper  21,  1904;  also  U.  S.  Geol.  Survey 
Butt.  529,  p.  180,  1913.' 

•LINDGREN,  WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.  U.  S.  Geol.  Survey  Prof.  Paper  42,  p.  198,  1905. 


CONTACT-METAMORPHIC  DEPOSITS  43 

Depths  at  Which  Contact-metamorphic  Ore  Deposits  Are 
Formed. — The  conditions  of  great  pressure  and  relatively  high 
temperature  under  which  contact-metamorphic  ore  deposits  are 
formed  rarely  prevail  simultaneously  near  the  surface.  A  number 
of  the  minerals  contained  in  contact-metamorphic  deposits  are 
not  ordinarily  formed  at  shallow  depths.  In  the  Philipsburg 
quadrangle,  Montana,  where  quartz  monzonites  cut  through 
sedimentary  rocks  that  range  in  age  from  early  Cambrian  to 
late  Cretaceous,1  they  have  caused  a  great  variety  of  alteration 
by  contact  metamorphism.  At  some  places  the  early  Paleozoic 
limestones  were  extensively  recrystallized  (marbleized)  and 
locally  replaced  by  masses  of  relatively  pure  magnetite.  At  other 
places  they  were  converted  to  garnet-epidote  rocks,  and  at  still 
others  they  were  changed  to  rocks  composed  of  calcite  with  a 
small  proportion  of  feathery  tremolite  evenly  distributed  through 
the  mass.  From  a  study  of  the  geologic  history  of  the  area  there 
is  good  reason  to  suppose  that  these  changes  took  place  under 
as  much  as  6,000  or  7,000  feet  of  sedimentary  rocks.  The  younger 
rocks,  which  must  have  been  nearer  the  surface  at  the  time  of 
the  intrusion,  were  metamorphosed  also,  but  they  do  not  con- 
tain ore  deposits  of  contact-metamorphic  origin.  Such  changes, 
therefore,  may  take  place  about  1^  miles  below  the  surface. 
In  the  Southwest  many  contact-metamorphic  garnet  zones  are 
associated  with  porphyries  that  have  a  microcrystalline  ground- 
mass  and  therefore  were  not  cooled  quickly  like  lavas.  In  the 
main  they  have  formed  at  depths  less  than  !}/£  miles  below  the 
surface.  Doubtless  a  mile  of  overlying  rock  is  sufficient  to  meet 
the  requirements  for  profound  metamorphism,  and  possibly 
even  less  is  necessary,  especially  where  the  intruded  rocks  are 
capped  by  flexible  and  relatively  impervious  shale  and  where 
extensive  fissuring  has  not  supplied  easy  paths  for  the  escape 
of  the  magmatic  solutions  to  the  surface.  Contact-metamorphic 
deposits,  although  showing  little  evidence  of  deposition  in  frac- 
tures, are  formed  in  the  zone  of  fracture,  yet  not  at  shallow  depths. 

Function  of  Mineralizing  Agents  and  Evidence  of  Their 
Activity. — The  part  played  by  the  "mineralizing  agents"  in  the 
formation  of  pegmatites  is  discussed  on  page  20.  These  sub- 
stances render  the  solutions  more  liquid  and  doubtless  aid  dif- 

1  EMMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana,  U.  S.  Geol.  Survey  Prof.  Paper  78,  p. 
188,  1913. 


44        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

fusion,  which  must  be  effective  where  large  crystals  are  formed 
from  solution.  In  the  process  of  contact  metamorphism  diffu- 
sion may  play  a  less  important  part.  Possibly  the  solutions, 
after  they  have  accomplished  contact  metamorphism,  gather  in 
the  larger  openings  outside  of  the  principal  metamorphic  zone 
and  escape  to  the  surface.  But  such  openings  do  not  seem 
to  have  been  present  in  all  places  where  contact-metamorphic 
deposits  have  been  formed,  and  they  may  not  be  essential  factors 
in  the  process.  The  exchange  of  material  may  take  place  to  some 
extent  by  diffusion,  the  metals  passing  through  the  solutions  at 
various  rates,  while  the  aqueous  solution  as  a  whole  does  not 
circulate  so  freely.  Water  vapor,  boron,  fluorine,  chlorine,  and 
other  volatile  substances  are  believed  to  take  an  active  part  in 
the  transfer  of  material,  and  if  they  do,  minerals  containing  these 
substances  should  be  expected  in  the  metamorphic  zone.  Of 
the  minerals  identified  in  contact-metamorphic  zones,  those  con- 
taining fluorine  are  fluor-apatite,  fluorite,  amblygonite,  topaz,  and 
vesuvianite;  those  containing  chlorine  are  chlor-apatite  and 
scapolite;  those  containing  boron  are  tourmaline,  ludwigite, 
axinite,  hulsite,  and  paigeite.  But  in  many  contact-meta- 
morphic deposits  these  minerals  are  rare  or  absent.  Contact- 
metamorphic  minerals  of  common  occurrence  that  contain  com- 
bined water  are  epidote,  biotite,  chlorite,  humite,  ilvaite,  musco- 
vite,  tourmaline,  allanite,  vesuvianite,  and  zoisite.  As  a  rule, 
however,  there  is  relatively  little  water,  combined  or  not  com- 
bined, in  contact-metamorphic  ore,  and  as  such  ore  contains 
relatively  few  cavities  which  would  indicate  that  water  was 
trapped  and  remained  behind,  there  is  a  strong  presumption  that 
water  or  steam  has  escaped  through  the  outer  metamorphosed 
zone  to  the  surface. 

Significance  of  Mineral  Associations  and  Synthetic  Experi- 
ments.— Certain  minerals,  such  as  quartz,  calcite,  pyrite,  chalco- 
pyrite,  and  galena,  are  formed  under  all  conditions  of  tempera- 
ture and  pressure  ranging  from  those  which  prevail  at  •  great 
depths  to  those  which  prevail  at  the  surface.  These  are  the 
"persistent"  minerals.  Other  minerals,  such  as  garnet,  vesu- 
vianite, tremolite,  pyroxene,  actinolite,  staurolite,  tourmaline, 
and  topaz  are  formed  only  under  conditions  of  high  tempera- 
ture and  pressure.  These  minerals  are  confined  essentially  to 
rocks  or  ore  deposits  formed  at  the  greater  depths.  Some  of  these 
minerals,  such  as  vesuvianite,  tourmaline,  and  staurolite,  have 


CONTACT-METAMORPHIC  DEPOSITS  45 

not  been  produced  in  the  laboratory;  others,  such  as  specularite 
and  cassiterite,  have  been  produced  only  at  high  temperatures 
or  under  considerable  pressure;  still  others  have  been  produced 
at  high  temperatures  in  streams  of  fluoride,  or  in  the  presence  of 
steam,  boron,  or  lithium  compounds.  Such  experiments  show 
that  normal  surface  conditions  are  not  favorable  for  the  genesis 
of  these  minerals.  The  same  conclusion  is  illustrated  by  de- 
structive experiments.  When  garnet,  vesuvianite,  and  many 
other  minerals  of  this  group  are  melted  and  the  melts  allowed  to 
cool,  they  will  break  up  into  other  mineral  compounds,  among 
which  are  anorthite,  olivine,  and  spinel.  These  and  other 
experiments  support  the  conclusion  that  contact-metamorphic 
deposits  are  formed  at  high  temperatures  and  pressures. 

Temperatures  and  Conditions  of  Solution  that  Form  Contact- 
metamorphic  Deposits. — The  temperatures  of  solidification  of  the 
deep-seated  rocks  are  in  general  lower  than  those  of  the  surface 
lavas,  for  the  deep-seated  rocks  retain  more  water  and  other  gases, 
which  materially  lower  the  point  of  precipitation.  Wright  and 
Larsen1  have  recently  made  some  experiments  that  have  a 
direct  bearing  on  the  problem.  Tridymite  is  formed  in  the 
laboratory  only  at  temperatures  as  high  as  800°  C.  At  lower 
temperatures  quartz  is  formed,  but  that  which  is  formed  above 
575°  is  of  the  trapezohedral-hemihedral  variety,  while  that 
formed  below  575°  is  probably  of  the  trapezohedral-tetartohedral 
variety.  The  two  varieties  of  quartz  show  consistent  differences 
in  the  maximum  double  refraction  and  other  properties.  Tridy- 
mite is  never  formed  under  the  conditions  that  prevail  at  great 
depths  but  is  peculiar  to  surface  lavas.  Some  of  the  quartz  of 
contact-metamorphic  deposits  is  of  the  variety  formed  at  tem- 
peratures below  575°C. 

Some  of  the  magmas  when  thrust  into  the  zone  where  contact- 
metamorphic  deposits  are  formed  must  be  at  least  as  hot  as  the 
lavas  that  reach  the  surface.  It  seems,  therefore,  that  these 
magmas  must  cool  through  a  considerable  range  before  they 
solidify.  As  the  conductivity  of  rocks  is  slight,  the  process  of 
cooling  must  be  slow;  consequently  there  is  a  long  period  of 
sweating  before  solidification  during  which  contact-metamorphic 
processes  may  operate.  The  solutions  pressing  outward  from 
the  magma  at  the  time  of  intrusion  are  probably  above  the  critical 

1  WRIGHT,  F.  E.,  and  LARSEN,  E.  S.:  Quartz  as  a  Geologic  Thermometer. 
Am.  Jour.  Sci.,  4th  ser.,  vol.  27,  pp.  421-447,  1909. 


46        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

temperature,  which  for  pure  water  is  358°C.  The  facts  above 
noted  suggest  the  probability  that  the  solutions  causing  contact 
metamorphism  are  in  part  in  the  gaseous  state.  The  competency 
of  the  solutions  to  penetrate  minute  openings,  as  indicated  by  the 
fact  that  contact-metamorphic  deposits  are  independent  of  well- 
defined  fissures,  may  be  due  to  their  gaseous  properties. 

Fissuring  During  Contact  Metamorphism. — Deep-lying  igne- 
ous bodies  cool  slowly.  During  the  period  of  cooling  fracturing 
and  fissuring  may  attend  metallization.  The  garnet  contact 
rock,  wherever  it  is '  formed,  is  relatively  impermeable,  and  un- 
less it  is  fractured  the  transfer  of  material  will  tend  to  follow 
other  paths.  But  fissuring  attending  the  movements  of  the 


*  S«S  *  ''-,  ~  "-    -  llV  ^  "*  * 


r^^"V»\  ty/  *=!'  S  *  /T..=  »  ™# 
u.-'-r ii -/,  =  v,,  *•/> . ' « //  =  *  « <•  *• : 


20  Feet 


Granite  Calcite.etc. 

Fia.  21. — Vertical  section  in  Cable  mine,  near  Anaconda,  Mont.,  showing 
recrystallization  of  limestone  after  faulting  of  granite  dike.  The  dike  is  an 
offshoot  of  granite  that  caused  contact  metamorphism. 


magma  may  brecciate  the  garnet  rock  and  any  small  dike-like 
apophyses  of  the  magma  which,  owing  to  their  smaller  size  and 
relatively  large  surface  of  radiation,  have  already  become  solid. 
Such  movements  are  illustrated  at  the  Cable  mine  (see  Fig.  21), 
some  15  miles  west  of  Anaconda,  Mont.,  where  Paleozoic  lime- 
stone and  shales  are  cut  by  intrusive  granite  (quartz  monzonite) 
that  sends  out  small  apophyses  into  the  sedimentary  rocks. 
Along  the  contact  great  angular  blocks  of  granite  and  of  garnet 
rock  are  surrounded  by  material  composed  of  coarse  calcite, 
pyrite,  chalcopyrite,  pyrrhotite,  and  magnetite.  The  relation 
as  indicated  by  Fig.  21  shows  that  the  metamorphism  and  exten- 
sive recrystallization  of  calcite  and  other  minerals  continued  after 
the  garnet  rock  had  formed  and  after  the  small  apophyses  of 
granite  had  solidified  and  had  been  fractured.  As  already  stated, 


CONTACT  METAMORPHIC  DEPOSITS  47 

fissuring,  though  it  may  be  a  contemporaneous  incident,  is  not  a 
necessary  condition  for  contact  metamorphism. 

Endomorphic  Changes. — At  some  places  near  intrusive  con- 
tacts the  composition  of  the  intruding  rock  has  changed,  pre- 
sumably as  a  result  of  absorption  of  the  rock  intruded.  Such 
changes  are  termed  endomorphic.  Doubtless  endomorphism  is 
an  effective  process  under  conditions  that  prevail  at  very  great 
depths.  Perhaps  chemical  analyses  of  selected  specimens  would 
show  that  the  intrusive  rocks  along  the  contacts  absorb  lime  and 
aluminum  from  the  intruded  rock.  At  some  places  fragments  of  the 
intruded  sediments  have  probably  been  completely  absorbed.1 
In  general,  however,  in  the  ore-bearing  contact-metamorphic 
zones  there  are  sharp  contacts  between  the  igneous  bodies  that 
caused  the  metamorphism  and  the  intruded  rocks,  rather  than 
broad  zones  of  material  in  which  igneous  rocks  and  molten  sedi- 
ments are  mingled. 

References 
CONTACT-METAMORPHIC  DEPOSITS 

BARRELL,  JOSEPH:  The  Physical  Effects  of  Contact  Metamorphism. 
Am.  Jour.  Sci.,  4th  ser.,  vol.  13,  p.  279,  1902.  Geology  of  the  Marysville 
Mining  District,  Montana — A  Study  of  Igneous  Intrusion  and  Contact 
Metamorphism.  U.  S.  Geol.  Survey  Prof.  Paper  57,  1907. 

BUTLER,  B.  S. :  Geology  and  Ore  Deposits  of  the  San  Francisco  District, 
Utah.  U.  S.  Geol.  Survey  Prof.  Paper  80,  1913. 

EMMONS,  S.  F. :  The  Cananea  Mining  District  of  Sonora,  Mexico.  Econ. 
Geol,  vol.  5,  pp.  312-356,  1910. 

EMMONS,  W.  H. :  A  Reconnaissance  of  Some  Mining  Camps  in  Elko, 
Lander,  and  Eureka  Counties,  Nevada.  U.  S.  Geol.  Survey  Bull.  408,  1910. 

EMMONS,  W.  H.,  and  CALKINS,  F.  C. :  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper  78, 
pp.  126-131,  221-223,  1913. 

KEMP,  J.  F. :  Ore  Deposits  at  the  Contacts  of  Intrusive  Rocks  and  Lime- 
stones, and  Their  Significance  as  Regards  the  General  Formation  of  Veins. 
Econ.  Geol.,  vol.  2,  pp.  1-13,  1907;  Cong.  geol.  internat.,  10th  sess.,  Compt. 
rend.,  pp.  519-531,  1906.  Garnet  Zones.  Min.  and  Sci.  Press,  vol.  92,  pp. 
220-221,  1906.  The  Iron  Ores  of  the  Iron  Springs  District,  Utah.  Econ. 
Geol.,  vol.  4,  pp.  782-791,  1909. 

KEMP,  J.  F.,  and  GUNTHER,  C.  G.:  The  White  Knob  Copper  Deposits, 
Mackay,  Idaho.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  38,  pp.  269-296,  1907. 

KEYES,  C.  R.:  Garnet  Contact  Deposits  of  Copper  and  the  Depths  at 
Which  They  Are  Formed.  Econ.  Geol,  vol.  4,  pp.  365-372,  1909. 

1  DALY,  R.  A. :  "Igneous  Rocks  and  Their  Origin,"  pp.  216-219,  345, 
1914. 


48        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

KNOPF,  ADOLPH:  Geology  of  the  Seward  Peninsula  Tin  Deposits,  Alaska. 
U.  S.  Geol.  Survey  Bull.  358,  1908. 

LAWSON,  A.  C.:  The  Copper  Deposits  of  the  Robinson  Mining  District, 
Nevada.  California  Univ.  Dept.  Geology  Bull,  vol.  4,  pp.  287-357,  1906. 

LEITH,  C.  K. :  Iron  Ores  of  Iron  Springs,  Utah  (reply  to  review  by  J.  F. 
KEMP  on  Contact  Metamorphism).  Econ.  Geol.,  vol.  5,  pp.  188-192,  1910. 

LEITH,  C.  K,  and  HARDER,  E.  C. :  The  Iron  Ores  of  the  Iron  Springs  Dis- 
trict, Utah.  U.  S.  Geol.  Survey  Bull.  338,  1908. 

LINDGREN,  WALDEMAR:  The  Character  and  Genesis  of  Certain  Contact 
Deposits.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  31,  pp.  226-244.  The  Genesis 
of  the  Copper  Deposits  of  Clifton-Morenci,  Arizona.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  35,  pp.  511-551,  1904.  The  Copper  Deposits  of  the  Clifton- 
Morenci  District,  Arizona.  U.  S.  Geol.  Survey  Prof.  Paper  43,  1905. 

LINDGREN,  WALDEMAR,  GORDON,  C.  H.,  and  GRATON,  L.  C.:  The  Ore  De- 
posits of  New  Mexico.  U.  S.  Geol.  Survey  Prof.  Paper  68,  1910. 

SPENCER,  A.  C.:  Magnetite  Deposits  of  the  Cornwall  Type  in  Pennsyl- 
vania. U.  S.  Geol.  Survey  Bull.  359,  1908. 

SPURR,  J.  E.,  and  GARREY,  G.  H.:  Ore  Deposits  of  the  Velardena  District, 
Mexico.  Econ.  Geol.,  vol.  3,  pp.  688-725,  1908. 

UGLOW,  W.  L. :  A  Review  of  the  Existing  Hypotheses  on  the  Origin  of  the 
Secondary  Silicate  Zones  at  the  Contacts  of  Intrusives  with  Limestone. 
Econ.  Geol,  vol.  8,  pp.  19-50,  215-234,  1913. 

UMPLEBY,  J.  B.:  The  Genesis  of  the  Mackay  Copper  Deposits.  Econ. 
Geol,  vol.  9,  pp.  307-358,  1914. 

VOGT,  J.  H.  L.:  Problems  in  the  Geology  of  Ore  Deposits.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  31,  pp.  125-169,  1901. 

WEED,  W.  H. :  Ore  Deposits  near  Igneous  Contacts.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  33,  pp.  715-746, 1902. 


CHAPTER  VI 

DEPOSITS  OF  THE  DEEP  VEIN  ZONE 

Occurrence.— Found  generally  in  or  near  intrusive  bodies  of  deep-seated 
igneous  rocks  that  have  been  deeply  eroded.  Not  genetically  related  to  sur- 
face lavas  and  intrusives  formed  near  the  surface.  Rarely  found  in  the 
younger  rocks. 

Composition. — The  minerals  are  approximately  the  same  as  those  formed 
in  contact-metamorphic  deposits,  but  quartz  is  as  a  rule  more  abundant. 
Gangue  minerals  include  garnet,  amphiboles,  pyroxenes,  and  micas.  Gan- 
gue  minerals  containing  elements  of  the  "agents  of  mineralization"  are 
commonly  present.  The  simple  sulphides  of  the  metals  are  frequently 
associated  with  metallic  oxides.  Gold,  tin,  iron,  zinc  and  copper  are  the 
most  important  metals  in  these  deposits.  Tungsten  and  molybdenum  also 
are  present  in  a  few  of  the  veins  of  the  deep  zone. 

Shape. — Some  of  the  deposits  are  tabular;  others  are  of  irregular  shape. 
Stockworks  and  stringer  leads  are  developed.  The  large,  regular  tabular 
bodies,  which  predominate  in  the  group  of  deposits  formed  nearer  the  surface, 
are  represented  but  are  proportionately  less  numerous  in  the  deep-vein  zone. 
Sheeted  zones,  bedding-plane  deposits,  saddles,  and  anticlinal  deposits  are 
developed. 

Size. — Many  of  the  individual  deposits  are  small;  some  are  large. 

Texture. — The  lodes  are  commonly  banded.  Vugs  are  locally  present. 
Fluid  inclusions  are  common.  Where  banded  rocks  are  replaced  by  ore,  the 
ore  may  retain  the  banding.  Comb  quartz  and  symmetrical  crustification 
are  not  unknown,  but  these  features  are  not  so  conspicuously  developed  as  in 
deposits  that  were  formed  nearer  the  surface.  Disseminated  ore  may  be 
formed. 

General  Features. — Deposits  of  the  deep  vein  zone  are  formed 
at  high  temperature  and  under  great  pressure,  in  and  along  fissures 
or  other  openings.  Although  this  group  is  represented  by 
numerous  valuable  deposits  in  the.  United  States,  it  is  as  a  whole 
less  important  economically  than  the  group  of  deposits  formed 
at  moderate  depth  and  lower  temperature.  Ore  veins  are  more 
commonly  formed  relatively  near  the  surface,  where  rocks  are 
more  easily  fractured  and  where  ascending  solutions  are  more 
readily  cooled  in  part  by  mingling  with  cold  surface  waters. 
The  Cornwall  tin  and  copper  deposits,  which  are  among  the  most 
notable  examples  of  the  deep  veins,  have  been  enormously  pro- 

49 


50        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

ductive.  In  the  United  States  the  gold  veins  of  the  Appalachian 
region  and  the  Homestake  ore  bodies  of  the  Black  Hills  are  the 
best-known  representatives. 

Aside  from  economic  considerations  this  class  of  ore  deposits 
is  of  great  scientific  interest,1  for  it  supplies  a  connecting  link 
between  pegmatite  veins  and  contact-metamorphic  deposits, 
on  one  hand,  and  ores  that  are  formed  in  and  along  fissures 
at  moderate  depth,  on  the  other.  Deposits  of  the  deep  zone, 
because  they  were  formed  at  high  temperatures,  are  allied  minera- 
logically  to  contact-metamorphic  deposits  and  to  pegmatites; 
because  they  were  formed  in  and  along  fissures,  they  are  allied 
to  vein  deposits  formed  near  the  surface. 

Occurrence. — Although  the  deposits  of  the  deep  vein  zone 
are  closely  allied  mineralogically  to  the  contact-metamorphic 
deposits,  they  differ  from  that  group,  in  that  they  are  clearly 
related  to  easily  recognized  fissures  or  other  openings  and  are 
confined  to  areas  where  fracturing  has  occurred  before  they  were 
formed.  As  a  rule  the  wall  rock  is  extensively  replaced  by  ore, 
but  such  replacement  is  not  so  extensive  as  to  obliterate  the 
relationship  of  these  deposits  to  openings  in  rocks. 

The  veins  are  in  or  near  igneous  rocks  and  probably  have 
been  deposited  by  solutions  that  were  exhaled  by  the  cooling 
magmas.  Some  of  them  cut  the  parent  rock  and  presumably 
were  deposited  soon  after  the  rock  had  become  rigid  enough  to 
break,  by  solutions  from  a  lower  portion  of  the  magma  not  yet 
solidified.  These  deposits  are  not  genetically  related  to  lavas  or 
to  superficial  intrusives,  because  pressures  are  moderate  where 
such  rocks  are  formed.  Many  deposits  of  this  group  are  in  or 
around  granitic  rocks,  but  the  intrusive  basic  rocks  less  com- 
monly supply  the  conditions  necessary  for  their  formation. 

Composition. — Deposits  of  the  deep  zone  carry  gold,  tin, 
copper,  iron,  tungsten,  and  other  metals.  Manganese  is  rare 
compared  with  the  amounts  found  in  lodes  formed  at  moderate 
depths.  The  deposits  contain  a  large  number  of  minerals,  most 
of  which  are  found  also  in  contact-metamorphic  deposits.  The 
commonest  of  these  are  garnet,  amphiboles,  pyroxene,  tourma- 
line, and  micas.  The  sulphides  include  pyrite,  chalcopyrite, 
pyrrhotite,  and  arsenopyrite.  Topaz,  magnetite,  ilmenite, 
specularite,  gold,  cassiterite,  wolframite,  and  molybdenite  are 

^INDGREN,  WALDEMAR:  The  Relation  of  Ore  Deposition  to  Physical 
Conditions.  Econ.  Geol,  vol.  2,  pp.  105-127,  1907. 


DEPOSITS  OF  THE  DEEP  VEIN  ZONE 


51 


developed  also.     Barite  and  other  sulphates  are  very  rare  or 
absent.     The  principal  minerals  are  listed  below. 


albite 

amphiboles 

andradite 

anhydrite 

ankerite 

apatite 

arsenopyrite 

bismuthinite 

biotite 

bornite 

calcite 

cassiterite 

chalcopyrite 

chlorite 


cinnabar 

cryolite 

diopside 

dolomite 

epidote 

fluorite 

gahnite 

galena 

garnet 

gold 

graphite 

hematite 

hornblende 

ilmenite 


lepidolite 

magnetite 

molybdenite 

muscovite 

orthoclase 

platinum 

pyrite 

pyrrhotite 

quartz 

rutile 

scapolite 

scheelite 

sericite 

siderite 


silver 

spinel 

specularite 

stibnite 

tellurides  (rare) 

topaz 

tourmaline 

tremolite 

willemite 

wolframite 

zinc  blende 

zincite 

zoisite 


These  minerals  may  be  deposited  in  open  spaces  or  they  may 
be  formed  by  replacement  of  the  wall  rock.  It  is  not  everywhere 
possible  to  determine  just  what  portion  of  a  deposit  has  filled  an 
open  space  and  what  portion  has  replaced  the  wall  rock,  but  in 
some  ores  the  texture  of  the  wall  rock,  bedding  planes,  jointing, 
mineral  boundaries,  etc.,  are  preserved  in  the  deposit  after  the 
chemical  and  mineral  composition  has  been  changed.  The 
crystals  of  certain  gangue  minerals  of  deposits  of  the  deep  vein 
zone  are  likely  to  be  larger  than  those  of  deposits  formed  by  hot 
waters  near  the  surface.  Thus  the  crystals  of  muscovite  in 
greisen1  are  much  larger  than  the  crystals  of  sericite  in  veins 
formed  at  moderate  depths.  Replacement  processes  are  discussed 
on  pages  218-229. 

Shape. — Because  they  are  formed  in  and  along  great  fissures 
some  of  the  deposits  of  this  group  are  broadly  tabular.  Gener- 
ally the  country  rock  along  a  fissure  is  replaced  by  ore  and  the 
boundaries  are  irregular  in  detail.  Where  the  fissures  are  small, 
closely  spaced,  and  not  parallel,  and  where  the  country  rock  be- 
tween the  fissures  has  been  partly  replaced,  a  large,  irregular, 
and  generally  low-grade  deposit  may  be  formed.  Examples  are 
the  stockworks2  of  Altenberg,  Saxony,  and  the  igneous  boss, 

1  Greisen  is  altered  granite  along  tin  veins.     It  has  been  recrystallized 
and  metamorphosed  by  hot  waters.  •  It  contains  mica,  topaz,  quartz,  and 
cassiterite. 

2  A  stockwork  is  a  deposit  made  up  of  many  small,  closely  spaced,  irregular 
veins,  with  mineralized  country  rock  between.     The  length  of  the  deposit  is 
not  much  greater  than  its  width. 


52        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

containing  closely  spaced  tin  veins,  at  Zinnwald,  near  by.  The 
irregular  deposits  in  schist  in  the  Appalachian  gold  belt  are 
figured  on  page  55. 

Size.  —  Many  of  the  deposits  of  this  class  are  small,  but  even 
the  small  deposits  may  be  so  closely  spaced  that  considerable 
bodies  of  rock  are  workable.  Some  of  the  deep  vein  deposits, 
however,  are  large.  The  tin  deposits  of  the  Cornwall  peninsula, 
England  —  for  example,  the  Dolcoath  lode-r-are  very  extensive 
both  vertically  and  along  the  strike.  The  Homestake  lode  in  the 
Black  Hills,  South  Dakota,  is  one  of  the  greatest  gold  deposits 
in  the  world.  Some  of  the  deposits  of  this  group  that  are 
worked  in  the  southern  Appalachians  are  small  and  if  they  were 
not  closely  spaced  would  be  of  comparatively  little  value. 

Texture.  —  Quartz  is  abundant  in  many  deposits  of  the  deep 
zones.  Much  of  it  contains  fluid  inclusions  with  gas  bubbles 
and  associated  solids.  Quartz  crystals  may  project  into  open 
spaces  or  vugs,  but  the  comb  structure  in  which  long  parallel 
crystals  of  quartz  alternate  with  layers  of  sulphides  and  other 
minerals,  characteristically  developed  in  open  spaces  near  the 
surface,  is  less  common  in  the  deep  vein  zone.  In  many  de- 
posits the  quartz  occurs  as  irregular  interlocking  grains,  like  the 
minerals  of  contact-metamorphic  deposits  and  some  igneous 
rocks.  In  some,  however,  it  is  banded  with  other  minerals. 
Garnet  is  not  everywhere  developed,  but  in  some  veins  it  is 
abundant.  Generally  it  shows  crystal  outlines.  In  some  de- 
posits, as  in  the  Lockhart  vein,  Dahlonega,  Ga..1  garnet  is  banded 
with  quartz  (Fig.  22).. 

The  filled  portions  of  the  lodes  may  differ  in  mineral  composi- 
tion from  the  portions  that  have  been  deposited  by  replacement. 
In  the  replacement  of  the  walls  the  heavy  silicates  may  be  abun- 
dantly developed,  but  quartz  is  generally  more  abundant  in  the 
part  that  represents  a  former  open  space.  Some  of  the  fractures 
that  have  been  filled  are  small  and  do  not  continue  regularly  very 
far  in  any  direction,  and  in  many  places  they  are  arranged  as  net- 
works of  small  fractures.  Some  of  them  may  be  mere  cooling 
cracks,  but  most  of  them  are  probably  due  to  relief  of  stresses. 
The  alterations  of  the  wall  rocks,  though  sometimes  more  intense, 
are  generally  not  so  extensive  as  in  deposits  formed  near  the  sur- 
face, where  rocks  are  more  readily  shattered. 


,  WALDEMAR:  The  Gold  Deposits  of  Dahlonega,   Ga.  U.  S. 
Geol.  Survey  Bull.  293,  p.  126,  1906. 


DEPOSITS  OF  THE  DEEP  VEIN  ZONE  53 

Depth  of  Formation. — The  basis  for  classifying  the  deep  vein 
deposits  and  the  two  groups  next  discussed  is  the  depth  at  which 
the  deposits  were  formed.  This  distinction  is  warranted,  be- 
cause depth  in  a  large  measure  controls  the  temperature  and  pres- 
sure and  to  some  extent  the  character  of  the  openings  and  the 
nature  of  replacement.  Moreover,  there  is  little  exact  informa- 


FIG.  22. — Ore  from  the  Lockhart  mine,  Dahlonega,  Georgia,  showing  quartz 
with  streaks  of  garnet,  dark  green  mica  and  hornblend.  (After  Lindgren,  U.  S. 
Geol.  Survey.) 

tion  as  to  the  temperature  and  pressure  prevailing  at  the  time  the 
deposits  were  formed.  If  knowledge  of  the  geologic  history  of 
a  region  that  contains  ore  deposits  is  adequate,  a  reasonably 
accurate  estimate  may  be  made  of  the  depth  of  formation. 
Depth,  however,  is  not  the  only  factor  that  influences  the  tem- 
perature and  pressure.  The  nature  of  the  fissures  and  especially 
the  character  of  the  rock  overlying  the  deposits  at  the  time  of 


54        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

their  formation  are  also  to  be  considered.  In  a  fissure  that  ex- 
tends freely  to  the  surface  the  pressure  is  hydrostatic  and  will  not 
be  greater  than  the  weight  of  a  column  of  water  as  high  as  the 
place  of  deposition  is  deep.  Where  openings  are  connected 
freely  with  the  surface,  solutions  at  temperatures  above  the 
boiling  point  at  that  pressure  could  not  persist;  the  water 
would  become  steam  and  escape.  On  the  other  hand,  in  open- 
ings that  are  not  connected  with  the  surface  the  pressure  is 
limited  only  by  the  crushing  strength  of  the  rocks,  and  the  tem- 
perature could  be  correspondingly  high.  Therefore,  deposits 
of  this  class,  which  are  normally  formed  at  great  depths,  may 
under  some  conditions  be  formed  nearer  the  surface.  Some  of 
these  deposits,  especially  where  the  containing  rock  is  capped  with 
shale,  a  rock  that  is  not  readily  fractured  and  that  is  relatively 
impermeable  to  solutions,  may  form  within  4,000  or  5,000  feet 
of  the  surface  or  even  less,  but  it  is  believed  that  most  deposits 
of  this  group  are  formed  at  depths  of  more  than  a  mile.  At 
Philipsburg,  Mont.,1  some  veins  of  this  group  were  formed  about 
7,000  feet  below  the  surface  at  the  time  of  their  deposition. 
In  the  southern  Appalachian  gold  belt  deposits  of  this  class 
were  probably  formed  15,000  feet  or  more  below  the  surface.2 

Origin  of  Openings. — Some  of  the  openings  that  have  been 
filled  in  the  deep  zone  have  doubtless  been  formed  by  the  relief 
of  compressive  stresses;  others  are  in  weak  rocks  and  have  been 
filled  at  great  depth,  probably  where  the  pressure  would  quickly 
close  any  large  openings.  In  the  southern  Appalachians,3 
where  the  veins  of  this  group  are  typically  developed,  as  stated 
above,  probably  more  than  15,000  feet  of  overlying  rock  has 
been  eroded  since  the  deposits  were  formed.  The  nature  of  the 
rocks  fractured  and  the  mass  of  the  overlying  rock  seem  to  have 
prevented  the  formation  of  long,  continuous  openings.  Some 
of  the  deposits  are  in  schist,  and  generally  such  rocks  do  not  have 
great  strength.  It  has  been  doubted  whether  spaces  at  depths 
so  great  could  have  remained  open  long,  and  they  must  have  been 
filled  quickly.  Becker,  in  his  discussion  of  the  fissures  filled  by 

1  EMMONS;  W.  H.,  and  CALKINS,  F.  C.:  The  Geology  and  Ore  Deposits  of 
the  Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper  78, 
p.  188,  1912. 

2LiNDGREN,  WALDEMAR:  The  Gold  Deposits  of  Dahlonega,  Georgia. 
U.  S.  Geol.  Survey  Bull.  293,  p.  59,  1906. 

3  BECKER,  G.  F. :  Gold  Fields  of  the  Southern  Appalachians.  U.  S.  Geol. 
Survey,  Sixteenth  Ann.  Rept.,  part  3,  pp.  251-331,  1895. 


DEPOSITS  OF  THE  DEEP  VEIN  ZONE  55 

such  veins,  suggests  that  the  force  of  crystallization  of  the 
minerals  contained  has  pushed  the  vein  walls  apart,  widening 
thin  fissures  as  they  were  filled  (see  page  176).  It  has  been 
suggested  by  Graton1  that  the  mineral-bearing  solutions  were 
injected  under  pressure  that  originated  in  the  intrusive  magma 
from  which  the  solutions  were  derived.  He  believes  that  the 
bodies  of  quartz  did  not  solidify  in  open  spaces  of  corresponding 


Amphibolite  Quartz 

FIG.  23. — Vertical  section  in  Schlegelmilch  mine,  York  County,  South 
Carolina,  showing  lenticular  bodies  of  quartz  in  amphibolite  schist.  (After 
Graton,  U.  S.  Gcol.  Survey.) 

dimensions  which  were  ready  to  receive  the  solutions,  but  that 
the  solutions,  pushing  their  way  along  what  may  in  many  places 
have  been  the  merest  cracks,  forced  the  walls  apart  and  made 
the  receptacles  in  which  their  load  was  deposited.  The  force 
of  crystallization2  may  have  aided  somewhat  in  expanding  the 

1  GRATON,  L.  C. :  Reconnaissance  of  Some  Gold  and  Tin  Deposits  in  the 
Southern  Appalachians.     U..S.  Geol.  Survey  Bull.  293,  p.  60,  1906. 

2  DUNN,  E.  J. :  Reports  on  the  Bendigo  Gold  Fields,  p.  25.     Victoria  Dept. 
Mines,  1896.     BECKER,  G.  F.,  and  DALY,  R.  A. :  The  Linear  Force  of  Grow- 
ing Crystals.     Washington  Acad.  Sci.  Proc.,  vol.  6,  pp.  283-288. 


56        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

openings,  but  according  to  Graton  the  principal  factor  was  the 
pressure  under  which  the  solutions  reached  the  zone  of  deposi- 
tion. The  structure  of  these  interfoliated  veins  is  indicated  by 
Fig.  23. 

Connection  with  the  Surface  at  the  Time  of  Deposition. — If  it 
is  assumed  that  some  of  the  deposits  of  the  deep  zone  are  related 
to  openings  which  were  formed  as  a  result  of  pressures  that  origi- 
nated in  the  magmas,  and  that  such  pressures  in  some  places 
exceeded  those  of  the  hydrostatic  head  at  the  same  depths,  it  is 
necessary  to  assume  also  that  the  systems  of  openings  did  not 
connect  freely  with  the  surface  at  the  time  of  deposition.  If  the 
spent  solutions  escaped  after  they  had  deposited  their  mineral 
load,  it  'was  through  small  spaces  such  as  capillary  openings, 
or  along  minute  cracks  where  friction  with  the  walls  served  to 
retard  or  hold  back  the  magmatic  waters.  Otherwise  the  pres- 
sure would  have  been  hydrostatic.  Under  such  conditions 
there  could  hardly  have  been  a  really  active  circulation  of  water, 
and  consequently  crustified  banding,  which  normally  is  best 
developed  in  veins  deposited  by  freely  circulating  waters,  is  not 
so  conspicuously  shown. 

In  the  Philipsburg  quadrangle,  Montana,1  there  are  certain 
gold-bearing  veins  that  carry  specularite  and  pyrrhotite;  in 
the  same  rocks  and  at  approximately  the  same  elevations  are 
other  veins  that  do  not  carry  minerals  of  the  deep  zone.  Still 
other  veins  near  by,  formed  at  the  same  time  and  under  approxi- 
mately the  same  load  of  overlying  rocks,  carry  tourmaline, 
quartz,  and  gold.  Pyrrhotite,  specularite,  and  tourmaline  are 
indicative  of  deposition  under  high  temperature  and  great 
pressure.  It  has  been  suggested  that  the  lodes  which  carry  the 
high-temperature  minerals  were  probably  formed  in  fissures  that 
did  not  extend  to  the  surface,  and  that  the  solutions  which  de- 
posited them  were  under  pressures  that  exceeded  those  of  the 
hydrostatic,  head.  Some  of  the  veins  near  Philipsburg,  which  do 
not  carry  minerals  characteristic  of  deep-seated  deposits,  were 
formed  at  the  same  depth  as  the  specularite-gold  and  tourmaline- 
gold  deposits,  but  the  greater  persistence  of  the  veins  along  both 
dip  and  strike,  their  more  nearly  tabular  form,  and  their  banded 
and  crustified  texture  are  features  supporting  the  hypothesis 
that  these  fissures  were  connected  with  the  surface  at  the  time 
they  were  formed  and  that  their  ores  were  deposited  by  circu- 

1  EMMONS,  W.  H.,  and  CALKINS,  F.  C. :  Op.  tit.,  p.  188. 


DEPOSITS  OF  THE  DEEP  VEIN  ZONE  57 

lating  waters  under  hydrostatic  head.  Both  groups  of  veins 
were  probably  formed  some  6,000  or  7,000  feet  below  the  surface 
at  the  time  of  deposition.  The  conditions  necessary  for  the 
development  of  the  minerals  of  the  deep  zone  might  exist  even 
nearer  the  surface,  in  places  where  the  seats  of  deposition  are  not 
connected  with  the  surface  by  fissures  (Fig.  .24).  But  owing  to 
the  more  extensive  fracturing  under  the  lighter  load  near  the 
surface,  such  conditions  are  exceptional. 

Function  of  Mineralizers. — Among  the  minerals  of  the  deep 
vein  zone  are  many  which  contain  fluorine,  chlorine,  boron, 
lithium,  or  other  elements  that  enter  into  the  composition  of 
the  compounds  which  have  been  called  the  mineralizing  agents. 
These  elements,  except  fluorine,  are  found  in  few  primary 
ores  in  the  vein  deposits  formed  under  conditions  that  pre- 


FIG.  24. — Section  showing  veins  of  the  deep  zone  type,  (B)  in  an  area  containing 
veins  formed  at  moderate  depths,  (A). 

vail  at  moderate  and  shallow  depths.  It  is  believed  that  they 
promote  diffusion,  and  owing  to  the  low  critical  temperatures  of 
many  of  the  compounds  that  contain  them  these  compounds 
remain  longer  in  the  gaseous  state.  The  mineralizers  probably 
give  the  solutions  greater  power  to  penetrate  minute  openings 
and  to  replace  relatively  impermeable  wall  rocks.  In  the  deep 
zones,  where  pressures  are  high  and  connections  with  the  sur- 
face are  less  direct,  the  elements  of  the  mineralizers  are  likely 
to  be  fixed  in  stable  mineral  compounds,  but  in  the  deposits 
formed  at  moderate  or  shaUow  depth  they  escape  more  readily 
to  the  surface. 

Age  of  Deposits  of  the  Deep  Zone. — The  deposits  of  this  group 
are  formed  under  conditions  of  pressure  which  at  the  time  of 
deposition  existed  normally  at  depths  more  than  a  mile  below 
the  surface.  The  deposits  therefore  generally  occur  in  rocks  that 


58        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

have  been  buried  under  a  mile  or  more  of  later  rocks  or  in  intru- 
sive masses  associated  with  such  rocks.  The  sedimentary  rocks 
which  have  been  so  deeply  buried  are  in  the  main  the  older  rocks. 
Lindgren1  has  brought  out  these  relations  in  his  analysis  of  the 
gold  production  of  North  America. 

The  deposits  of  this  group  are  found  in  the  pre-Cambrian 
and  early  Paleozoic  rocks  and  in  or  associated  with  Mesozoic 
and  early  Tertiary  intrusive  rocks,  more  commonly  than  with 
the  middle  and  late  Tertiary  intrusives,  which  in  few  places  have 
been  eroded  deeply  enough  to  expose  such  deposits.  Many  ore 
bodies  in  the  older  rocks,  formed  at  moderate  depth  and  at  low 
pressure  were  subsequently  buried  and  therefore  do  not  exhibit 
any  of  the  features  of  the  deposits  of  the  deep  zone.  Further- 
more, many  deposits  in  ancient  rocks  have  been  formed  in  late 
periods  near  the  surface.  Many  ore  deposits  inclosed  in  pre- 
Cambrian  rocks  are  associated  with  Tertiary  eruptives  and  have 
all  the  features  of  deposits  formed  at  moderate  depth. 

Gradations  into  Pegmatite  Veins. — Many  veins  of  the  deep 
zone  are  associated  with  deep-seated  intrusive  rocks  around 
which  pegmatites  seem  not  to  have  been  developed,  but  some 
are  closely  associated  with  pegmatites.  Typical  feldspar  peg- 
matites that  may  be  followed  into  profitable  gold-bearing  quartz 
veins  are  rare,  yet  there  are  gradational  phases  between  the  two 
classes  of  deposits.  The  data  bearing  on  the  origin  of  the  peg- 
matites, especially  on  their  gradation  into  gold-bearing  and  other 
quartz  veins,  have  been  summarized  and  discussed  by  Spurr.2 
There  is  much  evidence  to  show  that  in  certain  areas  such  as  the 
Silver  Peak  district,  Nevada,  and  the  Yukon  region,  Alaska, 
the  gold-bearing  veins  and  the  pegmatites  are  deposited  by  solu- 
tions originating  from  the  same  magmas. 

On  Mineral  Ridge,  near  Silver  Peak,  early  Paleozoic  strata  are 
complexly  injected  by  granitic  rocks  that  are  probably  of  late 
Jurassic  or  early  Cretaceous  age ;  at  some  places  these  rocks  are 
covered  by  Tertiary  lavas  and  sedimentary  rocks,  which,  how- 
ever, do  not  contain  deposits  of  the  metals.  The  granitic  rocks 
grade  into  alaskites  (rocks  containing  quartz  and  alkali  feldspar 
but  free  or  nearly  free  from  other  minerals).  Some  of  the 

LINDGREN,  WALDEMAR:  The  Geologic  Features  of  the  Gold  Production 
of  North  America.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  33,  pp.  790-844,  1902. 
Metallogenetic  Epochs.  Econ.  Geol.,  vol.  4,  pp.  409-420,  1909. 

2  SPURR,  J.  E. :  The  Ore  Deposits  of  the  Silver  Peak  Quadrangle,  Nevada. 
U.  S.  Geol.  Survey  Prof.  Paper  55,  p.  129,  1906. 


DEPOSITS  OF  THE  DEEP  VEIN  ZONE  59 

alaskite  carries  small  but  appreciable  quantities  of  gold.  By 
diminution  of  feldspar  some  of  the  alaskites  grade  into  pure  quartz 
veins.  The  quartz  is  full  of  fluid  inclusions.  In  the  Drinkwater 
ore  zone  (Fig.  25)  the  calcareous  country  rock  surrounding 
bodies  of  quartz  and  also  that  surrounding  bodies  of  alaskite  is 
similarly  metamorphosed,  and  near  the  contacts  hornblende, 
epidote,  quartz,  and  feldspar  have  been  developed  by  contact- 
metamorphic  processes.  Iron  oxides  and  sulphides  are  present 
in  the  ore. 

Gradation  into  Deposits  Formed  by  Hot  Solutions  at  Moder- 
ate Depths. — A  classification  of  ore  deposits  is  valuable  princi- 
pally as  a  convenient  instrument  for  purposes  of  description, 
comparison,  and  study.  The  several  classes  are  connected  by 
gradational  types,  and  nowhere  is  this  relationship  more  marked 


soo  feet 


FIG.    25. — Sketch  of   canyon   wall  east  of   Drinkwater   mine.     Silverpeak 
quadrangle,  Nevada,  showing  quartz  lenses  in  alaskite. 

than  at  the  hypothetical  line  separating  the  deposits  of  the  deep 
zone  from  deposits  formed  by  hot  waters  at  moderate  depths. 
Possibly  a  separation  between  the  two  groups  should  be  made  on 
the  basis  of  pressure — magmatic  or  hydrostatic — for  many  of 
the  deposits  of  the  deep  zone  were  formed  in  openings  that  did 
not  connect  freely  with  the  surface  and  under  pressure  that  prob- 
ably originated  in  the  magma,  whereas  most  ore  bodies  formed 
at  moderate  depth  were  deposited  in  openings  or  systems  of 
openings  that  presumably  were  connected  freely  with  the  sur- 
face at  the  time  the  solutions  filled  them  and  therefore  permitted 
the  more  ready  escape  of  spent  solutions  and  gases.  However, 
some  of  the  most  valuable  ore  bodies  that  carry  the  minerals  of 
the  deep  zone  in  typical  development  were  formed  in  and  along 
strong,  persistent,  and  nearly  vertical  fissures.  It  is  probable 


60        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

that  such  fissures  either  extended  to  the  surface  themselves  or 
connected  with  other  fissures  that  extended  to  the  surface  at  the 
time  of  deposition. 

Some  of  the  lodes  of  Cornwall — for  example,  the  Standard  lode, 
St.  Ives  Consols — have  been  followed  down  their  nearly  vertical 
dip  for  hundreds  of  feet.  The  Dolcoath  lode  extends  in  depth 
3,000  feet  or  more.  These  lodes  persist  also  along  the  strike 
and  must  have  been  deposited  along  fissures  that  had  great 
vertical  range.  The  minerals  of  the  ore  include  cassiterite  and 
tourmaline,  an  association  which  suggests  high  temperature 
and  pressure. 

In  the  Coeur  D'Alene  district,  Idaho,1  a  thick  series  of  quartz- 
ose  sedimentary  rocks  of  pre-Cambrian  age  is  intruded  by  great 
masses  of  monzonite  and  syenite.  The  sedimentary  rocks  are 
cut  by  persistent  lead-silver  lodes.  Some  of  them  have  been 
explored  along  the  strike  for  nearly  2  miles,  and  the  group  has  a 
vertical  range  of  more  than  4,000  feet.  The  lodes  dip  at  moder- 
ately high  angles.  As  shown  by  Ransome,  pyrrhotite  and  mag- 
netite, minerals  characteristic  of  the  deep  zone,  are  developed 
in  the  primary  ore  of  some  of  these  lodes.  In  the  Tiger-Poorman 
lode,  which  is  explored  on  the  lowest  level  in  the  district,  pyrrho- 
tite increases  toward  the  lower  levels.  The  Tiger-Poorman  is  of 
good  width,  extends  at  least  4,000  feet  along  the  strike,  is  at  least 
2,000  feet  in- vertical  extent,  and  dips  80°.  It  is  a  sheeted 
zone  of  overlapping  fissures  along  which  extensive  replacement 
has  occurred.  If  before  erosion  the  Tiger-Poorman  lode  extended 
upward  as  far  as  the  apex  of  other  veins  near  by,  the  pyrrhotite 
ore  in  it  was  formed  at  least  4,000  feet  below  the  surface. 

Some  lodes  in  the  Philipsburg  quadrangle,  Montana,  such  as 
the  Sunrise,  at  Henderson,  carry  pyrrhotite  in  variable  amounts, 
and  these  deposits  seem  to  have  been  formed  at  lower  tempera- 
ture and  pressure  than  the  pyrrhotite-specularite  veins  mentioned 
above.  Pyrrhotite  is  present  also  in  some  of  the  deep  veins  of 
California  but  is  not  abundant  nor  widespread.  There  is  much 
evidence,  therefore,  that  pyrrhotite  and  probably  magnetite 
also  are  minerals  that  link  veins  of  the  deep  zone  to  those  formed 
at  moderate  depth.  Like  garnet,  they  are  stable  under  the  deep- 
seated  conditions,  but  their  range  is  somewhat  greater  than  that 
of  garnet  and  other  heavy  silicates. 

1  RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Coeur  d'Alene  District,  Idaho.  U.  S.  Geol.  Survey  Prof.  Paper  62,  1908. 


DEPOSITS  OF  THE  DEEP  VEIN  ZONE  61 

Gradation  into  Contact  Metamorphic  Deposits. — Veins  of  the 
deep  zone  grade  also  into  contact  metamorphic  deposits.  In  some 
districts  the  mineral  composition  of  the  veins  changes  toward 
metamorphic  zones,  magnetite  and  pyrrhotite  becoming  more 
abundant  in  the  veins  that  are  situated  nearer  the  contacts 
while  in  the  deposits  of  contact  metamorphic  origin  magnetite, 
pyrrhotite,  and  heavy  silicates  are  present  in  force.  In  the 
Iron  Springs  deposits,  Utah,1  veins  and  breccia  deposits  in 
andesite  extend  into  the  larger  bodies  of  ore  developed  by  meta- 
morphism  of  sedimentary  rocks. 

References 
DEPOSITS  OF  THE  DEEP  VEIN  ZONE 

BUTLER,  B.  S. :  Geology  and  Ore  Deposits  of  the  San  Francisco  and  Adja- 
cent Districts,  Utah.  U.  S.  Geol.  Survey  Prof.  Paper  80,  p.  172,  1913. 

DERBY,  O.  A. :  On  the  Mineralization  of  the  Gold-bearing  Lodes  of  Pas- 
sagem,  Minas  Geraes,  Brazil.  Am.  Jour.  Sci.,  4th  ser.,  vol.  32,  pp.  185-190, 
1911. 

EMMONS,  W.  H. :  A  Genetic  Classification  of  Minerals.  Econ.  Geol.,  vol. 
3,  pp.  611-627,  1908. 

FERGUSON,  H.  G.,  and  BATEMAN,  A.  M.:  Geologic  Features  of  Tin 
Deposits.  Econ.  Geol.,  vol.  7,  pp.  263-279,  1912. 

HESS,  F.  L.,  and  GRATON,  L.  C. :  The  Occurrence  and  Distribution  of  Tin. 
U.  S.  Geol.  Survey  Bull.  260,  1905. 

LINDGREN,  WALDEMAR:  The  Relation  of  Ore  Deposition  to  Physical  Con- 
ditions. Econ.  Geol.,  vol.  2,  pp.  105-127,  1907.  Metallogenetic  Epochs. 
Econ.  Geol.,  vol.  4,  pp.  409-420,  1909.  Metasomatic  Processes  in  the  Gold 
Deposits  of  Western  Australia.  Econ.  Geol.,  vol.  1,  pp.  530-544,  1906.  Ore 
Deposition  and  Deep  Mining.  Econ.  Geol.,  vol.  1,  pp.  34-46,  1906. 

MACALESTER,  D.  A. :  Geological  Aspects  of  the  Lodes  of  Cornwall.  Econ. 
Geol.,  vol.  3,  pp.  363-380,  1908. 

SINGEWALD,  J.  T.,  JR.:  The  Erzgebirge  Tin  Deposits.  Econ.  Geol.,  vol. 
5,  pp.  166-177,  265-272,  1910. 

SPENCER,  A.  C.:  The  Juneau  Gold  Belt,  Alaska.  U.  S.  Geol.  Survey 
Bull.  287,  1906. 

SPURR,  J.  E. :  Ore  Deposits  of  the  Silver  Peak  Quadrangle,  Nevada.  U.  S. 
Geol.  Survey  Prof.  Paper  55,  pp.  99-123,  1906. 

WRIGHT,  F.  E.,  and  LARSEN,  E,  S.:  Quartz  as  a  Geologic  Thermometer. 
Am.  Jour.  Sci.,  4th  ser.,  vol.  27,  p.  147,  1909. 

!LEITH,  C.  K,  and  HARDER,  E.  C.:  The  Iron  Ores  of  the  Iron  Springs 
District,  Southern  Utah.  U.  S.  Geol.  Survey  Bull.,  338,  pp.  66-86,  1908. 


CHAPTER  VII 

DEPOSITS  FORMED  AT  MODERATE  DEPTHS  BY  HOT 
SOLUTIONS 

Occurrence. — In  or  near  igneous  rocks;  most  of  them  are  near  intrusive 
rocks. 

Composition. — Contain  a  great  variety  of  minerals.  Complex  sulpho- 
salts  of  antimony  and  arsenic  are  common;  metals  include  copper,  silver, 
gold,  lead,  zinc,  arsenic,  antimony,  subordinately  nickel,  cobalt,  bismuth, 
tungsten,  etc. 

Shape. — The  deposits  are  in  the  main  tabular  bodies  or  combinations  of 
tabular  bodies.  Sheeted  zones,  fracture  zones,  stockworks,  and  pipes  are 
developed.  In  limestone  many  of  the  ore  bodies  are  chambers.  "Saddle 
reefs"  and  bedding-plane  deposits  are  developed. 

Size. — Some  of  the  ore  bodies  are  large;  many  are  small. 

Texture. — The  veins  that  fill  fissures  are  generally  banded,  and  in  them 
comb  structure  and  drusy  cavities  with  symmetrical  crustified  banding  are 
common.  The  ore  which  replaces  the  wall  rocks  does  not  form  crusts  but 
may  be  banded.  Pseudomorphous  replacement  of  shales,  schists,  or  other 
banded  rocks  will  also  give  banded  ores  which  are  not  symmetrically  crus- 
tified. Replacements  of  homogeneous  rocks  may  be  banded  also  but  not 
crustified.  Valuable  bodies  of  disseminated  ores  belong  to  this  group;  in 
these  the  rock  is  cut  by  many  small  fractures  partly  filled  with  ore  minerals, 
and  the  rock  between  the  fractures  is  impregnated  or  peppered  with  little 
dots  of  ore. 

General  Features. — The  deposits  formed  at  moderate  depths 
by  hot  solutions  are  very  numerous,  and  many  of  them  are  of 
great  value.  Many  copper,  zinc,  lead,  gold,  and  silver  deposits 
in  the  American  Cordillera  belong  to  this  group,  which  includes 
nearly  all  the  more  valuable  copper  deposits  of  Nevada,  Colorado, 
Montana,  Utah,  Arizona,  New  Mexico,  and  other  Western 
States,  as  well  as  many  deposits  in  Mexico  and  British  Columbia. 
Many  of  the  deposits  fill  fissures  and  are  for  the  most  part 
tabular  or  rudely  tabular;  usually  there  has  been  extensive  hydro- 
thermal  metamorphism  of  the  wall  rock  along  the  fissures,  and 
in  a  large  number  of  deposits  the  wall  rock  is  replaced  by  ore. 
The  alteration  of  the  wall  rocks  of  these  deposits  is  discussed 
on  pages  237  to  251. 

At  moderate  and  shallow  depths  the  depositing  solutions  are 
mainly  liquid  and  the  openings  or  systems  of  openings  are  in 

62 


DEPOSITS  FORMED  AT  MODERATE  DEPTHS     63 

general  connected  freely  with  the  surface.  The  solutions  in  the 
fissures  move  to  places  of  less  pressure,  which  in  the  main  are 
upward.  In  some  districts  the  depositing  solutions  were  clearly 
ascending  waters,  and  as  the  ores  are  closely  associated  geo- 
graphically with  igneous  rocks  and  as  the  period  of  deposition 
nearly  everywhere  followed  a  period  of  igneous  intrusion,  the 
conclusion  is  inevitable  that  the  waters  were  hot.  Although 
the  solutions  are  believed  by  many  investigators  to  be  in  the 
main  of  magmatic  origin,  it  is  possible  that  some  of  the  deposits 
may  have  been  formed  by  hot  circulating  ground  water  of 
meteoric  origin;  others  must  have  been  deposited  in  fissures  that 
received  contributions  of  meteoric  waters  from  above  and  mag- 
matic water  from  below.  The  chilling  effect  of  meteoric  water, 
which  at  moderate  depths  would  collect  in  the  upper  parts  of  the 
fissures,  was  doubtless  a  potent  agent  for  rapidly  reducing  the 
temperature  of  the  ascending  metal-bearing  solutions  and  thereby 
causing  precipitation.  The  sources  of  waters  that  deposit 
metalliferous  ores  are  discussed  on  page  274. 

From  studies  of  geologic  structure  many  of  the  deposits  are 
known  to  have  been  formed  at  depths  of  not  much  more  than  a 
mile  below  the  surface  at  the  time  of  deposition.  Some  of  them — 
for  example  the  Granite  vein,  at  Philipsburg,  Mont. — appear  to 
have  been  formed  as  much  as  7,000  feet  below  the  surface. 
Others  have  probably  formed  less  than  a  mile  below  the  surface. 

Occurrence. — The  veins  of  this  class  are  usually  associated 
closely  with  igneous  rocks;  some  of  them  are  inclosed  in  walls 
of  igneous  rock;  others  are  in  sedimentary  beds  near  igneous  rocks 
(Fig.  26).  The  igneous  rocks  to  which  they  are  genetically 
related  range  from  acidic  to  basic  but  are  mostly  either  inter- 
mediate in  composition  (diorites,  diorite  porphyries)  or  grade 
toward  the  acidic  end  of  the  series  (granites,  alaskites,  grano- 
diorites,  monzonites,  quartz  porphyries).  A  considerable  number 
of  these  deposits,  however,  are  genetically  related  to  basic  rocks — 
for  example,  the  copper  deposits  of  the  Coronado  vein  at 
Morenci,  Ariz.,  the  silver  ores  at  Cobalt,  Ontario,  and  other 
deposits  of  gold  and  silver.  As  a  rule  the  deposits  are  less  than 
a  mile  away  from  igneous  rocks,  but  a  few  are  several  miles  from 
known  outcrops. 

The  deposits  of  the  deep  zone  are  in  the  main  genetically  re- 
lated to  the  granitoid  rocks;  the  contact-metamorphic  deposits 
are  related  to  the  granitoid  rocks  or  the  deep-seated  porphyries; 


64        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  lode  deposits  formed  at  moderate  depths  are  genetically 
related  to  the  granitoid  rocks,  to  deep-seated  porphyries,  or  to 
porphyries  formed  very  near  the  surface,  but  few  or  none  of 
them  have  been  formed  in  genetic  connection  with  lava  flows. 


1000  2000  3000  4000  Feet 


FIG.  26. — Sketch  of  area  near  Philipsburg,  Montana,  showing  veins  of  the  same 
system  in  both  igneous  and  sedimentary  rocks. 

Pegmatite  veins,  contact-metamorphic  deposits,  and  the  more 
typical  deposits  of  the  deep  vein  zone  (bearing  heavy  silicates, 
pyrrhotite,  and  magnetite)  are  generally  formed  in  or  near  igneous 
rocks.  Ore  bodies  formed  at  moderate  depths  by  hot  solutions, 


DEPOSITS  FORMED  AT  MODERATE  DEPTHS     65 

on  the  other  hand,  may  be  deposited  at  considerable  distances 
from  the  magma.  The  openings  near  the  surface  are  larger  than 
those  at  greater  depth,  and  the  solutions  circulate  in  them  more 
freely.  Nearly  all  such  solutions  are  highly  siliceous,  and  there- 
fore, in  accordance  with  well-known  chemical  laws,  they  do  not 
so  readily  attack  and  replace  quartzite  or  other  highly  siliceous 
noncalcareous  rocks  they  may  traverse.  Consequently  in  such 
rocks  they  may  carry  the  metals  far  from  their  sources.  Where 
the  siliceous  solutions  encounter  limestones,  however,  there  is 
usually  rapid  chemical  interchange  between  the  solutions  and 
the  wall  rock,  and  deposition  will  generally  take  place  relatively 
near  the  parent  magma. 

Composition. — The  metals  contained  in  this  group  of  deposits 
include  gold,  copper,  silver,  lead,  zinc,  antimony,  and  arsenic. 
Small  amounts  of  nickel,  cobalt,  bismuth,  manganese,  and  other 
metals  are  recovered  from  some  of  them. 

Minerals  found  in  deposits  formed  at  moderate  depths  by  hot 
solutions  are  listed  below.  In  this  list  some  secondary  sul- 
phides are  included.  Besides  these  there  are  many  minerals 
formed  by  decomposition  in  the  oxidizing  zone. 

ankerite  chalcopyrite  niccolite  rhodochrosite 

apatite  chert  opal  rhodonite 

argentite  chlorite  orpiment  sericite 

arsenopyrrte  cinnabar  orthoclase  siderite 

barite  cobaltite  pentlandite  stephanite 

bauxite  covellite  petzite  stibnite 

bismuth  dolomite  platinum  sylvanite 

bismuthinite  enargite  polybasite  tellurides 

bornite  fluorite  proustite  tennantite 

calaverite  galena  pyrargyrite  tetrahedrite 

calcite  gold  pyrite  tungstates 

celestite  molybdenite  quartz  zinc  blende 

chalcocite  muscovite  realgar 

A  large  group  of  minerals  are  formed  in  igneous  rocks,  in  pegma- 
tites, in  contact-metamorphic  deposits,  during  regional  meta- 
morphism,  and  in  deep  veins,  but  not  normally  in  veins  formed  at 
moderate  depths.  Among  them  are  amphiboles,  diopside,  garnet, 
graphite,  hornblende,  ilmenite,  pyrrhotite(?),  rutile,  scapolite, 
specularite,  spinel,  topaz,  and  tourmaline.  Magnetite,  which 
is  formed  in  these  deposits  under  certain  conditions  of  surface 
alteration,  is  rarely  if  ever  a  primary  mineral  in  the  deposits. 


66        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Shape. — Most  of  the  deposits  of  this  group  are  formed  in  and 
along  easily  recognized  fissures;  they  are  short  in  one  and  long  in 
two  dimensions.  They  may  be  vertical  or  flat  or  may  dip  at  any 
angle.  Several  small,  closely  spaced  fissures  form  a  sheeted  zone. 
Where  metallization  is  localized  by  the  intersection  of  two 
fissures,  or  a  dike  and  a  fissure,  or  a  sedimentary  bed  and  a  fissure, 
the  ore  body  is  long  in  one  and  short  in  two  dimensions.  Such 
deposits  are  termed  pipes  or  chimneys.  Stockworks,  ore  folds, 
and  bedding-plane  deposits1  are  also  included  in  this  group.1 

Size. — The  deposits  of  this  class  range  in  size  from  ore  bodies 
that  have  become  exhausted  in  mining  a  few  carloads  to  ore 
bodies  containing  millions  of  tons.  Some  of  the  fissures  are  filled 
only  here  and  there,  others  are  filled  with  material  that  is  of  too 
low  grade  to  work,  and  still  others  contain  workable  bodies  in  a 
larger  mass  of  low-grade  material.  Many  deposits  of  this  group 
are  workable  only  where  they  have  been  enriched  by  superficial 
alteration,  although  a  large  number  are  rich  in  the  primary 
concentration. 

Texture. — In  deposits  formed  at  moderate  depths  in  open 
fissures,  the  minerals,  are.  very  commonly  arranged  in  rude 
sheets  (Figs.  1-3),  quartz  and  sulphides  alternating,  and  the 
sheets  are  generally  parallel  to  the  walls  of  the  fissures  or  to 
surfaces  of  fragments  of  rock  in  the  veins.  This  structure,  on 
the  whole,  is  much  more  pronounced  than  in  the  veins  formed 
at  great  depths.  Where  a  vein  is  not  completely  filled  the  open 
space  is  lined  with  crystals  pointing  toward  the  center,  and  this 
position  is  in  general  characteristic  of  long  crystals  that  form 
crusts.  Banding  has  been  noted  in  replacement  deposits  also, 
and  locally  it  occurs  even  in  rocks  that  were  not  banded  before 
replacement,  but  it  is  relatively  rare  and  is  not  crustified.  Where 
symmetrical  crustification  or  comb  structure  appears  in  ore 
that  has  replaced  the  country  rock,  it  has  probably  been  devel- 
oped on  a  local  fracture  or  in  a  solution  cavity.  A  large  number 
of  replacement  deposits  and  replacement  veins  belong  to  this 
group.  They  are  discussed  on  pages  237  to  248.  In  many  of 
the  fissure  fillings  the  banding  is  symmetrical — that  is,  the  same 
series  is  shown  from  either  wall  to  the  center  (see  Fig.  27). 
More  commonly,  however,  such  symmetry  is  only  faintly  indi- 
cated or  is  entirely  lacking.  Veins  formed  at  all  depths  may 

1  Definitions  given  on  pages  184-190. 


DEPOSITS  FORMED  AT  MODERATE  DEPTHS     67 

be  brecciated  and  cemented  by  crustified  material  belonging  to 
a  later  period  of  deposition  (Fig.  28). 

The  great  disseminated  copper  ore  bodies  of  Bingham,  Utah; 
Ely,  Nev. ;  Morenci,  Ariz. ;  Santa  Rita,  N.  Mex. ;  and  other  similar 
"copper  porphyries"  belong  to  this  class.  In  these  deposits 
many  closely  spaced,  unsystematically  arranged  cracks  are 
filled  with  iron  and  copper  sulphides,  and  the  rock  between  is 


FIG.  27. — Section  of  banded  vein,  Finos  Altos  district,  New  Mexico.     (After 
Paige,  U.  S.  Geol.  Survey.) 

dotted  with  small  masses  of  the  sulphides.  The  metals  are  some- 
what regularly  distributed,  so  that  it  is  possible  to  work  the  entire 
low-grade  mass  at  a  profit  if  operations  are  conducted  on  a  suffi- 
ciently large  scale. 

Age. — Many  of  the  deposits  of  this  group  were  formed  in 
comparatively  late  geologic  time.  Many  of  the  epigenetic  de- 
posits associated  with  intrusive  rocks  that  were  formed  in 
Tertiary  time  are  included  in  this  class.  The  deposits  of  this 


THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


group  are  not  confined  to  rocks  invaded  by  Tertiary  intrusives, 
however,  but  include  also  large  ore  bodies  associated  with 
igneous  rocks  which  were  intruded  before  the  beginning  of 
the  Tertiary  period.  The  group  includes  also  some  deposits 
formed  in  early  Paleozoic  and  pre- 
Cambrian  time.  The  classification  is 
based  on  depth  of  formation  rather 
than  age:  the  deposits  may  have 
formed  at  any  time,  yet  when  the 
classes  of  deposits  are  compared,  the 
relationship  between  age  and  certain 
fairly  constant  characteristics  is  ap- 
parent :  the  younger  rocks  were  gener- 
ally under  light  loads  where  the  asso- 
ciated deposits  now  exposed  were 
formed,  but  older  rocks  may  have 
been  more  deeply  buried  at  the  time 
of  their  metallization. 

References 
DEPOSITS  FORMED  AT  MODERATE    DEPTHS 

EMMONS,  W.  H. :  A  Genetic  Classification 
of  Minerals.     Econ.  Geol,  vol.  3,  pp.  611- 
FIG.  28. — Sketch  of  specimen    627,  1908. 

showing  cementation  of  frag-  EMMONS  W.  H.,  and  CALKINS,  F.  C.: 
ments  of  galena  (a)  belonging  to  _  .  .  >»  iw  •  ,  ,  ™  -i- 

the  first  period  of  deposition  by  Geology  and  Ore  Deposits  of  the  Philips- 
a  crustified  material  belonging  burg  Quadrangle,  Montana.  U.  S.  Geol. 
to  the  second  period  and  de-  Survey  Prof.  Paper  78,  p.  187,  1913. 


posited  in  the  following  order: 
1,    Finely   banded     (crustified) 


nates  and  fine  quartz  (d)  con- 
taining  some  pyrite  (e)  and  a 
little  galena  (o).  (After  Spurr, 


LINDGREN,  WALDEMAR:  The  Relation  of 

brown"carbonltes"  (fe)72,"a  thin    Ore    Deposition     to     Physical     Conditions, 
crust  of   quartz   with  sulphides    Econ.    Geol.,    vol.    2,    pp.     105-127,     1907. 
(c)-,   3,    mingled    brown  ^carbo-    Metallogenetic  Epochs.  Econ.  Geol.,  vol.  4, 
>.  409-420,  1909. 

RANSOME,  F.  L. :  Geology  and  Ore   De- 

Garrey,  and  Batt,    U.   S.  Geol.    posits  of  the    Breckenridge  District,   Colo- 
Svney.)  rado      u  g  Q^  Survey  Prof.  Paper  75,  p. 

174,  1911. 

SPURR,  J.  E.:  Geology  of  the  Aspen  Mining  District,  Colorado.    U.  S. 
Geol.  Survey  Mon.  31,  1898. 

SPURR,  J.  E.,  GARRET,  G.  H.,  and  BALL,  S.  H.:  Geology  of  Georgetown 
District,  Colorado.    U.  S.  Geol.  Survey  Prof.  Paper  63,  1908. 

WEED,  W.  H.:  Geology  and  Ore  Deposits  of  the  Butte  District,  Mon- 
tana.    U.  S.  Geol.  Survey  Prof.  Paper  78,  1913. 


CHAPTER  VIII 

DEPOSITS   FORMED    AT    SHALLOW   DEPTHS   BY   HOT 
SOLUTIONS 

Occurrence. — In  igneous  and  sedimentary  rocks.  Many  are  in  or  near 
intrusive  rocks  consolidated  at  shallow  depths.  Common  in  regions  of  late 
igneous  activity,  especially  in  or  near  Miocene  and  later  intrusives;  in  rocks 
that  have  not  been  deeply  eroded  since  the  ores  were  deposited. 

Composition. — Minerals  include  simple  sulphides,  such  as  pyrite,  sphaler- 
ite, galena,  chalcopyrite,  stibnite,  and  cinnabar,  with  the  tellurides,  sele- 
nides,  and  complex  antimony  and  arsenic  sulphosalts.  Fluorite,  chalcedony, 
adularia,  barite,  and  alunite  are  commonly  present.  The  metals  include 
gold,  silver,  quicksilver,  antimony,  arsenic,  tungsten,  lead,  and  zinc.  Copper 
is  a  valuable  constituent  of  zeolitic  deposits  but  is  rarely  present  in  large 
quantities  in  sulphide  ores  formed  at  shallow  depths. 

Shape. — Many  are  simple  tabular  fissure  veins.  Ledges  and  irregular 
replacement  deposits  are  developed  in  shattered  rocks.  Irregular  veins  with 
ore  chambers  are  characteristic,  but  bedding-plane  deposits  and  saddles  are 
less  common  than  among  the  deposits  formed  at  moderate  depths. 

Size. — Some  deposits  are  small ;  others  are  large.  Bonanzas  are  frequently 
found  in  the  low-grade  ore. 

Texture. — Comb  structure  and  crustified  banding  are  common;  vugs  are 
nearly  always  present ;  many  deposits  are  formed  by  replacement.  Although 
hydrothermal  alteration  of  country  rock  is  usually  extensive,  the  ore  is  gen- 
erally deposited  in  or  relatively  near  the  fractures.  In  many  districts  lamel- 
lar quartz  is  developed  in  cleavage  cracks  of  calcite,  and  when  the  calcite  is 
dissolved  it  releases  a  quartzose  ore  with  peculiar  interpenetrating  blades. 

General  Features. — Deposits  formed  at  shallow  depths  by 
hot  solutions  constitute  one  of  the  most  valuable  classes  of  ore 
deposits.  They  have  produced  a  large  part  of  the  world's  silver 
and  considerable  amounts  of  gold.  The  zeolitic  copper  lodes 
that  are  here  included  are  among  the  most  valuable  sources  of 
copper.  Nearly  all  the  quicksilver  produced  is  obtained  from 
deposits  of  this  group,  and  also  a  little  lead  and  zinc.  These 
deposits  naturally  are  not  set  off  by  sharp  dividing  lines  from 
deposits  formed  at  moderate  depths,  and  there  are  numerous 
transitional  types. 

Occurrence. — The  deposits  are  found  in  all  kinds  of  igneous 
rocks — acidic  and  basic,  granular  and  glassy — also  in  sedimen- 


70        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tary  rocks.  Generally  they  are  in  or  near  masses  of  intruding 
rocks.  In  the  American  Cordillera  they  are  associated  at  a 
great  many  places  with  intrusive  porphyries  and  andesites. 
Most  of  those  found  in  flows  or  in  sedimentary  beds  are  not  far 
from  intrusive  masses.  Structurally  nearly  all  the  deposits  of 
sulphide  ores  are  fissure  fillings  or  formed  by  replacement  along 
fissures.  Bedding-plane  sulphide  deposits,  though  not  unknown, 
are  rare  in  this  class. 

The  extensive  zeolitic  copper  deposits  of  the  Lake  Superior 
region,  which  were  formed  in  intergranular  spaces  in  conglom- 
erates, in  vesicular  cavities  and  small  cracks  in  amygdaloids, 
and  by  replacement  of  both  of  these  rocks,  doubtless  have  a 
comparatively  complex  genesis,  some  features  of  which  are  as 
yet  obscure;  but  if  the  successive  metallized  beds  are  regarded 
as  having  been  filled  or  impregnated  one  after  another,  between 
the  periods  of  flows  and  before  tilting,  they  must  be  placed  in  this 
group,  although  structurally  they  are  unlike  the  common  types. 

Composition. — The  deposits  formed  at  shallow  depths  by  hot 
solutions  have  supplied  considerable  gold  and  enormous  quanti- 
ties of  silver.  In  many  places,  as  on  the  Comstock  lode  and  at 
Tonopah  and  Tuscarora,  Nev.,  both  gold  and  silver  are  found 
in  large  amounts  in  the  same  deposit.  There  are  some  deposits, 
also  (Cripple  Creek,  Colo.;  Goldfield,  Nev.),  in  which  gold  is 
found  almost  to  the  exclusion  of  silver,  and  a  few  in  which 
silver  only  is  important.  In  the  sulphide  ores  copper  is  generally 
subordinate,  although  not  a  little  is  recovered  as  a  by-product  of 
gold  and  silver  mining.  The  great  bodies  of  copper  sulphide 
ore,  such  as  are  typical  of  the  group  of  deposits  formed  at  moder- 
ate depths,  are  not  represented  in  this  class.  Much  copper, 
however,  is  obtained  from  zeolitic  lodes. 

Practically  all  the  quicksilver  deposits,  the  world  over,  belong 
to  this  group.  Lead  and  zinc,  though  generally  subordinate,  are 
present  in  many  of  these  deposits  and  are  commonly  recovered  as 
by-products  of  concentrating  or  smelting  the  gold  and  silver  ores. 

Among  the  common  constituents  of  these  deposits  adularia 
(vein-forming  orthoclase)  is  abundant  in  many  districts  (Tonopah, 
Nev.;  Cripple  Creek  and  Creede,  Colo.);  alunite  is  its  close 
associate  at  Goldfield,  Nev.,  and  in  some  other  districts;  and 
other  sulphates  are  commonly  developed.  Carbonates  are 
plentiful,  especially  calcite.  Zeolites  are  abundant  in  the  native 
copper  ores  of  Michigan.  The  list  below  includes  minerals  of  the 


DEPOSITS  FORMED  AT  SHALLOW  DEPTHS       71 

deposits  formed  at  shallow  depths,  as  well  as  some  secondary 
sulphides.  Besides  these,  many  minerals  are  formed  during 
decomposition  in  the  zone  of  oxidation. 


adularia  (valencianite) 

chalcopyrite 

marcasite 

sericite 

alunite 

chert 

muscovite 

silver 

analcite 

chlorite 

molybdenite 

stephanite 

ankerite 

cinnabar 

opal 

stibnite 

arsenopyrite 

cobaltite 

platinum 

stilbite 

barite 

copper 

polybasite 

stromeyerite 

bauxite 

dolomite 

proustite 

sylvanite 

bismuthinite 

nuorite 

pyrargyrite 

thuringite 

bornite 

galena 

pyrite 

tungstates 

calaverite 

gold 

quartz 

tellurides 

calcite 

kalgoorlite 

rhodochrosite 

tennantite 

celestite 

kaolin 

rhodonite     • 

tetrahedrite 

chalcedony 

magnesite 

realgar 

zeolites 

zinc  blende 

Some  minerals  that  are  stable  up  to  moderate  depths  are  not 
formed  near  the  surface.  The  heavy  silicates  characteristic  of 
deposits  formed  at  great  depths  are  lacking  in  this  group. 
Pyroxene,  amphiboles,  tourmaline,  topaz,  garnet,  biotite,  mag- 
netite, specularite,  and  pyrrhotite  are  practically  nowhere  present 
in  lode  ores  formed  at  shallow  depths,  although  some  of  these 
minerals  are  said  to  form  as  products  of  volcanic  sublimation. 
Rhodochrosite  and  manganiferous  calcite  are  .very  common,  as 
are  also  complex  silver-antimony  sulphides  and  silver-arsenic  sul- 
phides. Mineral  associations  are  discussed  on  pages  230  to  267. 

Shape. — As  the  deposits  of  this  group  are  generally  related  to 
fissures  they  do  not  differ  greatly  in  shape  from  the  deposits 
formed  at  moderate  depths;  tabular  bodies  or  combinations  of 
tabular  bodies  predominate.  Bedding-plane  deposits  of  sul- 
phide ore,  which  are  so  commonly  formed  where  limestones  are 
attacked  at  moderate  depths,  are  less  abundant  in  this  group, 
although  they  may  be  formed  at  shallow  depths  where  the  soluble 
beds  are  capped  by  shales  or  other  impermeable  rocks.  Anti- 
clinal deposits  and  "ore  folds"  are  rare.  Pronounced  hydro- 
thermal  alteration  of  the  wall  rock  is  common  and  is  as  a  rule 
attended  by  replacement  of  the  wall  rock  with  ore. 

Size. — The  deposits  range  in  size  from  small  veins  and  masses, 
such  as  some  of  the  ore  ledges  of  Goldfield,  Nev.,  to  enormous 
bodies  of  quartz  and  ore,  such  as  make  up  the  Comstock  lode. 
Precipitating  processes  are  especially  active  near  the  surface, 


72        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


and  the  metals  may  be  thrown  down  in  great  masses  that  are 
relatively  rich.  Indeed,  this  class  of  deposits  more  than  any 
other  may  be  called  the  " bonanza  group." 

Texture. — The  texture  of  these  deposits  is  in  the  main  like 
that  of  lodes  formed  at  moderate  depths.     Vugs  (Fig.  29)  are 
perhaps    more    common.     In    many    deposits    chalcedony    and 
quartz  form  bands  of  striking  beauty.     A  structure  common  in 
calcite  ores  in  several  deposits  in  the 
Western  States  shows  blades  of  calcite 
joining  at  angles  similar  to  the  cleavage 
angles  of  calcite.     Such  ores  are  found 
at  De  Lamar,  Idaho  (Fig.  30) ;  Marys- 
ville,  Mont. ;  Bullfrog  and  Manhattan, 
Nev.;  and  in  other    districts.     The 
calcite,  after  it  was  formed,  was  filled 
by  small  veinlets  of  quartz  occupying 
the  cleavage  planes  and  locally  cut- 
ting across  the  crystals  from  one  plane 
to  another.     After  the  lime  carbonate 
is  dissolved,  the  thin  blades  of  silica 
remain  as  pseudomorphs  of  the  calcite 
cleavage.     Where  the  calcite  is  man- 
ganiferous     many    of   the   blades  of 
FIG.  29. — Local  structure  in    silica  are  coated  by  sooty  manganese 
'   K'   &oloN'  WaTls'coSTof    oxide>  locally  carrying  gold. 

granite       containing 

References 


DEPOSITS  FORMED  AT  SHALLOW  DEPTHS 
BY  HOT  SOLUTIONS 

BECKER,  G.  F. :  Geology  of  the  Comstock 


reddish 

pyrite  in  cracks  perpendicular 
to  the  vein.  1,  Fragment  of 
altered  rock  wedged  in  fissure; 
2,  minutely  fissured  granite  with 
pyrite;  3,  calaverite;  4,  earlier 
generation  of  quartz;  5,  radial 
pyrite;  6,  later  generation  of 
quartz.  (After  Lindgren  and  Lode  and  the  Y\  ashoe  District.  U.  S.  Geol. 
Ransome,  U.  S.  Geol  Survey.)  Survey  Man.  3,  pp.  1-422,  1882.  Geology 
of  the  Quicksilver  Deposits  of  the  Pacific 
Slope.  U.  S.  Geol.  Survey  Mon.  13,  pp.  1-486,  1888. 

EMMONS,  W.  H. :  A  Genetic  Classification  of  Minerals.  Econ.  Geol.,  vol.  3, 
pp.  611-627,  1908. 

IRVING,  R.  D.:  The  Copper-Bearing  Rocks  of  Lake  Superior.  U.  S.  Geol. 
Survey  Mon.  5,  1885. 

LANE,  A.  C. :  The  Keweenaw  Series  of  Michigan.  Mich.  Geol.  and  Biol. 
Survey,  ser.  4,  vols.  1  and  2,  1911. 

LINDGREN,  WALDEMAR:  Orthoclase  as  a  Gangue  Mineral  in  a  Fissure  Vein. 
Am.  Jour.  Sd.,  4th  ser.,  vol.  5,  p.  418,  1899.  The  Occurrence  of  Stibnite  at 
Steamboat  Springs,  Nevada.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  36,  pp.  27- 


DEPOSITS  FORMED  AT  SHALLOW  DEPTHS      73 


31,  1905.  The  Relations  of  Ore  Deposition  to  Physical  Conditions.  Earn. 
Geol,  vol.  2,  pp.  105-127,  1907.  Metallogenetic  Epochs.  Econ.  Geol.,  vol.  4, 
pp.  409-420,  1909.  The  Gold  and  Silver  Veins  of  Silver  City,  De  Lamar, 
and  Other  Mining  Districts  in  Idaho.  U.  S.  Geol.  Survey  Twentieth  Ann. 
Rept.,  part  3,  pp.  65-256,  1899. 


FIG.  30. — Ore  from  De  Lamar  mine,  De  Lamar,  Idaho.  The  quartz  blades 
filled  cracks  of  calcite  or  barite,  which  subsequently  was  dissolved.  (After 
Lindgren,  U,  S.  Geol.  Survey.) 

LINDGREN,  WALDEMAR,  and  RANSOME,  F.  L. :  The  Geology  and  Gold  De- 
posits of  the  Cripple  Creek  District,  Colorado.  U.  S.  Geol.  Survey  Prof. 
Paper  54,  pp.  1-516,  1906. 

McCASKEY,  H.  D. :  Quicksilver.  U.  S.  Geol.  Survey  Mineral  Resources, 
1910,  part  1,  pp.  693-710,  1911. 

RANSOME,  F.  L.:  The  Geology  and  Ore  Deposits  of  Goldfield,  Nevada. 
U.  S.  Geol.  Survey  Prof.  Paper  66,  pp.  1-258,  1909. 

SPURR,  J.  E. :  Geology  of  the  Tonopah  Mining  District,  Nevada.  U.  S. 
Geol.  Survey  Prof,  Paper  42,  pp.  1-295,  1905. 


CHAPTER  IX 

DEPOSITS    FORMED    AT    MODERATE    AND    SHALLOW 
DEPTHS  BY  COLD  METEORIC  SOLUTIONS 

Occurrence. — Mainly  in  sedimentary  rocks — limestones,  shales,  and  sand- 
stones. Organic  matter  or  other  reducing  agents  are  commonly  present  in 
noteworthy  amounts.  Many  of  the  ores  occur  in  solution  cavities  and  in 
zones  of  brecciation  and  subordinately  in  fissures  and  faults.  Some  are 
formed  by  replacement. 

Composition. — The  principal  minerals  are  sulphides  of  zinc,  lead,  iron, 
and  copper,  with  their  alteration  products  and  oxides  of  iron  and  manganese. 
Gangue  minerals  include  calcite,  chert,  dolomite,  jasper,  barite.  Mineral 
simplicity  is  characteristic.  Heavy  silicates  are  absent,  and  complex 
antimony,  arsenic,  selenium,  and  tellurium  minerals  are  rarely  if  ever 
present.  The  metals  include  lead,  zinc,  copper,  uranium,  vanadium,  iron, 
and  manganese. 

Shape. — Large  tabular  upright  bodies  like  some  of  the  fissure  veins  genet- 
ically related  to  igneous  rocks  are  very  rare,  but  "sheet  ground"  is  developed 
in  bedding  planes,  forming  extensive  tabular  ore  bodies.  Crevices,  gash 
veins,  runs,  or  flats  and  pitches  are  characteristic  in  some  districts.  Many  of 
the  deposits  are  very  irregular,  especially  those  that  fill  solution  cavities. 

Size. — Some  of  the  bedding-plane  deposits,  the  disseminated  lead  deposits 
of  southeastern  Missouri,  and  deposits  in  "sheet  ground"  in  the  Joplin 
district,  are  very  large.  The  larger  bodies  are  parallel  to  bedding  planes  and 
in  rocks  not  tilted  are  flat-lying.  Many  of  the  deposits  are  small. 

Texture. — Solution  cavities,  crustified  banding,  and  symmetrically  lined 
vugs  are  common,  but  the  regularly  banded  quartz  veins  so  conspicuous 
among  deposits  genetically  related  to  igneous  rocks  are  rare.  Brecciated 
structure  is  common.  In  some  districts  "disseminated  ore"  is  developed, 
the  metallic  sulphides  being  sparingly  but  somewhat  regularly  distributed 
through  great  bodies  of  rock. 

General  Features. — The  deposits  formed  at  moderate  and 
shallow  depths  by  cold  meteoric  solutions  include  many  valuable 
ore  bodies,  among  them  the  lead  and  zinc  deposits  of  the  Missis- 
sippi Valley.  Most  of  them  are  far  from  outcrops  of  igneous  rock, 
and  some  are  more  than  a  hundred  miles  away.  It  appears  im- 
probable that  buried  igneous  rocks,  contemporaneous  with  or 
later  than  the  formations  that  contain  the  ore  are  concealed 
below  the  surface,  near  enough  to  the  zone  of  fracture  for  magmas 
to  have  contributed  water  or  mineral  salts  to  the  areas  con- 
taining these  deposits,  for  the  mineralized  areas  are  large,  and 
74 


DEPOSITS  FORMED  BY  COLD  SOLUTIONS        75 

extensive  igneous  activity  would  probably  be  attended  by  the 
eruption  of  lavas  at  one  place  or  another.  It  is  also  unlikely 
that  intrusives  many  miles  away  have  contributed  the  metals. 
In  the  West,  where  the  larger  ore  deposits  are  almost  invariably 
associated  with  igneous  rocks,  they  are  generally  in  or  grouped 
closely  around  intrusives.  Only  exceptionally  are  they  as  far  as 
one  mile  from  outcrops  of  igneous  rocks.  Finally,  in  some  of  the 
many  districts  containing  the  deposits  that  are  assumed  to  have 
been  formed  at  moderate  depths  by  cold  solutions  there  is  no 
evidence  of  tilting,  extensive  faulting,  or  other  deformation  such 
as  ordinarily  attends  igneous  activity. 

As  the  solutions  that  formed  these  deposits  were  meteoric 
waters,  not  heated  by  igneous  rocks,  they  were  cold  or  at  least 
not  warmer  than  meteoric  ground  water  would  become  by  cir- 
culating through  rocks  that  probably  have  the  normal  heat 
gradient  for  the  earth,  which  is  about  1°C.  for  30  meters.  As 
the  waters  were  cold,  the  country  rock  has  suffered  no  hydro- 
thermal  metamorphism,  and  those  changes  which  it  has  under- 
gone are  characteristic  of  weathering  or  they  are  such  as  may 
be  brought  about  by  hydrometamorphism  or  changes  that  take 
place  through  the  agency  of  ground  water  below  the  zone  of 
oxidation. 

The  solutions  that  have  not  been  affected  by  volcanism  have 
not  in  the  course  of  their  circulation  undergone  any  very  abrupt 
change  of  temperature:  the  range  of  temperature  through 
which  they  have  passed  is  controlled  by  the  depth  to  which  they 
have  descended.  The  investigations  of  Kemp,  Finch,  Lane, 
and  Rickard  have  shown  that  ordinarily  ground  water  does  not 
actively  circulate  even  at  moderate  depths,  except  in  rocks  that 
are  especially  permeable.  At  a  depth  of  half  a  mile  the  normal 
heat  increment  would  be  less  than  30°C.  and  the  temperature 
not  over  60°,  and  at  1  mile  it  would  not  be  100°C.  On  account 
of  this  narrow  range  of  temperature,  cooling  can  not  have  been 
so  potent  a  cause  of  precipitation  as  in  the  classes  of  ore  deposits 
formed  by  hot  solutions,  and  probably  it  is  almost  negligible. 
The  changes  in  pressure  also  are  much  less  than  those  affecting 
magmatic  waters. 

The  chief  cause  of  precipitation  is  not  decrease  in  temperature 
— indeed,  it  has  been  shown  that  some  of  the  deposits  of  this 
class  were  formed  by  descending  solutions,  whose  temperature 
would  increase  as  they  progressed  downward.  Precipitation  is 


76        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

brought  about  by  changes  in  chemical  environment,  rather  than 
by  great  changes  in  temperature.  The  chemical  changes  result 
in  forming  new  compounds  of  the  metals — compounds  that  are 
less  soluble  than  those  which  the  solutions  carried — by  changes  in 
the  chemical  character  of  the  solutions,  which  react  readily  with 
the  wall  rock.  Bituminous  limestones  and  carbonaceous  shales 
and  sandstones  are  especially  favorable  for  the  development  of 
deposits  of  this  class.  Recently  Siebenthal1  has  discussed  the 
chemistry  of  the  formation  of  such  deposits.  In  the  Joplin 
region  the  metals  may  have  been  carried  in  solution  as  sulphates, 
chlorides,  sulphides,  zincates,  carbonates,  or  bicarbonates. 
Precipitation  may  have  been  brought  about  by  hydrogen  sul- 
phide, on  the  escape  of  carbon  dioxide  near  the  surface. 

Occurrence. — The  deposits  of  this  group  are  found  in  both 
sedimentary  and  igneous  rocks,  but  the  best-known  examples 
are  in  sedimentary  rocks.  The  most  valuable  deposits  of  this 
class  in  the  United  States  are  the  lead  and  zinc  ores  in  limestone 
and  associated  chert  in  the  Mississippi  Valley.  Some  ores  are 
found  also  in  shale,  but  such  ore  bodies  are  small  compared 
to  those  in  limestone.  Where  shales  and  limestones  are  asso- 
ciated the  ore  is  commonly  deposited  in  the  limestone  near  the 
shale.  The  shale,  being  less  permeable,  tends  to  impede  circula- 
tion. If  the  solutions  are  rising  the  ores  are  likely  to  be  deposited 
below  a  shale  bed;  if  the  solutions  are  descending  they  are 
likely  to  be  deposited  above  the  shale.  In  the  lead  and  zinc 
region  of  southwestern  Wisconsin  most  of  the  deposits  are  above 
the  shale  bed  or  "clay  seam"  in  the  Galena  limestone,  or  just 
below  the  clay  seam,  where  the  rocks  are  fractured  somewhat  and 
solutions  may  pass  through  the  fractures. 

The  deposits  at  Joplin,  Mo.,  are  on  the  southwest  slope  of 
the  Ozark  uplift,  where  a  thick  series  of  limestone  and  sandstone 
is  capped  by  shale  which  dips  southwest  at  low  angles.  In  the 
Joplin  region  according  to  Siebenthal,  the  shale  has  been  eroded 
from  the  top  of  the  limestone.  The  solutions  travel  down  the 
dip  of  the  beds  but  are  impounded  by  the  shale  and,  being  unable 
to  move  downward  farther  along  the  dip,  rise  through  the  lime- 
stone near  the  shale-limestone  contact,  where  they  deposit  ores 
in  the  limestone  (see  Fig.  31). 

1  SIEBENTHAL,  C.  E. :  Origin  of  the  Zinc  and  Lead  Deposits  of  the  Joplin 
Region,  Missouri,  Kansas,  and  Oklahoma.  U.  S.  Geol.  Survey  Bull.  606, 
p.  42,  1915. 


DEPOSITS  FORMED  BY  COLD  SOLUTIONS        77 

In  the  Southwest  many  copper  deposits  occur  in  sandstone 
and  shale  far  removed  from  igneous  rocks.1  The  principal  ore 
minerals  are  chalcocite  and  bornite,  which  occur  in  a  gangue 
of  barite,  calcite,  and  subordinate  quartz.  The  copper  sulphide 
in  some  deposits  is  precipitated  on  coal  and  other  carbonaceous 
material  (Fig.  32).  The  associated  beds  in  many  places  contain 
sulphates  and  chlorides,  and  it  is  supposed  that  the  copper  was 
dissolved  out  of  the  rocks  and  concentrated  in  beds  and  along 
fissures,  where  precipitation  was  favored  by  the  presence  of 
organic  matter. 

The  precipitation  of  the  metals  by  organic  matter  is  not  the 
only  way  in  which  precipitation  may  be  accomplished.  Small 
amounts  of  lead  and  zinc  sulphide  may  be  dissolved  by  carbonic 
acid,  the  process  yielding  lead  and  zinc  carbonates  and  hydrogen 


FIG.  31. — Diagram  showing  stage  in  artesian  circulation,  Joplin  region,  Mo. 
A-E,  Stages  in  erosion  of  Pennsylvania  shale,  a,  Pennsylvania  shale;  b,  Missis- 
sippian  limestone;  c,  Chattanooga  (Devonian)  shale;  d,  Ordovician  and  Cambrian 
rocks;  e,  pre-Cambrian  rocks.  (After  Siebenthal,  U.  S.  Geol.  Survey.) 

sulphide.2  In  depth  in  the  presence  of  carbon  dioxide  the 
metallic  sulphides  are  not  precipitated,  but  when  the  solutions 
rise  again  near  to  the  surface  carbon  dioxide  escapes,  and  the 
metals  are  deposited  as  sulphides. 

1  EMMONS,   S.   F. :  Copper  in  the  Red   Beds  of  the   Colorado   Plateau 
Region.     U.  S.  Geol.  Survey  Bull.  260,  pp.  221-232,  1905. 

EMMONS,  W.  H.:  The  Cashin  Mine,  Montrose  County,  Colorado.  U.  S. 
Geol.  Survey  Bull.  285,  pp.  125-128,  1906. 

LINDGREN,  WALDEMAR:  Notes  on  Copper  Deposits  in  Chaff ee,  Fremont, 
and  Jefferson  Counties,  Colorado.  -IT.  S.  Geol.  Survey  Bull.  340,  pp.  157- 
174,  1908. 

LINDGREN,  WALDEMAR,  GRATON,  L.  C.,  and  GORDON,  C.  H.:  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Prof.  Paper  68,  pp.  48,  76-79, 
143-149,  163,  202-203,  1910. 

TARR,  W.  A.:  Copper  in  the  "Red  Beds"  of  Oklahoma.  Econ.  Geol,  vol. 
5,  pp.  221-226,  1910. 

FATH,  A.  E.:  Copper  Deposits  in  the  "Red  Beds"  of  Southeastern  Okla- 
homa. Econ.  Geol,  vol.  10,  p.  140,  1915. 

2  SIEBENTHAL,  C.  E. :  Op.  tit,  p.  52. 


78        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Uranium  and  vanadium  are  found  in  the  Southwest  in  deposits 
in  sandstone  far  removed  from  igneous  rocks.  The  metals 
are  believed  to  have  been  present  in  small  amounts  in  the 


FIQ.  32. — Section  of  chalcocite  (light)  replacing  coal  (dark)  Nacimiento 
district,  New  Mexico.  Upper  part  of  specimen  is  altered  by  oxidation.  (Re- 
drawn from  plate  by  Lindgren,  Graton,  and  Gordon,  U.  S.  Geol.  Survey.) 


Scale  of  Feet 

FIQ.  33. — Ore   in    the    Ruby    Trust    mine,    Granby,    Joplin    region,    Missouri. 
(After  Buckley  and  Buehler.) 

sandstone,  from  which  they  were  dissolved  by  ground  water. 
They  were  precipitated  at  favorable  places,  especially  in  the 
presence  of  organic  material,  such  as  decayed  trees  and  reeds. 


DEPOSITS  FORMED  BY  COLD  SOLUTIONS        79 

In  some  deposits  calcite  cement  in  sandstone  appears  to  have 
precipitated  the  metals. 

Iron  dissolved  from  country  rock  in  sulphate  or  carbonate 
solutions  may  be  precipitated  as  oxide  by  descending  waters. 
At  many  places,  small  veins  are  deposited  in  fissures  or  replace 
the  rock  along  fissures.  Manganese,  under  some  conditions  at 
least,  is  dissolved  more  readily  than  iron.  It  is  easily  precipi- 
tated as  oxide  when  acid  solutions  are  neutralized  by  alkaline 


nt 


FIG.  34. — Honeycomb  ore  from  the  Hazel  Green  mine,  Hazel  Green,  Wisconsin, 
ra,  Marcasite;  b,  zinc  blende;  g,  galena.     (After  Bain,  U.  S.  Geol  Survey.) 

rocks.  Many  manganese  deposits  have  been  formed  by  precipi- 
tation from  cold  solutions. 

Composition. — Deposits  formed  at  moderate  and  shallow 
depths  by  cold  solutions  yield  a  large  part  of  the  world's  sup- 
ply of  lead  and  zinc  and  some  copper  but  little  or  no  gold  or 
quicksilver,  and  only  a  little  silver.  The  ores  generally  are  of 
simple  composition  (Figs.  33  and  34).  As  pointed  out  by  Bain,1 

IBAIN,  H.  F.:  Some  Relations  of  Paleogeography  to  Ore  Deposition  in 
the  Mississippi  Valley.  Internal.  Geol.  Cong.,  Mexico,  1907.  The  Fluorspar 
Deposits  of  Southern  Illinois.  U.  S.  Geol.  Survey  Bull.  255,  pp.  61-67,  1905. 


80        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  complex  arsenic  and  antimony  minerals  are  generally 
absent  in  these  deposits.  Some  of  the  galena,  however,  carries 
antimony,1  and  in  some  regions  as  much  as  1  per  cent,  is  present. 
Minute  quantities  of  silver  may  also  occur  in  galena.  The 
uranium  and  vanadium  deposits  of  southwestern  Colorado  are 
included  in  this  class.  Deposits  of  iron  oxides  and  of  manganese 
oxides  are  formed  by  precipitation  in  shattered  zones  of  limestone 
and  of  other  rocks. 

The  minerals  that  are  found  in  deposits  of  this  class  are  listed 
below.  Of  these,  many  are  confined  to  oxidized  zones  near 
the  surface  and  are  regarded  as  alteration  products  of  primary 
minerals.  In  this  group  of  deposits  it  is  not  always  easy  to 
distinguish  between  primary  and  secondary  ores. 

alum  chalcedony  greenockite  pyrolusite 

anglesite  chalcocite  gypsum  pyromorphite 

ankerite  chalcopyrite  hematite  quartz 

aragonite  chert  hydrozincite  rhodochrosite 

aurichalcite  chlorite  kaolin  selenite 

azurite  chrysocolla  limonite  siderite 

barite  copper  malachite  smithsonite 

bauxite  covellite  manganite  sulphur 

bornite  cuprite  marcasite  uranium 

calamine  fluorite  millerite  compounds 

calcite  galena  opal  vanadium 

celestite  goethite  psilomelane  compounds 

cerusite  gold  pyrite  zinc  blende 

chalcanthite  goslarite 

Shape. — Large  rudely  tabular  bodies  are  common  in  the  de- 
posits of  this  class,  but  as  noted  above  they  generally  lie  along 
the  beds.  Great  veins  cutting  across  the  beds  are  rare.  In 
southwestern  Wisconsin  "crevices"  and  "gash  veins"  occur, 
but  these  are  thinner  and  less  persistent  than  normal  veins. 
In  that  region  flats  and  pitches  are  formed  where  beds  rich  in 
volatile  hydrocarbons  have  shrunk  and  let  down  limestone  beds 
above  them,  causing  the  limestone  to  fracture  along  horizontal 
and  steeply  dipping  planes  (see  Fig.  35). 

In  the  Joplin  region  some  of  the  deposits  have  formed  in  ancient 
caves,  in  limestone  sinks,  and  in  openings  between  fragments  that 
have  formed  superficial  chert  breccias  that  were  subsequently 
covered  by  later  beds.  Such  deposits  are  very  irregular  in  shape. 
Those  that  have  formed  in  solution  cavities  are  likely  to  be  long 

^IEBENTHAL,  C.  E. :  Op.  cit.,  p.  216. 


DEPOSITS  FORMED  BY  COLD  SOLUTIONS        81 

in  one  and  relatively  short  in  two  dimensions.  Deposits  in 
which  solutions  have  replaced  organic  matter  vary  widely  in 
shape,  which  depends  on  the  distribution  of  the  material  that 
caused  precipitation  and  on  the  extent  of  replacement. 

Size. — The  large  tabular  upright  ore  bodies  that  are  commonly 
referred  to  as  "true  fissure  veins"  are  conspicuous  in  many  dis- 
tricts where  the  ores  are  genetically  related  to  igneous  rocks  but 
are  rare  in  regions  where  the  deposits  were  formed  by  cold 
meteoric  waters.  Extensive  bedding-plane  deposits  are  formed 
in  such  regions;  the  Joplin  district  of  southwestern  Missouri  and 
the  lead  district  of  southeastern  Missouri  contain  bedding-plane 
deposits  that  are  as  large  as  the  great  veins  above  mentioned. 
Where  deposits  have  been  formed  in  ancient  underground  water 
channels  they  may  be  followed  here  and  there  over  distances 


FIG.  35. — Section   of   flats   and   pitches   of    Roberts   mine,   Linden,    Wisconsin. 
n,  m,  s,  North,  middle,  and  south  crevices.     (After  T.  C.  Chamberlin.) 

of  several  miles.  A  great  many  deposits  of  this  group,  however, 
are  small.  Among  the  hundreds  of  deposits  of  copper,  uranium, 
and  vanadium  .that  replace  organic  matter  in  the  Southwest,  few 
large  ore  bodies  have  been  developed.  While  many  primary 
iron  and  manganese  deposits  are  formed  by  deposition  from  cold 
solutions,  most  of  them  are  small.  On  the  other  hand,  some  of 
the  largest  iron-ore  deposits  have  been  formed  by  enrichment  of 
low-grade  protores  in  weathering. 

Texture. — The  texture  of  the  deposits  in  this  group  differs 
but  little  in  kind  from  that  of  deposits  formed  by  hydrothermal 
processes.  In  both  classes  crustified  banding  and  symmetri- 
cally lined  vugs  are  common.  Delicately  banded  quartz,  which 
is  so  common  in  deposits  formed  by  hot  waters,  is  rare  in  ores 
formed  by  cold  solutions,  'in  the  deposits  of  southwestern 
Wisconsin  crustified  bands  of  calcite  around  brecciated  fragments 


82        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

are  common.  Nodular  and  tabular  masses  are  formed  in  which 
the  order  of  deposition  is  marcasite,  sphalerite,  galena.  The 
great  bulk  of  the  ore  of  this  class,  however,  shows  little  crustifica- 
tion.  In  the  Joplin  region  much  of  the  ore  has  a  brecciated  struc- 
ture, consisting  of  chert  fragments  cemented  with  sphalerite. 
In  the  lead  district  of  southeastern  Missouri  most  of  the  ore  is 
disseminated  in  beds  of  carbonaceous  limestone.  Nodular 
bodies  and  geodes  are  of  common  occurrence  in  deposits  of  iron 
and  manganese  oxides. 

References 

DEPOSITS    FORMED    AT    MODERATE    AND   SHALLOW    DEPTHS 

BY  COLD  SOLUTIONS 

BAIN,  H.  F.:  Zinc  and  Lead  Deposits  of  Northwestern  Illinois.  U.  S. 
Geol.  Survey  Bull.  246,  1905.  Zinc  and  Lead  Deposits  of  the  Upper  Missis- 
sippi Valley.  U.  S.  Geol.  Survey  Bull.  294,  1906.  Sedi-genetic  and  Igneo- 
genetic  Ores.  Earn.  Geol.,  vol.  1,  pp.  331-339,  1906.  Some  Relations  of 
Paleogeography  to  Ore  Deposition  in  the  Mississippi  Valley.  Econ.  Geol., 
vol.  2,  pp.  128-144,  1907. 

BALL,  S.  H. :  The  Hartville  Iron-ore  Range,  Wyoming.  U.  S.  Geol.  Sur- 
vey Butt.  315,  pp.  200-203,  1906. 

BALL,  S.  H.,  and  SHALER,  M.  K. :  Mining  Conditions  in  the  Belgian  Congo. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  41,  pp.  197-201,  1911. 

BOUTWELL,  J.  M.:  Vanadium  and  Uranium  Deposits  in  Southeastern 
Utah.  U.  S.  Geol.  Survey  Bull.  260,  pp.  200-210,  1905. 

BUCKLEY,  E.  R. :  Geology  of  the  Disseminated  Lead  Deposits  of  St.  Fran- 
cis and  Washington  Counties,  Missouri.  Mo.  Bur.  Geol.  and  Mines,  vol. 
9,  1909. 

BUCKLEY,  E.  R.,  and  BUEHLER,  H.  A.:  Geology  of  the  Granby-Area. 
Mo.  Bur.  Geol.  and  Mines,  vol.  4,  2d  ser.,  1906. 

CHAMBERLIN,  T.  C.:  The  Ore  Deposits  of  Southwestern  Wisconsin.  Geol- 
ogy of  Wisconsin,  vol.  4,  pp.  367-568,  1882. 

EMMONS,  S.  F.:  Copper  in  the  Red  Beds  of  the  Colorado  Plateau  Region. 
U.  S.  Geol.  Survey  Bull.  260,  pp.  221-232,  1905. 

EMMONS,  W.  H.:  The  Cashin  Mine,  Colorado.  U.  S.  Geol.  Survey  Bull. 
285,  pp.  125-128,  1906.  A  Genetic  Classification  of  Minerals.  Econ.  Geol, 
vol.  3,  pp.  611-627,  1908. 

GALE,  H.  S. :  Geology  of  the  Copper  Deposits  near  Montpelier,  Idaho. 
U.  S.  Geol.  Survey  Bull.  430,  pp.  112-121,  1910.  Carnotite  in  Western 
Colorado.  U.  S.  Geol.  Survey  Bull.  340,  pp.  256-262,  1908. 

GRANT,  U.  S.:  Structural  Relations  of  the  Wisconsin  Zinc  and  Lead 
Deposits.  Econ.  Geol,  vol.  1,  pp.  233-242,  1905.  Report  on  Lead  and  Zinc 
Deposits  of  Wisconsin.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  Bull.  14, 
1906. 

HARDER,  E.  C.:  Iron  Ores,  Pig  Iron,  and  Steel.  U.  S.  Geol.  Survey 
Mineral  Resources,  part  1,  Metals,  pp.  81-89,  1908.  Manganese  Deposits  of 


DEPOSITS  FORMED  BY  COLD  SOLUTIONS        83 

the  United  States  with  Sections  on  Foreign  Deposits,  Chemistry,  and  Uses. 
U.  S.  Geol.  Survey,  Bull.  427,  pp.  1-208,  1910. 

HAWORTH,  ERASMUS,  CRANE,  W.  R.,  and  ROGERS,  A.  F. :  Special  Report 
on  Lead  and  Zinc.  Kansas  Univ.  Geol.  Survey,  vol.  8,  1904. 

HESS,  F.  L. :  A  Hypothesis  for  the  Origin  of  the  Carnotites  of  Colorado 
and  Utah.  Econ.  Geol,  vol.  9,  pp.  675-688,  1914. 

HEWETT,  D.  F.:  Vanadium  Deposits  in  Peru.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  40,  pp.  274-299,  1909. 

SIEBENTHAL,  C.  E. :  Origin  of  the,  Zinc  and  Lead  Deposits  of  the  Joplin 
Region,  Missouri,  Kansas,  and  Oklahoma.  U.  S.  Geol.  Survey  Bull.  606, 
1915. 

TARR,  W.  A. :  Copper  in  the  Red  Beds  of  Oklahoma.  Econ.  Geol.,  vol. 
5,  pp.  221-226,  1910. 

VAN  HISE,  C.  R. :  Some  Principles  Controlling  the  Deposition  of  Ores 
(discussion).  Am.  Inst.  Min.  Eng.  Trans.,  vol.  30,  pp.  27-177,  284-303, 
1901. 

VAN  HORN,  F.  B.:  Geology  of  Moniteau  County.  Mo.  Bur.  Geol.  and 
Mines  Bull.  3,  ser.  2,  pp.  80-97,  1905. 

WATSON,  T.  L. :  The  Lead  and  Zinc  Deposits  of  the  Virginia-Tennessee 
Region.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  36,  pp.  681-737,  1905. 

WINSLOW,  ARTHUR:  Notes  on  the  Lead  and  Zinc  Deposits  of  the  Missis- 
sippi Valley  and  the  Origin  of  Ores.  Jour.  Geol,  vol.  1,  pp.  612-619,  1893. 
Lead  and  Zinc  Deposits  of  Missouri.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  24, 
pp.  634-689,  1894.  The  Disseminated  Lead  Ores  of  Southeastern  Missouri. 
U.  S.  Geol.  Survey  Bull  132,  1896. 


CHAPTER  X 

SEDIMENTARY  DEPOSITS 

Occurrence. — In  sedimentary  rocks — gravels,  sands,  clays,  conglomerates, 
sandstones,  limestones,  shales,  etc. — usually  as  beds  or  parts  of  beds,  or 
disseminated  through  certain  beds.  All  are  oriented  with  the  contempora- 
neous strata.  Where  they  have  not  been  disturbed  they  are  flat-lying. 

Composition. — In  the  clastic  sedimentary  deposits  the  minerals  are  the 
residual  or  stable  minerals  of  older  rocks  and  older  deposits.  They  include 
gold,  platinum,  iron,  tin,  rutile.  zircon,  rare-earth  minerals,  gems,  and  other 
relatively  insoluble  materials.  In  beds  which  are  chemical  precipitates  iron 
and  manganese  are  the  most  important  metals.  Nonmetallic  substances 
are  coal,  phosphate  rock,  salines,  gypsum,  clays,  and  many  other  materials. 

Shape. — In  general  two  dimensions  are  great  and  one  is  relatively  small. 
Some  placers  are  long  in  one  and  short  in  two  dimensions.  Equidimensional 
deposits  are  rare  but  not  unknown. 

Size. — Some  are  small;  others  are  very  extensive. 

Texture. — Some  deposits  of  this  class  have  a  structure  characteristic  of 
sedimentary  rocks,  such  as  is  produced  by  sorting  in  water — bedding,  cross- 
bedding,  etc.  In  some  the  constituent  particles  are  rounded  by  wear.  Some 
contain  fossil  remains.  Included  anisodiametric  bodies  deposited  mechan- 
ically with  the  beds  are  generally  oriented  with  their  short  dimensions  across 
the  beds.  The  chemical  sediments  may  be  oolitic,  crystalline,  or  amorphous. 
Banded  structure  is  common  but  is  rarely  symmetrical  and  is  not  crustified. 
Comb  structure  and  vugs  lined  symmetrically  with  banded  crusts  are  never 
present  except  where  infiltration  has  taken  place  since  the  deposits  were 
formed. 

General  Features. — Weathering  and  erosion  are  processes  of 
mineral  segregation.  A  quartz  diorite  which  has  about  the 
composition  of  the  average  igneous  rock  is  composed  of  potash 
and  lime-soda  feldspars,  quartz,  muscovite,  biotite,  magnetite, 
augite,  and  other  ferromagnesian  minerals,  and  some  minor 
constituents,  such  as  titanite,  apatite  and  pyrite.  Normal 
weathering  will  tend  to  convert  the  rock  to  kaolin,  quartz,  and 
limonite;  wad  and  bauxite  also  may  form.  With  these  "end 
products"  of  weathering  titanite  and  some  apatite  will  remain. 
Ground  water,  which  generally  contains  carbon  dioxide,  will  dis- 
solve and  carry  away  alkalies  and  alkaline  earths  as  carbonates 
or  bicarbonates;  the  sulphur  is  converted  to  sulphates.  Some 
iron,  aluminum,  and  silica  also  are  dissolved,  but  more  slowly 

84 


SEDIMENTARY  DEPOSITS  85 

than  the  alkalies  and  alkaline  earths.  Phosphorus  and  titanium 
dissolve  very  slowly.  The  dissolved  materials  will  be  carried  to 
the  sea  and  form  organic  deposits  and  chemical  precipitates. 
Thus  are  supplied  materials  for  beds  of  limestone,  dolomite, 
chert,  iron-bearing  sediments,  gypsum,  phosphate  rock,  etc. 
The  waters  also  carry  in  suspension  or  roll  along  their  courses 
quartz,  kaolin,  limonite,  and  any  heavy  residual  materials  that 
may  be  present. 

If  the  rocks  decomposed  contain  deposits  of  metals,  such  as 
copper,  zinc,  tin,  and  platinum,  the  soluble  metals  like  copper  and 
zinc  may  be  carried  downward  and  reprecipitated,  or  they  may  be 
carried  to  the  sea  and  subsequently  precipitated  there.  The 
heavy,  insoluble  minerals,  if  any  happen  to  be  present,  especially 
those  which  resist  surface  decomposition,  like  platinum,  gold, 
and  tin  oxide,  will  generally  be  left  behind  in  the  beds  of  the 
streams  not  far  from  their  outcrops. 

Every  constituent  of  the  quartz  diorite  has  a  certain  economic 
value  if  sufficiently  concentrated,  and  when  conditions  are 
favorable  many  products  may  result  from  the  processes  of 
weathering,  transportation,  and  deposition.  But  erosion  seldom 
permits  complete  weathering,  and  the  material,  which  is  gener- 
ally disintegrated  mechanically  before  it  is  thoroughly  decom- 
posed chemically,  passes  to  the  sea  in  various  stages  of  segrega- 
tion. Thus  the  products  of  decomposition  are  generally  found  in 
various  stages  of  impurity. 

Sedimentary  deposits  are  valuable  sources  of  the  metals. 
From  them  are  obtained  more  than  half  of  the  world's  iron,  tin, 
and  platinum,  and  much  of  the  gold  and  manganese,  as  well 
as  gems,  many  other  nonmetallic  products,  and  hydrocarbons. 
Although  sulphides  are  present  in  many  sedimentary  beds,1  sul- 
phide ores  of  workable  grade  are  comparatively  rare  among 
sedimentary  deposits. 

Sedimentary  deposits  may  be  divided  into  those  concen- 
trated mechanically  and  those  concentrated  chemically.  De- 
posits concentrated  mechanically  include  basal  conglomerates 
of  iron  ore,  and  the  placers  of  platinum,  gold,  tin,  etc.  These 

1  ANDRUSSOW,  N. :  Le  mer  noire.  VII  Congres  G£ol.  Internal.  Guide 
des  excursions,  Excursion  XXIX,  1897. 

Doss,  BRUNO:  Melnikowit,  ein  neues  Eisenbisulphid,  und  seine  Bedeu- 
tung  fur  die  Genesis  der  Kieslagerstatten.  Zeitschr.  prakt.  Geologic,  Jahrg. 
20,  pp.  453-483,  1912. 


86        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

deposits  are  marine,  lacustrine,  or  fluviatile,  the  classification 
depending  on  the  nature  of  the  body  of  water  in  which  they  have 
found  rest.  Deposits  may  be  concentrated  chemically  as  a 
result  of  evaporation,  reduction,  or  oxidation,  or  by  organic 
agencies.  Examples  of  these  are  salt  beds,  coal,  gypsum,  and 
some  iron  ores. 

Many  deposits  of  sedimentary  origin  are  workable  in  their 
primary  state.  Others  supply  the  protores,  which  are  further 
concentrated  by  weathering  into  workable  ore  bodies. 

Occurrence. — Deposits  of  sedimentary  origin  may  be  found  in 
association  with  all  kinds  of  sedimentary  rocks.  Deposits  of 
mechanical  origin  are  likely  to  be  in  conglomerates,  sandstones 
and  shales;  deposits  of  chemical  origin  are  commonly  in  or  asso- 
ciated with  limestones,  shales,  or  fine-grained  sandstones. 


FIG.  36. — Section  showing  iron-ore  deposits  of  Iron  Mountain,  Missouri. 
The  iron  has  been  quarried  from  openings  at  the  top  of  the  hill  arid  mined 
underground  at  the  base  of  the  sedimentary  series,  where  it  formed  a  basal 
conglomerate.  (After  Crane,  Mo.  Geol.  Survey.) 

All  the  deposits  of  this  class  are  syngenetic,  or  "contem- 
poraneous" with  rocks  of  the  series  containing  them,  but  they 
are  younger  than  the  rocks  stratigraphically  below  and  older 
than  the  rocks  stratigraphically  above  them.  The  ore-bearing 
beds  may  be  much  younger  than  the  underlying  rocks  (see  Fig.  36) 
or  much  older  than  the  overlying  rocks  where  they  are  sepa- 
rated from  those  rocks  by  unconformities.  On  the  other  hand, 
the  deposits  may  form  a  continuous  series  with  the  underlying 
and  overlying  rocks.  The  mechanical  sediments  especially  con- 
glomerates are  commonly  unconformable  with  the  underlying 
rocks  and  therefore  may  be  considerably  younger;  the  chemical 
sediments  are  more  generally  contemporaneous — that  is,  they 
form  part  of  a  series  of  beds  that  were  deposited  continuously, 
without  interruptions  due  to  great  diastrophic  movements  and 
erosion. 

Deposits  of  sedimentary  origin  may  rest  upon  igneous  flows, 


SEDIMENTARY  DEPOSITS  87 

or  intrusive  rocks;  above  them  there  may  be  flows,  sills,  or  sheets 
of  igneous  rock. 

Some  sedimentary  deposits  are  superficial — that  is,  they  are 
not  covered  by  later  formations.  Examples  are  bog-iron  de- 
posits, some  beds  of  salt,  borax,  nitrates,  and  surface  placers. 
Other  deposits  of  this  class,  after  they  are  formed,  are  buried 
below  later  sediments  or  igneous  flows. 

Whether  chemical  or  mechanical,  all  sedimentary  deposits 
have  been  subjected  to  all  the  folding,  faulting,  or  other  de- 
formation which  has  affected  the  associated  beds  and  overlying 
formations  (see  Fig.  37).  They  are  flat-lying  only  where  they 
have  not  been  disturbed  by  subsequent  earth  movements. 

Composition. — The  principal  metals  of  sedimentary  deposits 
include  iron,  manganese,  gold,  platinum,  and  tin.  The 


Scale  of  Miles 

FIG.  37. — Section  across  Whiteoak  Mountain  syncline,  Chattanooga,  Ten- 
nessee, showing  "Rockwood"  or  Trenton  iron-ore  bed.  Sr,  "Rockwood" 
formation;  DC,  Chattanooga  shale;  Cp,  Fort  Payne  chert;  Cf,  Floyd  shale. 
(After  Burchard,  U.  S.  Geol.  Survey.) 

rare-earth  minerals  monazite  and  xenotime  should  be  mentioned 
also.  A  list  of  the  minerals  in  sedimentary  deposits  would 
include  essentially  all  minerals,  for  sedimentary  beds  are  made 
up  of  materials  from  geologic  bodies  of  all  classes,  mechanically 
disintegrated  and  weathered  in  varying  degrees;  but  because 
some  minerals  resist  weathering  more  than  others,  they  are 
more  common  and  relatively  more  abundant  in  mechanical  sedi- 
ments than  the  less  stable  minerals. 

Certain  minerals  that  are  stable  under  conditions  of  weathering 
are  listed  below. 

All  the  minerals  listed  below  are  dissolved  very  slowly  in 
ground  water.  Most  of  them  are  hard  and  therefore  not  readily 
disintegrated  mechanically.  Gold  and  platinum,  though  soft, 
are  malleable  and  therefore  withstand  abrasion.  Limonite, 
kaolin,  psilomelane,  pyrolusite,  and  bauxite  although  resistant 


88        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


MINERALS  STABLE  UNDER  WEATHERING  CONDITIONS 


Specific    gravity 

Hardness 

Quartz  (SiO2)  '  
Garnet  (composition  variable)  

2.65 
3  15-4.30 

7.0 

6  5-7  5 

Kaolin  (H4Al2Si2O9) 

2  60 

2  0-2  5 

Bauxite  (A12O3.3H2O) 

2  30-2  40 

2  5-3  5 

Limonite   (2Fe2O3.3H2O) 

3  60-4  00 

5  0-5  5 

Psilomelane  (H4MnO5)  
Pyrolusite  (MnO2)  
Magnetite  (Fe3O4)  

3.70-4.70 

4.73-4,86 
5.17 

5.0-6.0 
2.0-2.5 
5.5-6.5 

Chromite  (FeCr2O4)  

4.32-4.57 

5.5 

Hematite  (Fe2Os) 

4  90-5  30 

5  5-6  5 

Platinum  (Pt)  

21  00 

40-45 

Gold  (Au)  
Cassiterite  (SnO2)  
Monazite  ([Ce  La   Di]  PO4) 

19.00 
6,80-7.10 
4  90-5  30 

2.5-3.0 
6.0-7.0 
5  0-5  5 

Rutile  (TiO2) 

4  20 

5  0-6  5 

Zircon  (Zr  SiO4)  

4.70 

7.5 

to  chemical  changes,  are  not  resistant  to  mechanical  forces. 
Consequently  they  are  likely  to  be  broken  into  small  particles, 
carried  away  mechanically  from  coarser  clastic  sediments,  and 
finally  brought  to  rest  in  more  quiet  surroundings.  In  addition 
to  the  minerals  listed  above,  goethite,  gibbsite,  wad,  xenotime, 
and  ilmenite  should  be  mentioned. 

Kaolin,  quartz,  iron  aluminum  and  manganese  oxides,  and 
garnet  are  abundant  in  nature;  the  other  minerals  listed  above 
are  comparatively  rare.  The  common  minerals  are  in  general 
not  so  resistant  to  disintegration  as  the  rare  ones.  Muscovite, 
biotite,  and  apatite  also  resist  disintegration  but  are  less  stable 
than  the  minerals  named  above.  They  are  common,  neverthe- 
less, in  mechanical  sediments. 

Metalliferous  minerals  found  in  the  chemical  sediments  are 
siderite,  limonite,  goethite,  pyrite,  hematite,  glauconite,  chamo- 
site,  greenalite,  manganite,  wad,  and  psilomelane.  Gangue 
minerals  that  are  associated  with  these  are  chert,  quartz,  calcite, 
dolomite,  and  a  number  of  other  minerals,  as  a  rule  very  finely 
comminuted.  Of  these  kaolin  and  quartz  are  generally  the  most 
abundant.  Nonmetalliferous  minerals  in  chemical  sediments 
include  salt,  gypsum,  anhydrite,  calcium  phosphate,  borates, 
nitrates,  etc. 

One  group  of  sedimentary  deposits  that  exhibits  some  puzzling 


SEDIMENTARY  DEPOSITS  89 

features  should  be  mentioned  here.  These  are  the  marine  oolitic 
and  fossiliferous  iron  ores  of  the  so-called  Clinton  type.  They 
carry  hematite,  calcite,  siderite,  and  quartz.  The  iron  oxide 
occurs  as  fossils  of  shells  of  calcium-secreting  organisms  and 
doubtless  has  replaced  lime  carbonate.  But  because  these  shells 
are  uniformly  replaced  over  wide  areas  and  are  now  distributed 
throughout  beds  that  extend  down  the  dip  to  great  depths,  far 
below  the  level  of  active  oxidation,  it  is  believed  that  the  replace- 
ment of  lime  by  iron  occurred  before  the  shells  were  deeply  buried 
and  the  rock  consolidated,  probably  soon  after  the  shells  sank 
to  the  bottom  of  the  sea.  Thus  these  deposits,  though  formed 
by  replacement,  are  considered  sedimentary  and  syngenetic, 
just  as  dolomites  in  which  magnesia  has  replaced  lime,  probably 
under  nearly  similar  conditions,  are  classed  as  sedimentary 
beds. 

Shape. — Many  sedimentary  deposits,  especially  the  chemical 
and  organic  deposits,  approach  the  tabular  form  more  closely 
than  deposits  of  other  classes.  But  broadly  considered  they 
are  lenses,  for  they  must  pinch  out  somewhere,  however  extensive 
they  may  be.  In  general  the  thickness  is  comparatively  uni- 
form, at  least  more  nearly  uniform  than  in  deposits  of  other 
classes.  Many  placer  deposits  formed  along  streams  are  long 
in  one  and  short  in  two  dimensions.  Those  formed  on  relatively 
swift  streams  are  likely  to  be  irregular,  and  their  productive 
parts  may  lie  in  pockets. 

Size. — Many  of  the  deposits  of  sedimentary  origin  are  very 
extensive.  The  iron-bearing  formation  of  the  Mesabi  range, 
Minnesota,  is  traced  over  100  miles;  the  Penokee-Gogebic  iron 
range,  in  Michigan  and  Wisconsin,  extends  about  80  miles;  and 
the  Clinton  iron-bearing  formation,  which  contains  overlapping 
lenses  of  iron  ore,  extends  over  areas  of  many  hundreds  of  square 
miles.  Beds  of  coal,  gypsum,  or  rock  phosphate  may  extend  over 
thousands  of  square  miles — indeed,  the  most  extensive  mineral 
deposits  of  economic  value  belong  to  this  class.  On  the  other 
hand,  some  deposits  of  sedimentary  origin  are  very  small.  Some 
beds  of  bog  iron  and  of  bog  manganese  cover  only  a  few  acres, 
and  some  highly  profitable  placer  deposits  of  this  class  are  of 
small  extent.  Certain  very  extensive  beds  of  iron  ore  less  than 
2  feet  thick  have  been  worked.  Some  of  the  sedimentary 
ore  beds  have  great  thickness;  the  Biwabik  formation,  which  is 
the  protore  of  the  Mesabi  iron  deposits,  is  hundreds  of  feet  thick. 


90        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Texture. — Many  sedimentary  ores  are  banded  (Fig.  38).  The 
mechanical  sediments  may  show  bedding,  cross-bedding,  and  other 
evidences  of  sorting  by  water.  Their  constituent  particles  are 
commonly  corroded  or  rounded  by  abrasion.  Fossil  remains  of 
animals  or  plants  may  be  present;  in  clastic  sediments  the 
fossils  are  generally  broken.  As  the  deposits  are  formed  in 
water,  bodies  that  are  anisodiametric  (not  spherical)  will  lie 
with  their  longer  axes  approximately  horizontal  (Fig.  39); 
if  the  beds  are  subsequently  tilted,  these  bodies  will  still  lie 
approximately  parallel  to  the  bedding  planes.  If  heavy  minerals 
are  present  they  tend  to  be  concentrated  at  the  bottom  of  the 
bed.  Chemical  sediments  also  are  commonly  banded. 


FIG.  38. — Banded  ore  of  sedimentary  FIG.    39. — Section    of    sedimentary 

origin,    Kennedy   mine,    Cuyuna  range,  deposit.     Anisodiametric     bodies     lie 

Minnesota.     Chert    (light)    alternating  with  their  longer  axes  parallel  to  the 

with  hematite  (black).     (About  natural  bedding  planes, 
size.) 

Although  many  sedimentary  deposits  are  banded  or  bedded, 
they  are  unlike  deposits  that  fill  fissures  in  that  the  banding 
is  nowhere  symmetrically  crustified  (see  page  3).  The  open- 
ings in  these  deposits  are  intergranular  spaces  or  solution 
cavities.  Vugs,  which  are  common  in  veins,  are  lacking  in 
sedimentary  deposits  and  the  open  spaces  are  not  lined  by 
layers  or  crusts,  one  on  the  other,  like  cavities  of  veins.  The 
deposits  do  not  cross  the  bedding,  and  there  are  no  contempora- 
neous veinlets  extending  from  the  main  lodes  into  the  country 
rock,  such  as  those  that  extend  from  veins.  Many  sedimentary 
deposits  have  rather  sharp  contacts  with  the  associated  beds, 
whereas  replacement  deposits  more  commonly  grade  into  and 
finger  out  in  the  wall  rock.  Many  sedimentary  deposits — for 


SEDIMENTARY  DEPOSITS 


91 


example,  some  sedimentary  iron  ores — are  made  up  of  thin, 
closely  spaced  bands  of  ore  and  barren  country  rock. 

In  some  districts  epigenetic  deposits  replace  certain  beds 
partly  or  almost  completely.  These  deposits  exhibit  the  broader 
structural  relations  of  sedimentary  beds,  but  certain  details  of 
structure  serve  to  distinguish  them.  Such  deposits  are  discussed 


FIG.  40. — Diagram  showing  approximately  the  relative  abundance  of  various 
classes  of  primary  ores  and  protores  of  several  metals.  Broken  lines  indicate 
that  classes  of  deposits  are  of  little  or  no  value.  Broken  lines  with  long  dashes 
indicate  rare  deposits  or  deposits  of  subordinate  value.  Solid  lines  indicate 
valuable  deposits — the  value  of  the  class  being  shown  approximately  by  width 
of  the  line. 

on  page   197.     The  relative  abundance  of  various  classes  of 
deposits  of  certain  metals  is  indicated  by  Fig.  40. 

References 

SEDIMENTARY  DEPOSITS 

ALLEN,  R.  C.:  The  Occurrence  and  Origin  of  the  Brown  Ores  of  Spring 
Valley,  Wisconsin.  Mich.  Acad.  Sci.  Eleventh  Ann.  Rept.,  pp.  95-103, 1909. 


92        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

BAIN,  H.  F.:  Sedi-genetic  and  Igneo-genetic  Ores.  Econ.  GeoL,  vol.  1,  pp. 
331-339,  1906.  Some  Relations  of  Paleogeography  to  Ore  Deposition  in  the 
Mississippi  Valley.  Econ.  GeoL,  vol.  2,  pp.  128-144,  1907. 

BAYLEY,  W.  S. :  The  Menominee  Iron-bearing  District  of  Michigan.  U.  S. 
Geol.  Survey  Mon.  46,  1904. 

BURCHARD,  E.  F.:  The  Red  Iron  Ores  of  East  Tennessee.  Tenn.  Geol. 
Survey  Bull.  16,  pp.  1-173,  1913. 

BURCHARD,  E.  F.,  BUTTS,  CHARLES,  and  ECKEL,  E.  C. :  Iron  Ores  of  the 
Birmingham  District,  Alabama.  U.  S.  Geol.  Survey  Bull.  400,  1910. 

HARDER,  E.  C.:  The  "Itabirite"  Iron  Ores  of  Brazil.  Econ.  Geol,  vol.  9, 
pp.  101-111,  1914. 

HARDER,  E.  C.,  and  CHAMBERLIN,  R.  T. :  The  Geology  of  Central  Minas 
Geraes,  Brazil.  Jour.  GeoL,  vol.  23,  pp.  341-378,  385-424,  1915. 

HAYES,  A.  O. :  Wabana  Iron  Ore  of  Newfoundland.  Canada  Dept.  Mines, 
Geol.  Survey  Mem.  78,  pp.  62-92,  1915. 

HOPKINS,  T.  C. :  Cambro-Silurian  Limonite  Ores  of  Pennsylvania.  Geol. 
Soc.  America  Butt.,  vol.  11,  pp.  475-502,  1900. 

LEITH,  C.  K. :  Genesis  of  the  Lake  Superior  Iron  Ores.  Econ.  GeoL,  vol. 
1,  pp.  47-65,  1906. 

.    MCCALLIE,  S.  W. :  The  Iron  Ores  of  Georgia.     Ga.  Geol.  Survey  Bull. 
10A,  1900.     Fossil  Iron  Ores  of  Georgia.     Ga.  Geol.  Survey  Bull.  17,  1908. 

NEWLAND,  D.  H.:  The  Clinton  Iron-ore  Deposits  in  New  York  State. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  40,  pp.  165-184,  1909. 

NEWLAND,  D.  H.,  and  HARTNAGEL,  C.  A.:  Iron  Ores  of  the  Clinton  Form- 
ation in  New  York  State.  N.  Y.  State  Mus.  Bull.  123,  1908. 

PENROSE,  R.  A.  F.,  JR.:  Manganese:  Its  Uses,  Ores,  and  Deposits.  Ark. 
Geol.  Survey  Ann.  Re-pi,  for  1890,  vol.  1,  1892. 

PHALEN,  W.  C.:  Origin  and  Occurrence  of  Certain  Iron  Ores  of  North- 
eastern Kentucky.  Econ.  GeoL,  vol.  1,  pp.  660-669,  1906. 

SMYTH,  C.  H.,  JR.  :  On  the  Clinton  Iron  Ores.  Am.  Jour.  Sci.,  3d  ser., 
vol.  43,  pp.  487-496,  1892. 

VAN  HISE,  C.  R. :  The  Iron-ore  Deposits  of  the  Lake  Superior  Region. 
U.  S.  Geol.  Survey  Twenty-first  Ann.  Rept.,  part  3,  pp.  305-434,  1901. 

VAN  HISE,  C.  R. :  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey  Mon. 
47,  1904.  (The  Relation  of  Metamorphism  to  Ore  Deposits,  Chapter  12). 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K.:  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  52,  pp.  499-562,  1911. 

VEACH,  OTTO:  The  Clay  Deposits  of  Georgia.  Ga.  Geol.  Survey  Bull. 
18,  pp.  42,  261,  1909. 

VOGT,  J.  H.  L. :  Ueber  die  Sedimentation  der  Eisenerzlager  und  der  Eisen- 
glimmerschliefer.  Salten  og  Ranen,  1891,  pp.  214-224;  rev.  in  Zeitschr.  prakt. 
Geologic,  vol.  2,  pp.  30-34,  1894. 

WATSON,  T.  L. :  A  Preliminary  Report  on  the  Manganese  Deposits  of 
Georgia.  Ga.  Geol.  Survey  Bull.  14,  pp.  145-157,  1908.  A  Preliminary 
Report  on  the  Bauxite  Deposits  of  Georgia.  Ga.  Geol.  Survey  Bull.  11,  pp. 
119-130,  1904. 


CHAPTER  XI 
PRIMARY  ORE  SHOOTS 

Many  metalliferous  deposits  contain  bodies  of  workable  ore 
that  are  surrounded  by  lower-grade  unworkable  material.  The 
term  "ore  shoot"1  in  the  stricter  sense  is  applied  to  these  richer 
portions.  More  loosely,  the  term  is  applied  to  any  body  of 
valuable  ore  lying  in  the  gangue  or  country  rock.  A  body  very 
rich  and  of  good  size  is  called  a  "bonanza;"  a  thin  and  rudely 
tabular  body  of  richer  ore  is  called  a  "pay  streak;"  and  a  smaller 
body  not  very  large  in  any  dimension  is  a  "bunch"  or  "pocket." 

As  ore  shoots  are  the  most  valuable  portions  of  metalliferous 
deposits  they  are  naturally  of  great  interest  to  the  miner.  Their 
mode  of  occurrence  and-  genesis  may  become  apparent  only  after 
a  comprehensive  study  of  the  structural  and  historical  geology 
of  a  region  containing  them,  the  relations  of  the  deposits  to  the 
structure,  and  the  paragenesis  of  the  vein  matter. 

A  great  many  mineral  deposits — by  far  the  greater  number  of 
those  so  far  discovered — are  exposed  at  the  surface.  Their  out- 
crops may  be  rich  in  valuable  metals  or  they  may  be  leached  by 
surface  waters,  but  their  leached  portions  are  characteristic  and 
may  be  recognized  by  prospectors  who  have  observed  elsewhere 

1  PENROSE,  R.  A.  F.,  JR.:  Some  Causes  of  Ore  Shoots.  Econ.  Geol.,  vol. 
5,  pp.  97-133,  1910. 

VAN  HISE,  C.  R.:  Some  Principles  Controlling  the  Deposition  of  Ore. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  30,  pp.  27-177,  1900. 

IRVING,  J.  D. :  The  Localization  of  Values  in  Ore  Bodies  and  the  Occur- 
rence of  "Shoots"  in  Metalliferous  Deposits.  Econ.  Geol.,  vol.  3,  pp.  143- 
154,  1908.  Discussion  by  F.  C.  SMITH,  idem,  pp.  224-229;  R.  H.  SALES, 
idem,  pp.  326-331;  F.  L.  RANSOME,  idem,  pp.  331-337;  H.  V.  WINCHELL, 
idem,  pp.  425-428;  H.  SJOGREN,  idem,  pp.  637-643;  WALDEMAR  LINDGREN, 
idem,  vol.  4,  .pp.  56-61,  1909. 

PURINGTON,  C.  W.:  Ore  Horizons  in  the  San  Juan  Mountains.  Econ. 
Geol.,  vol.  1,  pp.  130-134,  1907. 

LINDGREN,  WALDEMAR,  and  RANSOME,  F.  L. :  Geology  and  Gold  Deposits 
of  Cripple  Creek,  Colo.  U.  S.  Geol.  Survey  Prof.  Paper  54,  pp.  271-496, 
1906. 

SPURR,  J.  E.,  and  GARREY,  G.  H.:  Economic  Geology  of  the  Georgetown 
Quadrangle,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper  63,  pp.  210-411, 
1908. 


94        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

similar  barren  outcrops  that  pass  into  valuable  deposits  beneath 
the  surface.  Profitable  deposits  as  a  rule  are  explored  as  they 
are  mined.  "Ore  against  ore"  is  an  old  Cornish  dictum.  It  is 
good  prospecting  to  follow  the  ore  body  as  far  as  practicable  and 
to  explore  adjoining  territory  for  similar  deposits.  The  ore  body 
is  followed  to  its  ends  on  drifts,  raises,  or  winzes.  These  work- 
ings are  generally  pressed  ahead  beyond  the  workable  deposit  or 
ore  shoot.  Valueless  vein  matter  near  a  deposit  is  prospected 
because  there  is  generally  a  fair  chance  of  finding  ore  in  association 
with  it. 

A  "blind"  deposit  is  one  that  does  not  crop  out.  The  country 
rock  on  all  sides  of  a  valuable  deposit  is  regarded  as  favorable 
ground  for  prospecting  unless  the  structural  relations  are  un- 
favorable. Thus  blind  deposits  may  be  discovered  near  those 
that  are  exposed. 

Ore  shoots  may  be  of  primary  or  secondary  origin.  A  great 
many  of  them  are  formed  by  superficial  alteration  and  enrich- 
ment; these  are  discussed  on  pages  124  to  169.  Many,  however, 
have  been  formed  by  primary  processes  at  the  same  time  as  the 
deposits  containing  them.  Primary  ore  shoots  are  discussed  in 
the  chapters  treating  the  several  classes  of  primary  ores,  the  open- 
ings in  rocks,  and  the  structural  features  of  epigenetic  deposits. 
In  this  chapter  some  of  the  causes  commonly  controlling  their 
formation  are  reviewed. 

.  If  magmatic  segregation  is  brought  about  by  gravity,  the 
heavier  metals  should  sink  as  they  do  in  bullion  or  matte  in  an 
ore  furnace.  At  Sudbury,  Ontario,  the  nickeliferous  pyrrhotite 
is  at  the  bottom  of  the  Sudbury  laccolith,  particularly  in  embay- 
ments  in  the  underlying  rock  (page  12)  and  in  dikes  that  make 
off  from  the  bottom  of  the  laccolith.  If  segregation  takes  place 
at  considerable  depth,  however,  the  metalliferous  matter  may 
be  injected  into  fissures  like  ordinary  dikes.  The  entire  dike 
may  be  the  deposit,  as  at  Iron  Mountain,  Wyo.,  where  a  dike  of 
titaniferous  iron  ore  over  a  mile  long  and  40  to  300  feet  wide  has 
been  thrust  into  anorthosite.  *  In  places  magmatic  segregation 
forms  bands  or  layers  of  ore  alternating  with  bands  or  layers 
of  valueless  minerals.  The  origin  of  these  deposits  is  not  clear. 
Much  remains  to  be  done  before  the  problems  of  distribution  of 
ore  shoots  segregated  in  igneous  rocks  are  all  solved. 

1  BALL,  S.  H. :  Titaniferous  Iron  Ore  of  Iron  Mountain,  Wyo.  U.  S.  Geol. 
Survey  Bull.  315,  pp.  206-214,  1907. 


PRIMARY  ORE  SHOOTS  95 

In  many  pegmatites  the  little  pockets  or  "nests"  that  contain 
gems  or  other  valuable  minerals  are  distributed  with  extreme 
irregularity.  In  some  pegmatite  veins,  however,  they  are  ar- 
ranged rudely  in  zones,  as  at  the  Mount  Mica  mine,  Paris, 
Maine1  (page  22).  Pipe-like  bodies  of  pegmatite  are  not  un- 
common. Butler2  has  described  a  tree-like  pegmatitic  body  of 
quartz  in  the  San  Francisco  region,  Utah,  in  which  ore  shoots  are 
located  on  the  branch-like  extensions  from  the  central  body. 

Contact-metamorphic  zones  are  zones  only  in  the  broad  sense. 
The  rocks  replaced  are  not  all  equally  hospitable  to  ore  deposition; 
some  are  more  easily  replaced  than  others,  and  in  these  the  re- 
placement is  likely  to  extend  farther  from  the  intruding  mass. 
Beds  of  pure  limestone  are  generally  more  extensively  replaced 
than  impure  limestones,  shales,  and  sandy  rocks.  Not  only  are 
the  zones  of  metamorphism  erratic  in  their  outlines,  but  the 
bodies  of  metalliferous  ores  within  such  zones  are  in  general 
irregularly  distributed.  In  some  districts  they  are  found  by 
prospecting  a  geologic  formation  at  a  horizon  that  experience  has 
shown  is  favorable  for  their  development.  Contact-metamorphic 
deposits  are  likely  to  be  developed  most  abundantly  in  regions 
where  igneous  intrusives  are  closely  spaced  and  in  connection 
with  intrusives  of  certain  types  (see  pages  29  to  47). 

Ore  veins  and  similar  deposits,  whether  they  were  formed  in 
the  deep  vein  zone,  at  moderate  depths,  or  near  the  surface,  are 
generally  not  uniformly  mineralized  but  vary  greatly  in  size 
and  richness.  Those  formed  at  moderate  depth  and  those  formed 
near  the  surface  more  commonly  contain  rich  primary  ore  shoots 
and  bonanzas  than  those  formed  at  greater  depth,  although  rich 
primary  deposits  may  be  formed  also  in  the  deep  vein  zone. 
Certain  depths  of  formation  are  more  favorable  than  other 
depths.  The  maximum  deposition  often  takes  place  also  where 
the  more  open  channels  are  provided  and  where  they  are  most 
favorably  situated.  These  channels  may  be  intergranular  spaces 
in  sedimentary  rock  or  openings  along  bedding  planes.  The 
richer  deposits  are  sometimes  found  where  the  rocks  were  most 
highly  shattered  when  the  ore  solutions  penetrated  them.  Thus 
shear  zones,  sheeted  zones,  or  areas  of  maximum  fracturing  and 

1  BASTIN,  E.  S. :  Geology  of  the  Pegmatites  and  Associated  Rocks  of 
Maine.     U.  S.  Geol.  Survey  Bull  445,  p.  84,  1911. 

2  BUTLER,  B.  S.:  Geology  and  Ore  Deposits  of  the  San  Francisco  and 
Adjacent  districts,  Utah.     U.  S.  Geol.  Survey  Prof.  Paper  80,  p.  125,  1913. 


96        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


brecciation  commonly  control  deposition.  The  planes  of  maxi- 
mum fracturing  may  be  along  faults,  at  crests  of  folds,  on  conjug- 
ated fissures,  or  along  certain  brittle  beds  or  dikes.  Intersections 
of  fissures  may  influence  deposition  favorably.  Where  the  fissures 
meet  at  a  small  angle  the  rocks  near  the  line  of  junction  are  likely 
to  form  thin  wedges  that  are  highly  shattered  and  more  easily 
replaced.1  The  mingling  of  solu- 
tions brought  together  at  the  junc- 
tion of  two  crossing  fissures  may 
promote  deposition.2  Suitable 
channels  may  be  provided  also 
when  dikes  or  any  other  brittle 
bodies  are  crossed  by  fissures,  and 
their  intersections  may  become 
sites  of  greater  mineralization  than 
other  parts  of  fissures.  One 
country  rock  may  be  more  favor- 
able to  deposition  than  another  on 
account  of  its  chemical  composition 
or  because  it  is  more  easily  frac- 
tured. If  a  fissure  carries  ore 
where  it  cuts  a  certain  dike  or  bed, 
ore  bodies  should  be  sought  for  at 
its  inter-section  with  similar  dikes 
or  beds  and  also  where  these  dykes 
or  beds  are  crossed  by  other  fissures. 
Some  long,  slender  ore  bodies  appear 
to  have  been  formed  along  channels 
where  ascending  waters  or  gases 
rose  to  the  surface  through  perme- 
able rocks,  to  issue  as  hot  springs. 
The  remarkable  ore  shoot  at  the  Anna  Lee  mine,3  Cripple  Creek, 
Colo.,  is  a  long,  nearly  vertical  body  of  ore  about  15  to  25  feet 

1  PENROSE,  R.  A.  F.,  JR.  :  Some  Causes  of  Ore  Shoots.     Earn.  Geol,  vol. 
5,  p.  110,  1910. 

2  VAN  HISE,  C.  R.:  Some  Principles  Controlling  the  Deposition  of  Ores. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  30,  p.  85,  1900. 

3 GROSS,  WHITMAN,  and  PENROSE,  R.  A.  F.,  JR.:  Geology  and  Mining 
Industries  of  the  Cripple  Creek  District,  Colorado.  U.  S.  Geol.  Survey, 
Sixteenth  Ann.  Rept.,  part,  2,  p.  205,  1895. 

LINDGREN,  WALDEMAR  and  RANSOME,  F.  L. :  Geology  and  Gold  Deposits 
of  Cripple  Creek,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper  54,  p.  448, 1906. 


Fio.  41. — Stereogram  of  Anna 
Lee  ore  chimney,  Cripple  Creek, 
Colorado.  (After  V.  G.  Hitts.) 


PRIMARY  ORE  SHOOTS  97 

in  diameter,  which  follows  a  nearly  vertical  basic  dike  that  is 
locally  brecciated  (see  Fig.  41).  It  has  probably  formed  where 
one  or  more  fissures  cross  the  dike.  At  the  Bassick  mine,  Colo- 
rado,1 a  nearly  vertical  ore  shoot  over  800  feet  long  occurs  in 
an  agglomerate  that  has  filled  the  neck  of  an  old  volcano.  The 
ore  deposit,  according  to  S.  F.  Emmons,  is  connected  genetically 
with  decadent  volcanic  activity. 

Some  rocks,  because  they  are  impervious  or  not  readily  frac- 
tured, obstruct  mineralizing  solutions  and  promote  deposition 
near  their  contacts.  Thus  at  Rico,  Colo.,2  and  at  the  Bremen 
mine,  New  Mexico,3  ore  shoots  are  formed  in  limestone  at  the 
contact  with  impervious  shale.  The  impermeable  gouge  in  some 
of  the  northwest  veins  at  Butte,  Mont.,4  has  locally  dammed 
back  the  solutions  and  prevented  continuous  mineralization. 

In  some  veins  the  ore  shoots  are  nearly  parallel  to  the  dips  of 
the  veins ;  in  others  they  plunge  to  one  side  or  the  other.  There 
is  no  universal  rule,  although  in  some  districts  the  larger  number 
of  ore  shoots  plunge  in  the  same  direction. 

Deposits  formed  at  moderate  and  shallow  depths  by  cold 
solutions  may  contain  either  primary  or  secondary  ore  shoots. 
Some  of  these  deposits  show  a  close  relationship  to  the  structure 
of  the  region.  In  the  Joplin  district5  the  more  valuable  deposits 
are  in  limestone  near  the  border  of  overlying  shale,  which  is 
believed  to  have  diverted  the  circulation  of  the  ore  solutions, 
causing  them  to  rise  to  the  surface.  Some  of  the  larger  deposits 
are  in  ancient  caves  that  were  formed  in  Mississippian  rocks 
before  the  Pennsylvanian  rocks  were  deposited  on  them;  others 
are  in  brecciated  zones  and  basal  breccia  beds.  In  the  Wisconsin 
region  conditions  for  deposition  were  favorable  in  shallow  basins 
or  synclines,  where  shrinkage  of  an  oil  shale  provided  openings 

1  EMMONS,  S.  F. :  Geology  of  Silver  Cliff  and  the  Rosita  Hills,  Colorado. 
U.  S.  Geol.  Survey  Seventeenth  Ann.  Rept.,  part  2,  p.  435,  1896. 

2  RICKARD,  T.  A.:  The  Enterprise  Mine,  Rico,  Colorado.     Am.  Inst.  Min. 
Eng.  Trans.,  vol.  26,  pp.  906-980,  1896. 

RANSOME,  F.  L.:  The  Ore  Deposits  of  the  Rico  Mountains,  Colorado. 
U.  S.  Geol.  Survey  Twenty-second  Ann.  Rept.,  part  2,  pp.  291-312,  1901. 

3  PENROSE,  R.  A.  F.,  JR.:  Some  Causes  of  Ore  Shoots.     Econ.  Geol,  vol. 
5,  p.  117,  1910. 

4  SALES,  R.  H. :  The  Localization  of  Values  in  Ore  Bodies  and  the  Occur- 
rence of  Shoots  in  Metalliferous  Deposits.     Econ.  Geol,  vol.  3,  p.  330,  1908. 

5  SIEBENTHAL,  C.  E. :  Origin  of  the  Zinc  and  Lead  Deposits  of  the  Joplin 
Region,  Missouri,  Kansas,  and  Oklahoma.     U.  S.  Geol.  Survey  Butt,  606,  p. 
26,  1915. 

7 


98        THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

for  deposition  and  also  a  precipitating  agent.1  The  occurrence 
of  deposits  formed  by  precipitation  in  and  around  organic  matter 
depends  on  the  distribution  of  such  matter,  which  in  turn  de- 
pends on  the  conditions  of  sedimentation  at  the  time  the  rocks 
inclosing  the  deposits  were  formed. 

Sedimentary  deposits  are  not  uniformly  metalliferous.  In 
placer  deposits  there  may  be  richer  sands  or  gravels  at  the  bottom 
of  the  bed,  just  above  the  bedrock  or  above  a  stiff  clay  seam. 
By  scour  and  fill  the  waters  of  the  stream  stir  up  the  gravel, 
allowing  the  heavier  part  to  sink  as  it  does  in  a  miner's  pan. 
Generally  not  all  the  river  bed  carries  gold.  As  a  rule  the  gold 
is  accumulated  in  streaks  and  in  largest  amounts  in  places  where 
the  stream's  velocity  is  checked.  On  a  meandering  stream  the 
placers  are  likely  to  accumulate  on  the  inner  sides  of  the  bends. 

Basal  conglomerates  of  iron  ore  (page  86)  may  be  classed  as 
ore  sh  Dts;  generally  they  occur  in  beds  that  lie  unconformably 
above  ron-bearing  formations. 

In  any  sedimentary  series  the  beds  vary  greatly  in  composi- 
tion. Beds  of  nearly  pure  iron  oxide  or  iron  carbonate  may  alter- 
nate with  chert.  The  individual  beds  range  in  width  from  less 
than  y±  inch  to  many  feet.  Such  variations  are  common  and 
are  attributed  to  changes  in  the  conditions  of  sedimentation. 
These  conditions  are  varied  and  complex,  and  their  causes  are  in 
large  part  obscure.  Structural  studies  may  show  where  the 
valuable  beds  are,  but  it  is  generally  impossible  to  predict  their 
changes  in  advance  of  exploration.  Ores  of  aluminum,  man- 
ganese, and  other  metals  may  be  segregated  in  beds  or  parts  of 
beds. 

From  this  brief  discussion  of  examples  of  primary  ore  shoots 
it  is  evident  that  the  occurrence  of  such  shoots  depends  on  a 
great  variety  of  conditions  and  causes.  The  controlling  factors 
in  one  district  may  not  control  in  another.  Each  district 
should  be  studied  as  a  separate  problem,  and  the  laws  that 
govern  ore  deposition  within  it  should  be  established  independ- 
ently. By  mapping  the  geology  the  structure  is  made  clear,  and 
then  the  structural  relations  .of  the  ore  bodies  are  apparent. 
Although  the  geologist  may  not  accurately  predict  in  advance 
of  exploration  where  the  richer  primary  ore  bodies  may  be  found, 

1  GRANT,  U.  S. :  Structural  Relations  of  the  Wisconsin  Zinc  and  Lead 
Deposits.  Earn.  Geol.,  vol.  1,  pp.  233-242,  1906;  Wis.  Geol.  and  Nat.  Hist. 
Survey  Bull.  14,  1906. 


PRIMARY  ORE  SHOOTS 


99 


he  may  indicate  the  more  favorable  places  to  look  for  them, 
and  in  a  district  where  rocks  are  adequately  exposed  at  the 
surface  or  where  underground  workings  are  extensive,  structural 
studies  are  almost  certain  to  yield  data  that  will  aid  in  pros- 
pecting and  developing  the  district.  The  narrow  study  of  the 
deposits  themselves  without  regard  to  the  structure  of  the  region 
that  contains  them  is  likely  to  lead  to  error. 

The  laws  controlling  superficial  alteration  and  the  develop- 
ment of  secondary  ore  shoots  are  by  no  means  simple,  but  they 
are  more  easily  interpreted  than  those  controlling  the  deposition 
of  primary  ore  shoots.  These  laws  are  treated  in  Chapter  XV 
(pages  124  to  169).  They  should  be  constantly  in  the  mind  of 
one  who  is  seeking  to  ascertain  the  causes  for  the  occurrence  and 
distribution  of  the  primary  ores  and  of  ore  shoots  in  a  mining 
district. 


CHAPTER  XII 
DEFORMATION  OF  ORE  DEPOSITS 

The  treatment  of  ore  deposits  in  this  volume  recognizes  three 
groups  of  processes — the  deposition  of  ore  bodies,  the  deformation 
of  ore  bodies  and  the  superficial  alteration  and  enrichment  of 
ore  bodies.  Many  ore  deposits,  however,  have  not  been  de- 
formed, and  a  considerable  number  are  not  appreciably  enriched 
by  superficial  alteration.  Some  deformed  deposits  and  some 
that  are  not  deformed  are  workable,  though  superficial  enrich- 
ment has  not  enhanced  their  value.  Other  ore  bodies  after 
their  deposition  have  been  both  deformed  and  enriched. 

Deformation  is  essentially  a  physical  process,  involving  mass 
movement,  but  it  may  be  attended  by  some  chemical  changes. 
Superficial  alteration  and  enrichment  are  in  the  main  chemical 
processes,  involving  molecular  movement,  although  some  mass 
movement  may  attend  chemical  changes.  The  deformation  of 
ore  deposits  is  merely  incidental  to  the  deformation  of  the  con- 
taining rocks.  Economic  investigations,  however,  are  largely 
studies  of  the  geologic  structures  of  ore-bearing  areas,  and  struc- 
tural geology  warrants  a  more  extended  treatment  in  connection 
with  economic  studies  than  can  be  given  here.  For  discussions 
of  structural  geology  the  reader  is  referred  to  standard  textbooks 
of  geology.1 

The  character  of  primary  ore  bodies  depends  in  large  measure 
on  their  depth  at  the  time  of  their  deposition.  Depth  is  a  factor 
no  less  important  in  deformation.  The  earth's  crust  may  be 
regarded  as  divided  into  three  zones,  characterized  by  the  nature 
of  deformation — a  zone  of  fracture  near  the  surface,  where  all 

xSee  especially  CHAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:  "Geology," 
Vol.  I,  Processes,  1905. 

LEITH,  C.  K,  "Structural  Geology,"  1913. 

VAN  HISE,  C.  R.:  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey 
Mori.  47,  1904. 

PIRSSON,  L.  V.,  and  SCHTJCHERT,  CHARLES:  "A  Textbook  of  Geology," 
vol.  1,  Physical  Geology,  1915. 

LAHEE,  F.  H.:  "Field  Geology,"  1916. 
100 


DEFORMATION  OF  ORE  DEPOSITS  101 

rocks  will  break;  a  zone  of  flowage  below  the  surface,  where  even 
the  stronger  rocks  are  not  strong  enough  to  hold  spaces  open 
under  the  pressures  that  prevail;  and  a  zone  of  combined  frac- 
ture and  flowage,  between  these  two,  where  strong  rocks  break 
and  weak  ones  flow.  This  conception  of  deformation  has  been 
developed  principally  by  Van  Hise  and  Leith. 

Rocks  differ  greatly  in  crushing  strength.  Shales  will  flow 
at  shallow  depths;  quartzites  and  igneous  rocks  are  strong  enough 
to  hold  fractures  open  several  miles  below  the  surface.  The 
zone  of  combined  fracture  and  flowage  is  of  great  extent,  em- 
bracing most  of  the  zone  that  comes  under  human  observation. 

Faulting  is  characteristic  of  the  zone  of  fracture,  and  in  areas 
of  rocks  deformed  at  shallow  depths  normal  faults  are  generally 
more  common  than  reverse  faults,  although  the  latter  are  not 
unknown,  even  at  the  surface.  In  the  zone  of  combined  frac- 
ture and  flowage  thrust  faulting  or  reverse  faulting  is  conspicu- 
ously shown.  In  the  zone  of  flowage  close  folding  is  character- 
istic. There  are  great  areas  in  the  United  States  where  normal 
faulting  is  common  and  reverse  faulting  with  folding  is  rare, 
and  other  areas  where  folding  and  thrust  faulting  are  common 
and  normal  faulting  is  practically  unknown.  Thus  there  are 
deformation  provinces,  each  characterized  by  a  certain  type 
of  deformation,  just  as  there  are  petrographic  provinces  and 
metallogenic  provinces. 


CHAPTER  XIII 

FAULTING  AND  FOLDING  OF  ORE  DEPOSITS 
FAULTING   OF   ORE   DEPOSITS 

General  Features. — Faulting  is  a  process  of  great  interest  in 
economic  geology,  because  some  ore  deposits  occur  along  faults 
and  many  are  displaced  by  faults.  The  mineralization  of  faults 
is  discussed  on  pages  192-196.  The  present  chapter  is  concerned 
principally  with  faulting  as  a  process  of  deformation  of  rocks  and 
ores.  The  subject  presents  many  difficulties.  We  do  not  know 
the  nature  of  all  the  stresses  that  result  in  faulting  or  what  be- 
comes of  faults  as  they  pass  downward,  yet  by  detailed  mapping 
of  districts  it  may  be  possible  to  follow  faulted  ore  bodies  and 
predict  their  position  accurately.  At  Butte,  Mont.,  the  com- 
plexity of  the  faulting  is  appalling;  yet,  equipped  with  abundant 
data  gained  through  years  of  careful  mapping,  the  geologists  of 
the  companies  that  operate  there  are  enabled  to  locate  ore  bodies 
with  great  precision  in  advance  of  exploration.  In  many  dis- 
tricts the  data  are  sufficient  to  work  out  systems  that  aid  in 
exploration. 

The  various  elements  of  faults  and  their  nomenclature  have 
recently  been  discussed  in  considerable  detail.1  In  1909  the 
Geological  Society  of  America  appointed  a  committee  to  investi- 
gate the  subject.  This  committee  has  recommended  a  compre- 
hensive nomenclature,2  the  adoption  of  which  would  bring  about 

^PURR,  J.  E.:  "Geology  Applied  to  Mining,"  1904.  The  Measurement 
of  Faults.  Jour.  Geol,  vol.  5,  p.  723,  1897. 

RANSOMS,  F.  L.:  The  Direction  of  Movement  and  Nomenclature  of 
Faults.  Econ.  Geol.,  vol.  1,  p.  777,  1906. 

TOLMAN,  C.  F.:  Graphic  Solution  of  the  Fault  Problem.  Min.  and  Sci. 
Press,  1911.  How  Should  Faults  be  Named  and  Classified?  Econ.  Geol., 
vol.  2,  pp.  506-511,  1907. 

REID,  H.  F.:  Geometry  of  Faults.  Geol.  Soc.  America  Bull.,  vol.  20,  pp. 
171-196,  1909. 

CHAMBERLIN,  T.  C.:  The  Fault  Problem.  Econ.  Geol.,  vol.  2,  pp.  585- 
601,  704-724,  1906. 

2  REID,  H.  F.,  DAVIS,  W.  M.,  LAWSON,  A.  C.,  and  RANSOME,  F.  L. :  Report  of 
the  Committee  on  the  Nomenclature  of  Faults.  Geol.  Soc.  America  Bull, 
vol.  24,  pp.  163-186,  1913. 

102 


FA  ULTING  AND  FOLDING  OF  ORE  DEPOSITS    103 

uniform  usage  of  terms  and  thus  aid  in  the  correlation  of  data. 
Some  of  the  following  definitions  and  figures  are  taken  from  its 
report. 

A  fault  is  a  fracture  along  which  there  has  been  notable  dis- 
placement. Any  fracture  is  accompanied  by  some  movement, 
otherwise  there  would  not  be  a  fracture;  but  the  term  "fault"  is 
not  applied  to  movements  at  right  angles  to  a  fracture  plane, 
but  only  to  those  where  it  can  be  shown  that  one  or  the  other  wall 
has  been  moved  along  the  fracture  plane  (see  Figs.  42,  43,  44). 
Fault  problems  are  in  the  main  simple  problems  of  solid  geome- 
try, and  many  interesting  theoretical  combinations  may  be  solved 
as  problems  of  construction.  But  notwithstanding  their  mathe- 
matical simplicity,  the  solution  of  many  fault  problems  in  the 
field  is  difficult  because  the  data  are  generally  insufficient  to  show 
precisely  the  direction  and  amount  of  the  movements.  Where 


FIG.  42. — Normal  fault.       FIG.  43. — Reverse  fault.       FIG.  44. — Horse  in  fault. 


an  ore  body  is  cut  off  by  a  fault,  only  a  careful  consideration  of 
all  the  data  gained  by  accurate  mapping  will  suffice,  and  even 
then  it  is  not  always  possible  to  locate  the  other  end  of  the  faulted 
body. 

The  fault  strike  is  the  direction  of  the  intersection  of  the 
fault  surface  with  a  horizontal  plane — that  is,  a  level  line 
along  a  fault.  The  fault  dip  is  the  inclination  of  the  fault 
surface  measured  at  right  angles  to  the  strike  on  the  plane  of  the 
fault.  The  hade  is  the  inclination  of  the  fault  surface,  measured 
from  the  vertical;  it  is  the  complement  of  the  dip.  The  hanging 
wall  is  the  upper  wall  of  the  fault.  The  foot  wall  is  the  lower  wall 
of  the  fault.  A  fault  block  may  move  perpendicular  to  the 
strike  of  a  fault  plane,  or  parallel  to  it,  or  its  path  may  make  an 
acute  angle  with  the  strike.  If  one  part  of  the  block  moves 
farther  in  a  given  direction  than  another  part,  the  block  is  said 
to  rotate. 


104      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

There  are  two  ways  of  defining  the  displacement  caused  by  a 
fault :  the  apparent  relative  displacement  of  a  bed  may  be  defined 
by  naming  the  distance  between  its  two  disrupted  portions 


7 


FIG.  45. — Elements  of  faults.  The  upper  and  lower  surfaces  of  the  blocks 
are  horizontal;  the  end  faces  are  vertical  and  at  right  angles  to  the  fault  strike. 
a,  b,  c,  and  d  lie  in  the  fault-plane;  e,  f,  and  k,  in  the  end  face.  Let  the  point 
originally  adjacent  to  a  move  to  b;  then  ab  =  slip  or  net  slip;  cb  =  dip  slip; 
ac  =  strike  slip;  bd  =  perpendicular  slip;  ad  =  trace  slip;  fk  =  throw;  ek  = 
heave. 

measured  in  any  chosen  direction,  such  as  the  vertical  distance 
between  the  two  portions,  measured  in  a  shaft,  or  the  perpen- 


FIG.  46.— Slip  and  shift.  The  plane  beg  is  perpendicular  to  ac;  df  is  parallel 
to  ac.  ab,  slip  or  net  slip;  ac,  strike  slip;  be,  dip  slip;  eg,  throw;  bg,  heave;  de, 
shift;  df,  strike  shift;  fe,  dip  shift. 

dicular  distance  between  the  lines  of  intersection  of  the  two  por- 
tions with  the  fault  plane;  or  the  actual  relative  displacement 
of  the  two  sides  in  certain  directions  may  be  defined.  The  ap- 


FAULTING  AND  FOLDING  OF  ORE  DEPOSITS    105 

parent  displacement  is  usually  measured  directly;  the  actual 
displacement  must  generally  be  worked  out  later. 

The  slip  is  the  relative  displacement  of  formerly  adjacent 
points  on  opposite  sides  of  the  fault,  measured  on  the  fault  sur- 
face (Figs.  45,46).  The  meaning  of  strike  slip  and  dip  slip  is 
indicated  on  the  diagrams.  The  shift  is  the  relative  displace- 
ment of  regions  outside  the  dislocated  zone.  If  the  fault  is  a 
clean-cut  fracture  and  there  is  no  bending  of  the  strata,  the  slip 
and  shift  are  identical.  Throw  and  heave  refer  to  the  displace- 
ment of  the  edge  of  a  disrupted  bed,  as  measured  on  the  vertical 
section  of  a  block. 

"Displacement"  and  "dislocation"  are  given  no  technical 
meaning  but  may  be  applied  to  a  relative  movement  of  bodies 
on  the  two  sides  of  the  fault,  measured  in  any  direction,  if  that 


FIG.  47. — Apparent  vertical  and  horizontal  displacements.  If  the  section 
is  at  right  angles  to  the  strike  of  the  fault  and  the  fault  movement  is  down  the 
dip,  the  dip  slip  is  net  slip,  ac,  the  vertical  component,  is  the  throw;  be,  the 
horizontal  component,  is  the  heave. 

direction  is  specified,  or  to  the  change  in  position  of  a  bed  or  other 
feature,  caused  by  the  fault  movement  (Fig.  47). 

In  discussing  sedimentary  rocks  (or  other  rocks  that  have 
planes  of  reference)  the  following  terms  are  used : 

A  strike  fault  is  one  whose  strike  is  parallel  to  the  strike  of  the 
strata.  A  dip  fault  is  one  whose  strike  is  approximately  at  right 
angles  to  the  strike  of  the  strata.  An  oblique  fault  is  one  whose 
strike  is  oblique  to  the  strike  of  the  strata.  A  bedding-plane 
fault  is  one  whose  surface  is  parallel  with  the  bedding  of  the 
stratified  rocks. 

The  separation  of  the  bed  or  vein  or  of  any  recognizable  plane 
is  the  distance  between  the  corresponding  surfaces  of  the  dis- 
rupted bed  or  other  tabular  body,  measured  between  corre- 
sponding surfaces  on  the  two  sides  of  the  fault,  in  any  indicated 


106      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

direction.  The  vertical  separation  is  the  separation  measured 
along  a  vertical  line.  The  horizontal  separation  is  the  separation 
measured  in  any  indicated  horizontal  direction. 

The  normal  horizontal  separation  of  a  bed  or  other  surface  is 
its  horizontal  separation  measured  at  right  angles  to  the  strike 


FIG.  48. — Plan  of  oblique 
fault,  ab,  offset  or  normal 
horizontal  separation;  ac, 
horizontal  separation  along 
fault;  be,  gap. 


FIG.  49. — Plan  of  oblique 
fault.  ab,  offset;  ac,  hori- 
zontal separation  along  fault; 
fee,  overlap. 


of  the  bed.  It  is  frequently  determined  from  the  outcrops  of 
the  bed  at  the  surface  of  the  ground,  and  is  then  usually  called 
the  offset  of  the  bed  (see  Figs.  48  and  49). 


F  SECTION  AB  SECTION  CD 

FIG.  50. — Reverse  fault  due  to  horizontal  displacement. 

Normal  faults  are  those  along  which  the  hanging  wall  appears 
to  have  been  depressed  relatively  to  the  foot  wall. 

Reverse  faults  are  those  along  which  the  hanging  wall  appears 
to  have  been  raised  relatively  to  the  foot  wall. 

The  terms  "normal"  and  "reverse"  designate  the  apparent 
displacement  of  the  two  parts  of  a  dislocated  bed  or  other  recog- 


FAULTING  AND  FOLDING  OF  ORE  DEPOSITS    107 

nized  surface  in  a  vertical  plane  at  right  angles  to  the  fault 
strike.  It  does  not  follow  that  in  an  oblique  fault  a  horizontal 
line  at  right  angles  to  the  fault  strike  would  be  lengthened  if  the 
fault  were  normal  or  shortened  if  it  were  reverse.  This  may  be 
illustrated  by  Fig.  50,  which  shows  an  oblique  strike  fault,  ap- 
parently reverse,  that  is  formed  by  horizontal  movement. 
Frequently  nothing  more  than  the  apparent  displacement  of 
the  strata  can  be  determined.  The  terms  "normal"  and 
"reverse"  faults  are  used  merely  for  purposes  of  description 
and  not  for  the  purpose  of  indicating  extension  or  contraction. 

Overthrusts  are  reverse  faults  with  low  dip.  In  some  over- 
thrusts  the  dip  slip  is  great,  amounting  to  several  miles. 

A  fault  block  is  a  mass  bounded  on  its  sides,  completely  or  in 
part,  by  faults.  A  horst  is  a  mass  elevated  relatively  to  the  sur- 
rounding masses  and  separated  from  them  by  faults.  A  graben 
is  a  mass  depressed  relatively  to  the  surrounding  masses  and 
separated  from  them  by  faults.1 

A  fault  mosaic  is  an  area  divided  by  intersecting  faults  into 
blocks  that  have  settled  in  varying  degrees. 

Fault  strice  are  scratches  on  the  walls  of  faults  formed  by  abra- 
sion of  hard  particles.  Striae  show  the  direction  of  movement 
along  the  walls.  It  is  not  safe  to  assume  that  all  the  faulting 
movement  was  in  the  direction  indicated  by  the  striae;  obviously 
the  last  movement  only  may  be  recorded.  On  some  faults 
two  sets  of  striae  cross,  showing  different  movements  at  different 
times.  Fault  grooves  are  undulations  deeper  than  striae  but 
similarly  formed.  Because  they  usually  record  larger  stresses 
they  have  greater  significance  as  indicating  direction  of  movement. 

Searching  for  Faulted  Segments. — Where  detailed  mapping 
does  not  show  the  amount  or  direction  of  movement  along  a 
fault,  certain  general  rules  are  sometimes  applied.  These  rules 
are  based  on  experience  in  different  areas  and  if  used  intelligently 
are  of  much  service.  They  should  be  applied  only  when  mapping 
of  beds,  dikes,  or  other  horizons  of  reference  fails  to  show  the 
direction  of  displacement.  Fault  problems  are  simple  enough 
if  the  data  at  hand  are  adequate,  but  for  many  problems  all  that 
can  be  done  is  to  make  the  most  intelligent  use  of  inadequate 
data.  Four  sets  of  conditions  may  be  considered:  (1)  faulting 
of  one  homogeneous  formation,  (2)  faulting  of  flat  tabular  bodies, 

1  The  material  beginning  at  this  point  is  not  taken  from  the  report  of  the 
committee  cited  on  page  102. 


108      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

(3)  faulting  of  inclined  tabular  bodies,  (4)  faulting  of  intersecting 
bodies. 

1.  If  faulting  has  dislocated  a  homogeneous  body — for  ex- 
ample, a  great  uniform  mass  of  igneous  rock — it  may  not  be 
possible  to  show  even  approximately  how  much  movement  has 
taken  place.  The  walls  of  fissures  may  be  striated  or  scratched 
by  hard  particles  rubbing  against  them,  or  they  may  be  "case- 
hardened"  or  "  slickensided "  by  attrition,  or  rounded  boulders 
or  friction  breccia  may  be  found  in  and  along  the  fissures.  All 
these  features  suggest  that  there  has  been  movement  parallel  to 
the  fissure,  although  it  is  not  possible  to  prove  any  displace- 
ment because  there  are  no  horizons  of  reference.  Here  there  is 
a  noteworthy  difference  in  nomenclature  that  grows  out  of  the 
natural  limitations  of  field  study.  In  surface  mapping  of  areas 
that  do  not  contain  mineral  deposits  as  a  rule  only  fissures  that 
show  displacement  are  mapped  as  faults,  because  friction  breccia, 
slickensiding,  and  striated  faces  are  not  so  readily  discovered  at 
the  surface.  But  underground,  where  fresh  rocks  are  more 
generally  exposed,  these  evidences  of  movement  may  be  con- 
spicuous, and  although  there  is  no  way  of  measuring  the  move- 
ment, many  investigators  will  term  a  fissure  that  shows  them, 
a  fault. 

Suppose  that  a  vein  in  a  homogeneous  rock,  say  in  granite,  is 
cut  off  at  a  fault.  It  is  important  to  find  the  other  part  of  the 
displaced  ore  body.  Such  problems  come  up  frequently  in 
many  districts.  If  the  fault  involves  other  veins  or  several 
rocks,  detailed  mapping  of  the  surface  or*underground  may  show 
both  the  direction  and  the  amount  of  separation.  If  the  fault 
involves  no  other  rocks  or  ore  bodies,  mapping  will  not  suffice. 
A  short  drift  along  the  fault  may  show  that  the  faulted  end  of 
the  vein  is  curved  (Fig.  51),  and  obviously  this  curved  end  will 
point  toward  the  other  part  of  the  vein.  Again,  in  one  direction 
from  the  ore  body  the  fault  zone  may  carry  "  drag  ore  "  or  crushed 
and  brecciated  vein  matter  (Fig.  52),  whereas  in  the  other  direc- 
tion it  may  be  barren.  On  many  faults,  however,  the  vein  is 
not  curved  near  the  break,  and  on  some  no  drag  ore  is  shown; 
still  others  may  show  drag  ore  along  the  fault  in  both  directions 
as  it  is  followed  away  from  the  ore  body.  Under  these  conditions, 
in  regions  where  normal  faults  prevail,1  exploration  is  directed 

1  In  the  western  United  States  most  of  the  faults  that  displace  the  Ter- 
tiary ores  are  normal  faults. 


FAULTING  AND  FOLDING  OF  ORE  DEPOSITS    109 


on  the  assumption  that  the  fault  is  normal — that  is,  that  the  hang- 
ing wall  of  the  fault  has  dropped.     This  rule  is  justified  by  ex- 


FIG.  51. — Vein  curved  near  fault. 


Fio.  52. — Drag  ore  along  fault  zone. 


perience.     A  tabulation  of  all  faults  that  involve  the  ore  bodies  in 
a  number  of  regions  of  Tertiary  ore  deposits  shows  that  more 


FIG.   53. — Duplication   of   vein   by  faulting.      On   one  level  parallel   drifts   are 
run  on  the  same  vein.     The  fault  is  a  broad  zone. 

than  85  per  cent,  are  normal  faults.  At  Butte,  Mont.,  and  in 
some  other  districts,  however,  some  of  the  veins  are  cut  off  by 
reverse  faults.  Each  district  presents  a  separate  problem. 


110      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Many  faults  are  not  clean-cut  fissures  but  closely  spaced  par- 
allel fractures  along  which  the  movement  has  been  distributed 
(Fig.  53).  Commonly  much  material  is  ground  up  in  such  a  zone, 
and  waters  attacking  such  material  will  alter  it  to  "clay," 
making  it  slippery  and  "heavy"  ground.  At  some  places  it  is 
difficult  to  hold  up  this  ground,  and  for  this  reason  drifting  along 
faults  is  avoided  in  some  mines  as  far  as  practicable.  It  is  com- 
mon practice  to  drive  a  drift  through  the  fault,  continuing  it 
beyond  in  the  direction  the  vein  would  take  if  it  were  not  faulted, 
and  then  to  crosscut  to  one  side  or  the  other  for  the  vein. . 

In  many  districts  there  is  a  kind  of  rhythmic  succession  of 
faults,  the  planes  of  a  system  being  nearly  parallel  and  the  spac- 
ing, throws,  and  displacements  of  several  faults  are  of  the  same 
order  of  magnitude.  Conspicuous  examples  may  be  seen  at 
Tonopah  (Wandering  Boy  and  Montana-Tonopah  veins)  and 
Bullfrog,  Nev. 

The  relations  reviewed  above  emphasize  the  importance  of 
detailed  mapping,  even  in  an  area  of  a  homogeneous  rock,  before 
extensive  exploration  is  undertaken  to  find  a  faulted  vein. 

2.  In  flat-lying  bedded  rocks  there  are  horizontal  planes  of 
reference,  and  all  movements  that  are  not  in  the  planes  of  the 
beds  are  shown.     Movements  parallel  to  the  beds  will  not  be 
recorded  by  offset  and  can  not  be  measured  except  where  some 
crosscutting  feature,  such  as  a  dike  or  an  older  fault,  is  involved 
in  the  movement.     Faults  in  flat-lying  beds  are  generally  normal. 
Not  all  faulting  movements  are  directly  downward  along  the 
plane  of  the  fault— that  is,  at  90°  to  the  strike  of  the  fault. 
There  may  be  a  horizontal  element,  as  shown  along  the  fault 
plane  by  fault  striae  that  are  inclined  to  the  dip  of  the  plane, 
indicating  that  the  downward  pull  of  gravity  was  accompanied 
by  lateral  stress.     As  lateral  stress  is  generally  relieved  partly  by 
tilting  or  folding  the  beds,  it  is  generally  safe  to  assume  that  if 
beds  are  flat  after  faulting,  the  faults  have  been  normal  and  there 
has  not  been  much  horizontal  movement. 

3.  Areas  of  tilted  bedded  rocks  may  contain  either  normal  or 
reverse  faults.     Faulted  beds  are  generally  tilted  (Fig.  54)  or 
folded. 

In  tilted  beds  faults  are  marked  by  offsets  of  the  beds  except 
where  the  planes  of  movement  are  parallel  to  the  beds.  The 
greater  the  angle  between  the  bedding  planes  and  the  fault  the 
greater  will  be  the  apparent  displacement  with  the  same  move- 


FAULTING  AND  FOLDING  OF  ORE  DEPOSITS    111 

ment.  The  problems  of  this  group  are  often  more  difficult  than 
those  of  the  groups  discussed  above,  because  the  faults  com- 
monly lie  with  the  beds  (Fig.  55)  or  make  only  small  angles  with 


FIG.  54. — Section  of  bedding-plane  deposit  cut  by  normal  faults,  Combination 
mine,  Philipsburg  quadrangle,  Montana. 

them.  As  a  rule  in  field  practice  it  is  possible  usually  to  identify 
only  beds  or  horizons.  It  is  an  exception  when  a  particular  point 
on  a  bed  on  one  side  of  a  fault  can  be  correlated  with  a  correspond- 


joo  Feet 


FIG.  55. — Plan  of  Headlight  vein,  near  Philipsburg,  Montana.  The  vein 
cuts  across  the  bedding  of  the  country  rock  and  is  displaced  by  faults  that 
follow  the  bedding  plants. 

ing  point  on  the  other  side,  and  when  the  cross  section  is  studied 
the  ends  that  are  separated  along  the  fault  are  commonly 
regarded  as  having  joined  before  the  faulting.  This  assumption 
is  warranted  only  in  areas  where  the  fault  movements  are  at 


112      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

right  angles  to  the  strikes  of  the  faults  and  on  sections  also  normal 
to  the  strike.  In  such  areas  fault  striae  would  all  be  parallel  to 
the  dip,  but  the  areas  where  such  conditions  exist  are  in  the 
minority.  Here,  again,  the  investigator  may  profit  by  study  of 
other  faults  in  the  same  area.  If  it  is  found  that  all  other  faults 
in  the  area  are  normal  it  is  highly  probable,  though  not  certain, 
that  the  fault  under  consideration  is  normal,  and  if  in  the 
absence  of  striae  on  the  fault  plane  it  is  found  that  on  other 
faults  in  the  area  the  striae  are  nearly  parallel  to  the  dip,  or  if 
structural  studies  show  that  horizontal  movement  is  not  great 
along  other  faults  in  the  area,  the  most  probable  hypothesis  is 
that  the  downward  element  is  the  most  important  in  the*  fault 
under  consideration. 

A  fault  that  is  located  exactly  on  the  wall  of  a  tabular  ore  body 
can  not  displace  it.  But  as  most  "tabular"  ore  bodies,  so-called, 
bend  more  or  less,  and  most  faults  also  have  curved  planes,  a 
fault  that  is  at  most  places  parallel  to  a  thin  ore  body  may 
terminate  the  ore  body  by  even  a  slight  bend.  It- is  good  practice 
to  drift  along  or  (in  heavy  ground)  near  such  a  fault  and  to  cross- 
cut in  the  walls  at  short  intervals.  Strike  faults  are  character- 
istic of  areas  of  close  folding  and  thrust  faulting,  and  they  are 
by  no  means  rare  in  regions  where  normal  faulting  predominates. 

4.  The  intersection  of  two  planes  is  a  line.  Sedimentary 
beds  cut  across  by  veins  or  dikes  or  containing  "ribbons  of  ore" 
may  afford  horizons  of  reference  marked  off  by  lines.  If  a 
fault  cuts  across  such  a  line,  all  the  elements  may  be  deter- 
mined after  the  two  parts  of  the  line  or  linear  ore  body  have  been 
found.  In  field  practice,  however,  the  end  of  a  line  is  more  diffi- 
cult to  discover  than  a  plane  or  tabular  body.  The  approxi- 
mate direction  of  faulting  may  re'adily  be  determined,  but  to 
determine  the  amount  of  faulting  is  more  difficult.  Unless 
numerous  horizons  of  reference  and  numerous  exposures  are 
available,  the  task  of  finding  the  faulted  segment  of  a  small  linear 
body  is  almost  hopeless.  Where  tabular  masses  are  faulted, 
however,  the  problem  is  to  find  the  plane  or  zone  that  may  carry 
ore  rather  than  the  exact  points  or  lines  that  are  cut  by  faults. 
In  homogeneous  rocks,  where  "lines"  or  "ribbons"  of  ore  are 
faulted,  striae  may  show  the  direction  of  movement  but  not  its 
amount.  Where  this  set  of  conditions  prevails  the  limitations 
of  field  study  are  obvious. 


FAULTING  AND  FOLDING  OF  ORE  DEPOSITS    113 

FOLDING  OF  ORE  DEPOSITS 

When  rocks  are  broken  in  blocks  and  faulted,  they  are  gener- 
ally tilted  and  commonly  they  are  flexed  or  folded.  Rocks 
may  be  folded  in  the  zone  of  fracture  by  movement  along  many 
small  faults  or  along  joints.  This  is  commonly  the  case  where 
brittle  rocks  are  deformed  near  the  surface.  If  rocks  containing 
ore  deposits  are  folded  in  the  zone  of  fracture,  any  deposits  of 
brittle  minerals  they  contain,  such  as  quartz,  will  be  folded  by 
fracture. 

When  rocks  are  folded  in  the  zone  of  flowage  there  is  a  move- 
ment of  their  minute  particles  attended  by  recrystallization  and 
changes  in  the  thickness  of  the  beds.1  In  the  zone  of  flowage 
the  mineral  character  of  the  beds  or  deposits  is1  generally  exten- 
sively altered.  These  changes,  due  to  folding  in  the  zone  of 
flowage  and  recrystallization,  are  phenomena  attending  dynamic 
metamorphism  of  the  rock  or  ore  body.  Folding  may  take  place 
without  dynamic  metamorphism,  but  dynamic  metamorphism  is 
practically  always  attended  by  much  folding.  Deformation  of 
ore  deposits  by  folding  in  the  zone  of  flowage  is  treated  in  Chapter 
XIV,  on  the  dynamic  metamorphism  of  ore  deposits. 

1  LEITH,  C.  K. :  Structural  Geology,  p.  109,  1913. 

VAN  HISE,  C.  R. :  Principles  of  Pre-Cambrian  Geology.  U.  S.  Geol. 
Survey  Sixteenth  Ann.  Rept.,  part  1,  pp.  631-664,  1896. 


CHAPTER  XIV 
DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS 

General  Character. — Any  ore  deposit,  however  formed,  may 
be  metamorphosed  by  dynamic  processes.  The  terms  "dynamic 
metamorphism,"  "regional  metamorphism,"  and  "dynamo-re- 
gional metamorphism"  are  frequently  used  interchangeably  in 
describing  these  processes.  "Regional  metamorphism"  is  often 
used  to  imply  that  a  considerable  body  or  mass  of  rocks  is  affected. 
As  deep  burial  and  pressure  are  necessary  conditions  for  dynamic 
metamorphism,  it  follows  that  the  processes  are  not  selective  and 
that  all  rocks  which  were  present  within  the  metamorphic  mass 
at  the  time  of  dynamic  metamorphism  were  subjected  to  pressure, 
although  they  generally  show  the  effects  of  pressure  in  different 
ways  and  in  different  degrees. 

Owing  to  its  loss  of  heat  and  to  other  causes,  the  earth  is 
shrinking.  The  central  part  shrinks  more  rapidly  than  the  outer 
portion  of  the  lithosphere.  Consequently  the  outer  shell, 
drawn  inward  by  gravity,  must  wrinkle  in  order  to  fit  the  interior. 
As  a  result  the  lithosphere  or  shell  is  warped  or  folded  to  form 
the  mountain  ranges;'  the  rocks  are  subjected  to  great  stresses, 
acting  horizontally,  or  rather  tangentially,  along  the  great  circles 
of  the  earth.  Rocks  that  are  not  deeply  buried  may  be  broken 
into  huge  blocks,  and  at  the  surface  of  the  earth  these  blocks  are 
moved  about  more  or  less  freely  as  independent  masses.  Conse- 
quently fracturing  and  faulting  take  place  on  a  large  scale 
when  the  rocks  near  the  surface  are  deformed  by  compressive 


Rocks  that  are  deeply  buried  are  held  down  by  the  superin- 
cumbent load  and  can  not  move  about  so  freely  as  independent 
blocks.  At  great  depths,  or  where  the  overlying  load  is  suffi- 
ciently heavy,  the  stresses  are  greater  than  the  crushing  strengths 
of  the  rocks,  which,  however,  vary  greatly,1  as  is  shown  by  the 
following  table: 

JVAN  HISE,  C.  R.:  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey 
Mon.  47,  pp.  1011-1013,  1904. 

LEITH,  C.  K:  Rock  Cleavage.     U.  S.  Geol.  Survey  Bull.  239,  1905. 
114 


DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS  115 

Pounds  per  square  inch 

Granite 15,000  to  30,000 

Sandstone 8,000  to  12,000 

Lime'stone 3,000  to  14,000 

Shale 1,000  to    3,000 

Clay  and  mud 0  to      500 

Strong  granite,  which  weighs  about  165  pounds  a  cubic  foot, 
is  rigid  enough  to  bear  a  mass  of  granite  about  5  miles  high.  If 
factors  like  differential  strains  and  the  weakness  of  mortar  are 
disregarded,  a  block  of  granite  in  the  lower  course  of  a  stone 
monument  would  theoretically  bear  the  strain  of  the  weight  of 
5  miles  of  granite  above  it.  But  if  a  sufficient  weight  were  placed 
above  it,  the  lower  course  would  fail.  Sandstone  and  limestone 
would  fail  under  much  lighter  loads,  and  shale  would  fail  under 
a  load  lighter  still.  When  a  rock  is  so  deeply  buried  that  the 
weight  above  it  exceeds  its  crushing  strength,  open  spaces  or  con- 
tinuous fractures  will  be  closed  by  the  failure  of  the  rock,  which 
acts  somewhat  as  a  viscous  mass  and  is  said  to  be  deformed  by 
"flowage." 

At  considerable  depths  rocks  will  hold  spaces  open  under 
greater  pressures  than  near  the  surface,  because  their  openings 
may  contain  water,  and  the  water  pressure  counterbalances  some 
of  the  pressure  on  the  rocks.  When  corrections  are  made  for 
this  factor  and  for  increased  rigidity  due  to  lateral  support,1 
it  appears  that  while  some  shales  may  flow  at  depths  less  than 
1,500  feet,  the  strongest  rocks  would  probably  not  flow  at  depths 
of  considerably  more  than  6  miles.2  If  a  mass  composed  of 
several  formations  that  differ  in  strength  is  deformed  by  pressure, 
some  of  the  rocks  may  flow  while  others  fracture  (see  Fig.  56). 
The  rock  mass  is  then  in  the  zone  of  combined  fracture  and  flow- 
age.3  This,  is  a  deep  zone;  as  already  stated,  some  clays  and 
shales  will  flow  almost  at  the  surface,  but  granite  and  other 
strong  rocks  might  fracture  rather  than  flow  at  depths  of  several 
miles.  Very  commonly  masses  composed  of  rocks  of  two  or 

1  ADAMS,  F.  D. :  An  Experimental  Contribution  to  the  Question  of  the 
Depth  of  the  Zone  of  Flowage  in  the  Earth's  Crust.  Jour.  Geol,  vol.  20,  pp. 
97-118,  1912. 

2LEiTH,  C.  K:  "Structural  Geology,"  p.  3,  1913. 

VAN  HISE,  C.  R. :  Principles  of  North  American  Pre-Cambrian  Geology 
U.  S.  Geol.  Survey  Sixteenth  Ann.  Rept.,  part  1,  pp.  581-845,  1896. 

3  VAN  HISE,  C.  R. :  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey 
Mon.  47,  p.  748,  1904. 


116      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

more  kinds  will,  after  deformation  by  pressure  under  load,  ex- 
hibit structure  characteristically  found  in  the  zone  of  combined 
fracture  and  flowage.  The  crushing  strength  of  ore  bodies 


FIG.  56. — Plan  of  part  of  115-foot  level,  Milan  mine,  New  Hampshire.      The 
pyritic  quartzose  copper  ore  fractured  while  the  mica  schist   "flowed." 

depends  upon  the  component  minerals  and  their  arrangement. 
In  general  ore  bodies  are  stronger  than  argillaceous  rocks  (see 
Fig.  56). 


3lnche; 


FIG.  57. — Sulphide  ore  metamorphosed  by  pressure,     a,  quartz;  b,  pyrite;  c, 
chlorite  schist. 

Rocks  that  are  deformed  under  great  load  by  pressure  (ana- 
morphism) undergo  certain  characteristic  changes.  The  brittle 
minerals,  such  as  quartz  and  feldspar,  are  mashed,  cemented, 


DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS  117 

and  recrystallized.  Mica,  chlorite,  and  amphibole  are  generally 
developed.  These  platy  or  fibrous  minerals  are  arranged  gener- 
ally with  their  long  dimension  normal  to  the  direction  of  greatest 
pressure,  thus  giving  the  rock  schistosity  or  slaty  cleavage  (see 
Fig.  57).  Extensive  recrystallization  may  take  place,  with  the 
development  of  garnet,  staurolite,  ottrelite,  andalusite,  and  other 
heavy  silicates.  These  minerals  are  sometimes  called  the  "por- 
phyritic  minerals"  of  schist,  and  in  many  schistose  rocks  they 
are  not  arranged  parallel  to  the  schistosity.  The  processes  of 
anamorphism  are  treated  at  length  by  Van  Hise,  Leith,  and  others 
in  papers  cited  above. 

Orientation  of  Ore  Bodies. — Elongated  or  flat  minerals  show 
a  strong  tendency  to  be  oriented  with  the  longer  dimensions 
parallel  to  the  direction  of  least  pressure,  and  the  same  tendency 
is  shown  by  ore  bodies  that  are  inclosed  in  yielding  rocks.  Quartz 
veins  are  broken  into  smaller  bodies  which  generally  lie  approxi- 
mately with  the  schistosity.  In  an  area  that  is  undergoing  meta- 
morphism  and  that  contains  quartz  veins  striking  in  several  direc- 
tions, the  veins  which  strike  across  the  direction  of  least  pressure 
are  more  likely  to  be  broken  into  smaller  bodies  than  those  which 
lie  nearly  in  the  direction  of  least  pressure.  In  the  Ducktown 
district,  Tennessee,  and  in  the  Ellijay  quadrangle,  Georgia,1 
the  schists  contain  thousands  of  small  quartz  masses  many  of 
which  are  probably  portions  of  ruptured  veins.  At  a  few  places 
these  veins  are  bent  into  sharp  folds,  but  generally  they  are  broken 
to  form  short  lenticular  or  spindle-shaped  masses,  some  of  them 
not  much  longer  than  they  are  wide.  A  number  of  quartz  lenses 
in  alignment  with  the  schistosity,  and  approximately  in  line,  may 
represent  the  separated  portions  of  larger  masses. 

Large,  thick  masses  of  the  harder  rock  may  not  be  broken 
apart,  and  separated,  but  will  generally  be  lengthened  in  the 
direction  of  the  schistosity. 

Dynamically  metamorphosed  deposits  may  have  been  faulted 
near  the  surface  before  they  were  deeply  buried;  they  may  have 
been  faulted  after  any  part  or  all  of  the  great  load  of  overlying 
rock  was  removed  by  erosion;  or  they  may  have  been  faulted 
when  the  deposits  and  the  inclosing  country  rock  were  in  the 
zone  of  combined  fracture  and  flowage.  Faults  formed  in  the 
zone  of  combined  fracture  and  flowage  should  not  penetrate  the 

1  PHALEN,  W.  C.:  On  a  Peculiar  Cleavage  Structure  Resembling  Stretched 
Pebbles  near  Ellijay,  Georgia.  Jour.  Geol,  vol.  18,  pp.  554-564,  1910. 


118       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

weaker  or  "incompetent"  rocks  for  any  considerable  distance, 
but  such  rocks  may  flow  along  the  plane  of  separation.  If  flowage 
has  occurred  it  may  generally  be  shown  by  mapping  the  schis- 
tosity  of  the  country  rock  along  the  fault  and  especially  at 
the  two  ends,  of  the  broken  ore  body.  In  the  Milan  mine,  New 
Hampshire,  a  fault  laps  around  both  ends  of  the  displaced  ore 
body  much  like  the  letter  S,  and  it  does  not  penetrate  the  walls 
of  schist  beyond  the  two  ends  of  the  broken  ore  body  (Fig.  56). 
In  the  schists  the  fault  is  tight.  If  the  area  containing  the  ore 
bodies  had  been  subjected  to  more  intense  deformation  after  fault- 
ing, doubtless  the  evidence  of  faulting,  which  is  now  preserved, 
would  have  been  obliterated  in  the  schists  between  the  two  ends. 

Ore  bodies  in  the  shape  of  carinate  folds,  and  close  folds  of 
the  commoner  types  may  have  been  metamorphosed  by  pressure 
or  they  may  have  replaced  older  tabular  beds  that  were  metamor- 
phosed. The  deformation  of  rock  masses  composed  of  formations 
of  various  strengths  is  discussed  by  Willis,1  and  by  Leith  and 
Mead.2  In  experiments  with  layers  of  different  waxes  under 
pressure  Willis  produced  structures  resembling  those  exhibited  by 
some  dynamically  metamorphosed  ores. 

Chemical  Changes  During  Metamorphism. — When  ore  bodies 
under  load  are  subjected  to  heavy  stresses  great  changes  may 
take  place,  but  they  probably  do  not  involve  the  introduction  of 
large  amounts  of  material.  This  view  is  not  shared  by  those 
who  regard  the  "segregated  vein"  as  a  body  of  ore  brought  to- 
gether during  dynamic  metamorphism  by  solutions  searching 
great  masses  of  rock  and  concentrating  in  a  smaller  mass  the 
metals  which  before  metamorphism  were  widely  scattered  through 
the  great  masses. 

Bastin3  has  taken  averages  of  hundreds  of  analyses  of  shales, 
slates,  pelites,  and  schists  and  found  certain  clearly  expressed 
chemical  relations  which  recur  throughout  the  different  series, 
exhibiting  various  degrees  of  metamorphism.  His  averages  of 
analyses  indicate  that  little  material  is  added.  The  mineral 
changes  are  due  to  rearrangement  of  the  elements  of  the  shale  or 

1  WILLIS,    BAILEY:  The   Mechanics   of    Appalachian  Structure.     U.    S. 
Geol.  Survey  Thirteenth  Ann.  Rept.,  part  2,  pp.  211-281,  1892. 

2  LEITH,  C.  K,  and  MEAD,  W.  J.:  "Metamorphic  Geology,"  pp.  161-168, 
1915. 

3  BASTIN,   E.   S. :  Chemical  Composition  as  a  Criterion  in  Identifying 
Metamorphosed  Sediments.     Jour.  Geol,  vol.  17,  pp..  445-472,  1909. 


DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS  119 

slate  rather  than  to  the  introduction  of  new  elements.  Although 
these  investigations  indicate  that  there  is  but  little  gain  of 
material  during  dynamic  metamorphism  of  aluminous  sedi- 
mentary rocks,  a  loss  of  material  may  take  place  during  metamor- 
phism, especially  losses  of  the  more  soluble  substances,  such  as 
lime  carbonate.'1 

Although  the  ore  and  gangue  minerals  have  a  greater  crushing 
strength  than  the  aluminous  minerals  of  quartzitic  shale  and  are 
therefore  more  competent  to  hold  spaces  open,  the  same  princi- 
ples will  probably  apply  to  them.  The  processes  which  operate 
in  the  regional  or  dynamic  metamorphism  of  ore  bodies  are  solu- 
tion, reprecipitation,  mashing,  dehydration,  deoxidation,  and  ce- 
mentation. These  changes  are  attended  by  the  formation  of  com- 
plex minerals  of  high  specific  gravity  that  occupy  less  space  than 
simple  minerals.  The  elements  are  rearranged  within  the  ore 
body;  there  may  be  losses,  but  probably  little  material  is  added. 
However,  where  igneous  bodies  intrude  rocks  at  great  depths 
under  heavy  load,  igneous  metamorphism  may  take  place,  and 
it  may  be  attended  by  the  addition  of  much  material.  Many 
investigators  believe  that  waters  from  deep  sources  migrate 
considerable  distances  through  rocks  deeply  buried  in  the  zone  of 
flowage. 

Mineral  Composition. — As  deposits  of  any  character  may  be 
metamorphosed  by  pressure,  the  metamorphosed  deposits  contain 
a  great  variety  of  minerals.  Many  of  these  minerals  were  doubt- 
less formed  by  primary  processes  and  have  endured  throughout 
the  metamorphic  changes,  but  others  have  been  formed  while  the 
deposits  have  been  deeply  buried  and  compressed.  Garnet, 
chlorite,  epidote,  zoisite,  mica,  and  amphibole  are  very  com- 
monly present.  Even  if  they  do  not  occur  in  the  primary  de- 
posits, one  or  all  of  them  may  be  formed  during  metamorphism. 
The  sulphides  are  probably  little  changed,  at  least  in  kind.  By 
dehydration  and  reduction,  hematite  and  magnetite  are  assumed 
to  be  developed.  By  some  investigators  pyrrhotite  is  believed 
to  be  characteristic  of  these  deposits,2  but  in  some  of  the  metamor- 
phosed pyritic  deposits  it  is  lacking. 

1  LEITH,  C.  K.,  and    MEAD,   W.   J. :   "  Metamorphic   Geology,"   p.   226, 
1915. 

2  KLOCKMAN,  F. :  Ueber  den  Einfluss  der  Metamorphose  auf  die  mineral- 
ische  Zusammensetzung  der  Kieslagerstatten.     Zeitschr.  prakt.  Geologie,  vol. 
2,  pp.  13,  153,  1904. 


120      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Texture  and  Paragenesis. — When  rock  matter  is  metamor- 
phosed by  heavy  pressure  in  the  zone  of  flowage  it  tends  to  be- 
come schistose.  The  particles  of  hard  minerals,  such  as  quartz, 
may  be  rotated  and  oriented  so  that  their  short  dimensions  lie  in 
the  direction  of  greatest  stress;  new  crystals  will  form,  and  their 
short  axes  also  will  lie  in  the  direction  of  greatest  stress.  The 
micas,  chlorites,  amphiboles,  and  other  platy  or  fibrous  minerals, 
all  lying  with  their  greater  dimensions  parallel  and  also  parallel 
to  the  long  dimensions  of  quartz  or  other  hard  minerals,  give 
character  to  the  schistose  texture  or  slaty  cleavage.1  Although 
some  ores  show  schistose  texture  it  is  not  so  commonly  developed 


FIQ.  58. — Polished  surface  of  schistose  ore  metamorphosed  by  pressure,  Milan 
mine,  New  Hampshire. 

in  perfection  in  them  because  micaceous  or  fibrous  minerals  are 
less  common  in  metamorphosed  ore  bodies  than  they  are  in  many 
metamorphosed  rocks.  Most  of  the  sulphides,  moreover,  recrys- 
tallize  readily,  and  continued  metamorphism  will  cause  recrys- 
tallization  and  obliteration  of  any  schistose  texture  that  may 
have  been  previously  formed.  In  certain  metamorphosed  pyritic 
bodies  of  Maine  and  New  Hampshire,  contained  in  chloritic 
schists,  the  ore  near  the  margins  of  the  deposits  is  composed  of 
quartz,  pyrite,  and  chlorite  and  shows  a  well-defined  schistosity 
(Fig.  57).  The  central  portions  of  the  lodes,  which  are  composed 
of  quartz  and  pyrite  without  chlorite  or  other  platy  or  fibrous 
minerals,  show  no  schistosity  whatever,  although  the  pyrite  when 
»LEITH,  C.  K:  "Structural  Geology,"  p.  76,  1913. 


DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS  121 

examined  microscopically  is  seen  to  be  shattered  and  recemented.1 
If  two  sulphides  like  chalcopyrite  and  pyrite  are  present,  these 
may  be  mashed  and  partly  recrystallized  to  form  a  banded 
ore — a  sulphide  schist  (Fig.  58) .  If  the  flaky  or  fibrous  materials, 
such  as  mica,  chlorite,  and  amphibole,  are  contained  in  the  ore, 
these  will  generally  be  oriented  parallel  to  the  direction  of  schis- 


FIG.  59. — Schistose  ore  from  Deer  Isle,  Maine,  showing  parallel  bands  of 
garnet,  chlorite,  sericite,  and  sulphides,  a,  Chorite  and  sericite;  b,  fractured 
garnet;  c,  pyrite,  zinc  blende,  and  galena.  . 

tosity  in  the  country  rock.     Garnet  bands  may  be  extensively 
shattered  (Fig.  59). 

When  an  association  of  minerals  has  been  compressed  under  a 
load  sufficient  to  permit  the  softer  minerals  to  flow  and  the  harder 
and  stronger  minerals  to  break,  the  soft  minerals  will  fill  cracks 
and  cement  the  fractured  particles  of  the  harder  minerals. 


Scale  of  Feet 


FIG.  60. — Plan  of  ore  deposit  at  Rammelsberg,  Germany. 

Adams,2  in  a  series  of  experiments  on  deformation,  placed 
various  crystals  in  strong  tubes  which  were  filled  with  wax  or 
some  other  plastic  substance  before  they  were  sealed.  Under 
great  pressure  the  softer  minerals  were  deformed  by  flowage  and 
recrystallization.  The  ease  with  which  flowage  structure  was 

1  EMMONS,  W.  H. :  Some  Ore  Deposits  of  Maine  and  the  Milan  Mine,  New 
Hampshire.     U.  S.  Geol.  Survey  Bull.  432,  p.  19,  1910. 

2  ADAMS,  F.  D. :  An  Experimental  Investigation  into  the  Action  of  Differ- 
ential Pressure  on  Certain  Minerals  and  Rocks,  Employing  the  Process 
Suggested  by  Prof.  Kick.     Jour.  Geol,  vol.  18,  pp.  489-525,  1910. 


122      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

produced  was  found  to  vary  inversely  with  the  hardness  of  the 
minerals. 

In  the  Rammelsberg  deposits,  in  the  Harz  Mountains,  Ger- 
many (Figs.  60,  61),  the  soft  sulphides,  sphalerite,  chalcopyrite, 
galena,  and  arsenopyrite,  are  pressed  out  in  streaks  and  squeezed 
around  the  harder  pyrite  masses;  the  structure  is  like  that  of  a 
gneiss.1 

Ore  bodies,  like  rocks,  have  doubtless  suffered  all  degrees  of 
dynamic  action.  In  some  the  minerals  may  be  merely  cracked 
and  bent.  In  others,  where  material  for  forming  platy  or 
fibrous  minerals  was  available  and  where  pressures  were  suffi- 
cient, the  ore  may  have  become  schistose.  High  temperature 
and  pressure  cause  sulphides  to  recrystallize,  and  as  the  most  com- 
mon of  these  are  approximately  isodiametric,  the  crystals  do  not 


FIG.  61. — Cross- section  showing  ore  deposit  at  Rammelsberg,   Germany. 

readily  produce  a  schistose  texture.  In  sulphide  deposits  schistos- 
ity  is  doubtless  developed  in  the  earlier  stages  of  metamorphism. 
Subsequently,  if  pressure  and  heat  become  sufficiently  great,  the 
sulphide  ore  will  recrystallize  and  schistosity  will  be  destroyed. 

Dynamically  metamorphosed  sulphide  deposits  rarely  show 
vugs  lined  with  banded  crusts.  If  the  wall  rock  was  hydrother- 
mally  altered  when  the  primary  ores  were  formed,  the  platy 
hydrothermal  minerals,  such  as  mica  and  chlorite,  will  be  recrys- 
tallized  and,  by  pressure,  oriented  so  that  their  longer  axes  lie 
in  the  direction  of  the  schistosity  of  the  region. 

Deposits  Inclosed  in  Schists  but  not  Dynamically  Meta- 
morphosed.— Many  ore  bodies  in  schists  have  not  been  meta- 

1  LINDGREN,  WALDEMAR,  and  IRVING,  J.  D. :  The  Origin  of  the  Rammels- 
berg Ore  Deposit.  Econ.  Geol.,  vol.  6,  pp.  303-313,  1911. 


DYNAMIC  METAMORPHISM  OF  ORE  DEPOSITS  123 

morphosed.  They  were  deposited  after  the  metamorphism  of 
the  schists  and  do  not  exhibit  the  structure  characteristic  of 
metamorphosed  deposits.  Some  districts  of  schistose  rocks  con- 
tain deposits  formed  before  metamorphism  and  also  deposits 
formed  after  metamorphism.  In  New  England1  both  types  of 
deposits  are  represented. 

Age  Relations  of  Dynamically  Metamorphosed  Deposits. — As 
deep  burial  is  necessary  for  dynamic  metamorphism,  the  older 
rocks  and  ores  are  more  commonly  affected  by  such  processes 
than  the  younger  rocks  and  ores,  which  in  general  have  not  been 
so  deeply  buried.  Ore  deposits  formed  in  late  geologic  time  at 
moderate  and  shallow  depths  are  rarely  metamorphosed  by 
dynamic  processes.  But  deposits  formed  in  earlier  geologic 
time,  even  at  the  surface,  may  after  deep  burial  be  intensely 
metamorphosed  by  pressure;  some  of  the  iron  ores  of  the  Ver- 
milion district,  Minnesota,  and  of  the  Marquette  district,  Mich- 
igan, that  were  concentrated  by  surface  weathering,  have  been 
deeply  buried  and  converted  by  pressure  into  schistose  hematites. 

1  EMMONS,  W.  H. :  Some  ore  deposits  of  Maine  and  the  Milan  mine,  New- 
Hampshire.  U,  S.  Geol.  Survey  Bull.  432,  pp.  14-22,  1910. 


CHAPTER  XV 

SUPERFICIAL  ALTERATION  AND  ENRICHMENT  OF  ORE 
DEPOSITS 

General  Features. — The  earth  is  commonly  regarded  as  com- 
posed of  a  core — the  lithosphere — surrounded  by  two  shells,  the 
hydrosphere  and  the  atmosphere.  The  atmosphere  or  air  con- 
tains also  water,  and  the  hydrosphere,  or  water  sphere,  contains 
some  air.  Both  air  and  water  penetrate  the  lithosphere.  Weath- 
ering results  where  rocks,  air,  and  water  come  together — where 
rocks  of  the  lithosphere  are  attacked  by  air  and  water. 

Rocks  and  ores  exposed  to  air  and  water  at  or  near  the  surface 
of  the  earth  break  down  and  form  soluble  salts  and  minerals  that 
are  stable  under  surface  conditions.  Few  minerals  that  are  long 
exposed  to  air  and  water  remain  unaltered;  some,  however,  are 
much  more  resistant  to  weathering  than  others  and  these  be-' 
come  concentrated  when  material  .associated  with  them  is  re- 
moved. Weathering  usually  precedes  erosion,'  and  many  valu- 
able beds,  such  as  clay,  sand,  iron  ore,  and  placer  gold,  become 
concentrated  through  weathering  and  aggradation  working  to- 
gether. The  subject  treated  here  is  the  weathering  of  material 
in  place,  and  particularly  the  leaching,  and  enrichment  of  mineral 
deposits  by  weathering.1 

By  weathering,  many  low-grade  ores  and  protores  are  con- 
verted into  valuable  deposits.  Enrichment  may  be  brought 
about  by  solution  and  removal  of  valueless  material,  leaving  the 
weathered  material  in  a  more  concentrated  state;  or  it  may  be 
brought  about  by  solution  of  valuable  materials  and  their  precipi- 

1  For  more  extended  discussions  of  rock  weathering  the  student  is  referred 
to  the  following  papers : 

MERRILL,  G.  P.:  "Rocks,  Rock  Weathering,  and  Soils,"  1897. 

CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3ded.  U.  S.  Geol.  Survey 
Bull.  616,  1915. 

VAN  HISE,  C.  R.:  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey 
Mon.  47,  1903. 

EMMONS,  W.  H.:  The  Enrichment  of  Ore  Deposits.  U.  S.  Geol.  Survey 
Bull.  625,  1917. 

LEITH,  C.  K.,  and  MEAD,  W.  J.:  "Metamorphic  Geology,"  1915. 

WATSON,  T.  L. :  The  Granites  of  Georgia.  Ga.  Geol.  Survey  Bull.  9-A,  1903. 
124 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  125 

tation  in  depth.  Some  metals  are  readily  dissolved  near  the 
surface  in  oxygenated  waters,  and  are  readily  precipitated  at 
depths  where  air  is  excluded  and  where  the  solutions  are  ren- 
dered neutral  by  reacting  on  ores  and.  rocks.  Many  sulphide 
deposits  are  leached  of  metals  near  the  surface  but  are  enriched 
near  the  water  level  where  air  is  excluded  and  precipitation  takes 
place.  Concentration  by  weathering  and  related  processes  is 
secondary  enrichment.  Because  the  chemical  process  in  rocks 
is  notably  different  from  chemical  processes  in  the  presence  of 
appreciable  sulphides,  it  is  practicable  to  treat  them  separately. 

Ores  formed  by  the  weathering  of  rocks  are  ordinarily  derived 
from  material  of  very  low  grade;  many  investigators  consider 
them  a  separate  class  of  primary  ore.  The  geologic  processes, 
however,  are  closely  related  to  those  by  which  workable  sulphide 
deposits  are  concentrated  from  lean,  unworkable  protore.  The 
material  weathered,  whether  rock  or  ore,  determines  the  kind 
and  in  general  the  value  of  the  product  of  weathering.  Thus 
iron  ore  will  result  from  thorough  weathering  of  iron-rich  rocks, 
such  as  peridotite,  diabase,  greenalite,  and  cherty  iron  carbonate 
•rocks.  Bauxite  will  form  from  nepheline  syenites  and  other 
aluminum-rich  rocks.  These  rocks  that  are  especially  rich  in 
certain  metals  are  classed  as  protores  of  those  metals. 

The  processes  of  weathering  and  also  of  sulphide  enrichment 
are  closely  allied  to  the  deposition  of  ore  bodies  at  moderate 
depths  by  cold  solutions.  There  is  this  difference,  however: 
the  ore  bodies  mentioned  are  formed  at  places  where  no  metallif- 
erous rock  may  have  existed  previously,  and  many  though  not 
all  are  formed  at  or  near  places  where  there  was  some  reducing 
agent  such  as  carbonaceous  material.  Metalliferous  products 
of  weathering  and  sulphide  enrichment  occupy  in  the  main  the 
spaces  formerly  occupied  by  lower-grade  metalliferous  material 
from  which  the  workable  deposits  were  derived.  By  weathering 
and  enrichment  some  small  bodies  of  ores  may  be  formed  in 
and  along  cracks  outside  of  the  older  parent  metalliferous  rock, 
but  these  are  generally  of  only  subordinate  importance.  They 
form,  however,  a  gradational  type  between  the  secondary  deposits 
and  the  primary  deposits  formed  at  shallow  or  moderate  depths 
by  cold  solutions-  (pages  74  to  83). 

WEATHERING  OF  ROCKS 

The  term  weathering  includes  all  the  processes  by  which  rocks 
near  the  surface  exposed  to  water  and  air  are  gradually  decom- 


126      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

posed  and  broken  up.  These  processes  are  active  mainly  above 
the  water  level.  Rain  water  carries  oxygen  and  some  carbon 
dioxide,  which  render  it  an  active  solvent;  changes  of  tempera- 
ture, frost,  and  vegetation  loosen  the  material  and  render  it  more 
permeable.  The  alkalies  in  rocks  are  readily  dissolved,  especially 
sodium.  Alkaline  earths  are  attacked  also,  and  calcium  and 
magnesium  go  into  solution.  Silica  dissolves  less  readily,  but 
the  alkalies  render  the  solutions  more  active  solvents  of  silica. 
The  metals  iron  and  aluminum  are  slowly  dissolved,  iron  in  gen- 
eral more  rapidly  than  aluminum.  New  minerals  are  formed, 
especially  oxides,  hydroxides,  and  carbonates;  most  carbonates, 
however,  are  unstable  under  conditions  of  thorough  weathering. 

As  a  rule  the  weathering  of  igneous  rocks  will  increase  the  vol- 
ume where  expansion  is  possible,1  but  this  increase  is  only  tempo- 
rary, for  by  solution  material  is  removed.  Most  ores  that  have 
been  concentrated  by  weathering  show  much  pore  space  due  to 
the  removal  of  material  by  solution.  In  many  weathered  rocks 
the  pore  space  amounts  to  50  per  cent,  or  more.  The  develop- 
ment of  pore  space,  however,  weakens  the  rock,  and  the  pore 
space  may  be  partly  eliminated  by  slumping. 

Of  the  minerals  attacked  by  weathering  some  are  compara- 
tively stable.  Gold,  platinum,  magnetite,  chromite,  garnet, 
cassiterite,  rutile,  monazite,  and  several  others  are  not  readily 
dissolved  and  will  accumulate  in  residual  bodies  and  placers. 
Quartz  is  not  strongly  attacked,  but  the  alkali,  alkali  earth,  and 
iron  silicates  dissolve  more  readily.  As  a  general  rule  the  sili- 
cates containing  little  silica,  such  as  olivine  and  enstatite,  will 
be  changed  more  readily  than  feldspars,  and  feldspars  more 
readily  than  quartz.  Consequently  the  basic  rocks — gabbro, 
peridotite,  and  others — are  the  more  readily  altered.  Such 
rocks  on  weathering  yield  many  metalliferous  products.  Kaolin, 
bauxite,  gibbsite,  ferric  hydroxides,  and  some  manganese  oxides 
are  fairly  stable  under  surface  conditions,  and  rocks  composed  of 
these  minerals  are  but  slowly  attacked.  Many  silicates  digested 
in  water  give  alkaline  reactions,  and  if  digested  in  water  charged 
with  carbon  dioxide,  their  loss  is  very  appreciable. 

During  rock  weathering2  silica  is  released  and  carbonates  of 

1  MERRILL,   G.  P.:  The  Principles  of   Rock  Weathering.     Jour.   Geol., 
vol.  4,  pp.  704-724,  1896. 

2  CLARKE.  F.  W.:  The  Data  of  Geochemistry,  3d  ed.     U.  S.  Geol.  Survey 
Bull.  616,  p.  481,  1916. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  127 


lime,  iron,  magnesia,  and  alkalies  may  be  formed.  The  iron 
carbonate  is  readily  oxidized  to  ferric  hydroxide.  The  lime, 
magnesia,  and  alkali  salts  remain  in  solution,  to  be  carried  away, 
together  with  some  silica.  The  portion  of  the  rock  that  is  not 
dissolved  is  hydrated;  the  feldspars  are  altered  to  kaolin,  the 
magnesian  minerals  to  talc  or  serpentine,  and  the  iron  to 
hydrated  ferric  oxides;  quartz  grains  are  dissolved  very  slowly. 
In  the  following  table  are  shown  analyses  of  diorite  from  Albe- 
marle  County,  Virginia.1  The  concentration  of  aluminum  and 
iron  and  the  loss  of  lime,  magnesia,  and  soda  are  noteworthy. 
The  large  loss  on  ignition  of  the  altered  rock  indicates  extensive 
hydration  during  weathering. 

ANALYSES  OF  FRESH  AND  ALTERED  DIORITE 


Fresh 

Altered 

SiO2 

46  75 

42  44 

A12O3 

17  61 

25  51 

Fe203  \ 
FeO     J    
MgO  

16.79 
5.12 

19.20 
0.21 

CaO  

9.46 

0.37 

Na,O 

2  56 

0  56 

K2O 

0  55 

0  49 

Loss  on  ignition 

0  92 

10.92 

P205  
MnO  

0.25 

0.29 

100.01 

99.99 

Thorough  weathering  may  convert  a  basic  rock  to  a  mantle  of 
workable  iron  ore.  Such  deposits  are  termed  lateritic  ores.  In 
eastern  Cuba  lateritic  iron  ores  are  extensively  developed.2  The 

1  MERRILL,  G.  P.:  "Rock  Weathering  and  Soils,"  pp.  224-225. 

2L.EiTH,  C.  K.,  and  MEAD,  W.  J.:  Origin  of  the  Iron  Ores  of  Central  and 
Northeastern  Cuba.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  42,  pp.  90-102, 
1912.  Additional  Data  on  Origin  of  Lateritic  Iron  Ores  of  Eastern  Cuba. 
Am.  Inst.  Min.  Eng.  Bull.  103,  pp.  1377-1380,  July,  1915.  "Metamorphic 
Geology,"  p.  391,  1915. 

KEMP,  J.  F.:  The  Mayari  Iron-Ore  Deposits,  Cuba.  Am.  Inst.  Min. 
Eng.  Bull.  98,  pp.  129-154,  February,  1915. 

LITTLE,  J.  E.:  The  Mayari  Iron  Mines,  Oriente  Province,  Island  of 
Cuba,  as  Developed  by  the  Spanish-American  Iron  Co.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  42,  pp.  152-169,  1911. 


128      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

deposits  cap  intrusive  bodies  (Fig.  62),  and  are  from  less  than 
1  foot  to  80  feet  thick.  They  are  situated  on  plateaus  from 
1,500  to  2,000  feet  above  sea  level.  The  ore  grades  downward 
into  serpentine,  an  altered  basic  intrusive  rock  containing  only 
a  little  bauxite,  kaolin,  and  other  minerals.  Leith  and  Mead 
have  calculated  the  mineral  constituents  of  the  ore  and  deter- 
mined the  amount  of  pore  space  at  different  depths.  These  data 
they  used  in  constructing  the  diagram  shown  in  Fig.  63.  Below 


FIG.  62. — Sketch  showing  limits  of  ore  bodies,  Mayari,  Cuba.     (After  Little.) 


29  feet  serpentine  is  encountered.  By  the  removal  of  magnesia 
and  silica  the  pore  space  is  much  increased  near  the  surface  and 
the  serpentine  rock,  which  carries  only  7.10  per  cent,  of  iron  at 
a  depth  of  29  feet,  becomes  an  ore  carrying  46.39  per  cent,  of 
iron.  These  and  similar  changes  may  be  compared  with  those 
effected  in  the  alteration  of  greenalite  in  the  Mesabi  range, 
Minnesota  (page  307),  and  the  weathering  of  nepheline  syenite 
in  Arkansas  to  form  bauxite  (page  506). 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  129 

HYDROMETAMORPHISM 

Decomposition  may  take  place  at  considerable  depths,  below 
the  level  of  ground  water  and  below  the  zone  of  active  oxidation. 
Close  examination  of  many  of  the  rocks  that  appear  nearly  fresh 
shows  that  uralite,  chlorite,  serpentine,  talc,  or  hydromica  have 
formed  from  older  silicate  minerals.  Such  changes,  which  take 
place  below  the  zone  of  oxidizing  decomposition,  are  commonly 


Surface  Percentage  Composition  by  Weight 

0      5      10     15     20     25    30     35     40     45     50     55    60     65     70     75     80     85    90     95 


1 

"fstef  j 

«\ 

5 
7 
9 

I" 

fa  13 
c 

S15 

|l7 
19 
21 
23 
25 
27 
29 

ppvkl 

|v|%ii^> 

\ 

!_(. 

ss 

BY 

SUL 

U  1  1 

JN 

•  » 

• 

• 

/ 

V           < 

\ 

~"l  ^f^^^^n^^^ 

Miscellaneous       Kaolin. 

<&fe 

'/ 

^^^ 
Baazi 

to 

m 
. 

H 

JE 

H 

3mat 

' 

tte        L 

mi 
~~ 

mon 

S£ 

ite 

"^•j>eij.VF7gg 

ESSg 

Quart 

•rT^^T 
Z        I 

.rrs---^ 

5erp 

^ 

mtlnt 

(Ni.Co.Cr.M'n'ls) 

FIG.  63. — Diagram  showing  mineral  changes  in  alteration  of  serpentine  to  iron 
ore  by  weathering,  Mayari,  Cuba.     (After  Leith  and  Mead.) 


termed  "hydrometamorphism."  Some  basic  rocks  (peridotites, 
hornblendites,  pyroxenites)  are  almost  entirely  converted  into 
secondary  minerals,  commonly  into  serpentine.  As  the  surface 
is  worn  away  the  hydrometamorphosed  rocks  are  exposed  to 
actively  oxidizing  solutions,  and  more  extensive  changes  take 
place.  Weathering  and  hydrometamorphism  of  rocks  may  be 
compared  respectively  to  surface  oxidation  and  deep  enrichment 
of  sulphide  ores. 


130      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


SUPERFICIAL   ALTERATION   AND    ENRICHMENT    OF    SULPHIDE 
DEPOSITS 

General  Features. — Many  sulphide  deposits  show  charac- 
teristic changes  from  the  surface  downward.1  The  outcrop  and 
the  upper  part  of  the  oxidized  portion  of  a  deposit  may  be  poor. 
Below  this  there  may  be  rich  oxidized  ore;  still  farther  down, 
rich  sulphide  ore;  and  below  the  rich  sulphides,  ore  of  relatively 
low  grade  (see  Fig.  64).  This  lowest  ore  is  commonly  assumed 
to  be  the  primary  ore,  from  which  the  various  kinds  of  ore  above 
have  been  derived.  The  several  kinds  of  ore  have  a  rude  zonal 
arrangement,  the  so-called  zones  being, 
like  the  water  table,  highly  undulatory 
(see  Fig.  65).  They  are  related 
broadly  to  the  present  surface  and 
generally  to  the  hydrostatic  level  but 
may  be  much  more  irregular  than 
either,  for  they  depend  in  large  meas- 
ure on  the  local  fracturing  in  the 
lode,  which  controls  the  circulation 
of  underground  waters.  Any  zone 

FIQ.    64.— Section      showing  °  /       • 

a  tabular  sulphide  ore  deposit    may  be  thick  at  one  place  and  thin  or 
with  changes  due  to  superficial    eyen  absent  at  another.     The  zone  of 

alteration  and  enrichment. 

oxidized   ore  is  generally  above  the 

water  level.     The  zone  of  secondary  sulphides  in  moist  countries 
is  in  general  below  the  water  level. 

EMMONS,   S.   F.:  The  Secondary  Enrichment  of  Ore   Deposits.     Am. 
Inst.  Min.  Eng.  Trans.,  vol.  30,  pp.  177-217,  1901. 

WEED,    W.    H.:  The    Enrichment    of    Gold    and    Silver    Veins.     Idem, 
pp.  424-448. 

.    VAN  HISE,  C.  R.:  Some  Principles  Controlling  the  Deposition  of  Ores. 
Idem,  pp.  27-177. 

KEMP,  J.  F. :  Secondary  Enrichment  in  Ore  Deposits  of  Copper.     Econ. 
Geol.,  vol.  1,  pp.  11-25,  1906. 

RANSOME,  F.  L.:  Criteria  of  Downward  Sulphide  Enrichment.     Econ. 
Geol.,  vol.  5,  p.  205,  1910. 

PENROSE,    R.  A.   F.,  JR.:  The  Superficial  Alteration  of  Ore  Deposits. 
Jour.  Geol.,  vol.  2,  pp.  288-317,  1894. 

TOLMAN,  C.  F. :  Secondary  Sulphide  Enrichment.     Min.  and  Sci.  Press., 
vol.  106,  pp.  38-43,  141-145,  178-181,  1913. 

EMMONS,  W.  H.:  The  Enrichment  of  Sulphide  Ores.     U.  S.  Geol.  Survey 
Bull.  529,  1913. 

EMMONS,  W.  H. :  The  Enrichment  of  Ore  Deposits.     U.  S.  Geol.  Survey 
Bull.  625,  pp.  1-503,  1917. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  131 

All  these  zones  except  that  of  the  primary  ore  are,  broadly  con- 
sidered, continually  descending.  Through  more  rapid  erosion 
at  some  particular  part  of  the  lode  any  one  of  these  zones  may  be 
exposed,  and  hence  an  outcrop  of  ore  of  any  character  is  possible. 

Level  of  Ground  Water. — The  terms  "water  table"  and  "level 
of  ground  water"  are  generally  used  to  describe  the  upper  limit 
of  the  zone  in  which  the  openings  in  rocks  are  filled  with  water. 
This  upper  limit  of  the  zone  of  saturation  is  not  a  plane  but  a 
warped  surface.  It  follows  in  general  the  topography  of  the 
country  but  is  less  accentuated.  It  is  not  so  deep  below  a  valley 
as  below  a  hill  but  rises  with  the  country  toward  the  hilltops  and 


Fiu.  65. — Vertical  section  of  Granite-Bimetallic  vein,  Philipsburg,   Montana. 


in  general  is  higher  there  than  in  the  valleys.  Although  the 
water  in  the  zone  of  saturation  does  not  move  rapidly,  it  is  not 
stationary.  If  there  is  a  lower  outlet,  it  will  move  toward  that 
point.  Its  movements  are  slow,  however,  and  it  may  follow 
a  very  circuitous  route  before  it  issues  again  at  the  surface.  It 
follows  the  paths  of  least  resistance,  and  if  these  are  downward 
the  water  may  sink  to  great  depths  before  it  rises,  under  pressure, 
to  make  its  exit  at  some  point  which  is  lower  than  that  at  which 
it  first  entered  the  belt  of  saturation. 

As  the  country  is  eroded  the  water  level  moves  downward  and, 
within    certain   limits,    it   changes    with   the   seasons.     In   dry 


132       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

years  it  is  deeper  than  in  wet  years,  and  in  dry  seasons  it  is 
deeper  than  in  wet  seasons.  Thus  the  water  table  may  be 
considered  a  kind  of  indicator  that  registers  the  differences  be- 
tween the  loss  or  leakage  of  the  zone  of  saturation  and  addition 
from  the  surface.  There  is  a  zone  that  is  above  ground-water 
level  in  dry  periods  but  below  it  in  wet  periods,  and  in  moist 
hilly  countries  this  zone  may  be  of  considerable  vertical  extent. 
The  difference  of  altitude  between  the  top  of  the  zone  of  satu- 
ration in  a  wet  year  and  in  a  dry  year  is  normally  greater  under 
the  hilltops  than  on  the  slopes  and  in  the  valleys.  Thus  the 
water  table  oscillates,  though  in  general  it  moves  downward 
with  degradation  of  the  land  surface  (see  Fig.  66). 

Vadose  Circulation. — Of  the  rain  that  falls  on  the  surface  a 
part  is  drained  off  by  rills  and  streams,  another  part  is  evaporated, 
and  still  another  part  soaks  deep  into  the  ground,  passes  down- 
ward, and  is  added  to  the  water  of  the  zone  of  saturation.  The 
downward  movement  of  such  water  toward  the  zone  of  saturation 
has  been  termed  the  "vadose"  circulation  (from  vadus,  shallow).1 
The  depth  or  thickness  of  the  vadose  zone  is  variable,  for  its 
lower  limit  depends  on  the  variable  level  of  ground  water.  In 
moist  hilly  countries  its. depth  varies  from  a  few  feet  to  several 
hundred  feet.  In  arid  regions,  where  the  rainfall  is  low  and 
evaporation  is  rapid,  it  may  extend  to  much  greater  depths.  It 
is,  in  the  main,  a  zone  of  solution;  consequently  its  rocks  are 
open  and  circulation  within  it  is  comparatively  rapid. 

Deeper  Circulation. — The  circulation  of  the  water  in  the  zone 
of  saturation  depends  on  the  relief  of  the  country  and  on  the 
number,  continuity,  spacing,  and  size  of  the  openings  in  the  rocks. 
Under  hydrostatic  head  the  waters  in  this  zone  move  to  points 
of  less  pressure  and  issue  at  points  lower  than  those  of  entry. 
If  the  deposit  is  tight  and  there  are  no  deep  outlets  the  principal 
movement  is  shallow,  following  down  the  grade  of  the  water 
table.  As  a  rule  movement  in  the  deeper  zone  is  much  slower 
than  in  the  vadose  zone,  because  the  openings  are  less  numerous 
and  also  because  they  are  smaller,  so  that  friction  on  their  walls 
is  greater.  Below  the  water  table,  moreover,  the  openings  are 
already  filled  with  water.  As  shown  in  deep  mines  the  under- 

1  POSEPNY,  FRANZ  :   "The  Genesis  of  Ore  Deposits,"  p.  18,  1902. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  133 

ground  circulation,  in  many  places  is  exceedingly  sluggish.1  In 
some  rocks,  however,  under  favorable  structural  conditions, 
surface  waters  are  conducted,  in  porous  beds  or  along  fractured 
zones,  several  thousand  feet  below  the  surface.  In  other  rocks 
little  or  no  water  is  collected  at  depths  of  more  than  a  few 
hundred  feet. 

Region  of  Nearly  Stagnant  Water. — The  zone  of  the  deeper 
circulation  varies  greatly  in  depth  and  vertical  extent.  Its  water 
is  discharged  at  points  that  are  not  lower  than  the  lowest  altitude 
of  the  country,  and  if  numerous  points  of  discharge  are  located 
along  a  lode  that  crops  out  at  several  different  altitudes  there  will 
be  a  considerable  lateral  movement  of  the  waters  toward  these 
points,  for  the  solutions  move  to  points  of  less  pressure.  If 
lower  rocks  are  saturated  and  their  openings  are  filled,  the  solu- 
tions descending  from  above  will  find  any  lateral  outlet  that  is 
available.  Where  there  are  structural  features  that  afford  pas- 
sages like  inverted  siphons  there  may  be  a  considerable  movement 
of  water  below  the  lowest  outlet,  but  where  the  spacing  of  open- 
ings along  the  lode  is  fairly  regular  the  circulation  becomes  less 
and  less  vigorous  as  depth  increases  below  the  lowest  outlet. 
There  is  thus  a  division,  probably  everywhere  somewhat  in- 
definite, between  the  sluggish  deeper  circulation  and  a  zone  of 
static  or  nearly  stagnant  waters  below  it.  There  is  much  evi- 
dence that  in  some  rocks  the  top  of  this  zone  lies  within  a  few 
hundred  feet  of  the  surface  or  even  less,  but  where  there  are  deep 
open  fissures  it  may  be  much  deeper.2 

Pulsating  Movements  of  Underground  Waters. — The  under- 
ground circulation  of  water  is  normally  downward  from  the  top 
of  the  vadose  zone  to  the  water  table ;  thence  by  less  direct  routes 
to  greater  but  generally  undetermined  depths  below  the  water 
table;  after  that  laterally,  and  perhaps  upward,  to  openings 
that  are  lower  than  the  points  of  entrance.  This  circulation, 
however,  is  not  to  be  regarded  as  a  uniform,  steady  flow.  Many 
springs,  probably  most  of  those  that  are  fed  by  underground 

1  KEMP,  J.  F. :  The  Role  of  the  Igneous  Rocks  in  the  Formation  of  Veins, 
in  POSEPNY,  FRANZ:  The  Genesis  of  Ore  Deposits,  pp.  681-809,  1902. 

FINCH,  J.  W. :  The  Circulation  of  Underground  Aqueous  Solutions  and 
the  Deposition  of  Lode  Ores.  Colo.  Sci.  Soc.  Proc.,  vol.  7,  pp.  193-252,  1904. 

RICKARD,  T.  A. :  Waters  Meteoric  and  Magrnatic.  Min.  and  Sci.  Press, 
June  27,  1908. 

2  FINCH,  J.  W. :  The  Circulation  of  Underground  Aqueous  Solutions  and 
the  Deposition  of  Lode  Ores.     Colo.  Sci.  Soc.  Proc.,  vol.  7,  p.  216,  1904. 


•134      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


meteoric  waters,  issue  more  copiously  during  and  immediately 
after  rainy  seasons  than  during  dry  seasons.  At  times  of  drought 
some  of  them  will  cease  to  flow.  There  are  sound  reasons  for 
supposing  that  the  normal  movement  of  ground  water  is  not 
steady,  even  at  depths  below  the  ground- water  level.  After 
a  rainy  season  the  water  level  is  raised  and  the  additional  pres- 
sure on  the  water  in  the  zone  of  saturation  will  cause  it  to  move 
more  rapidly  to  points  of  less  pressure  and  to  issue  at  any  avail- 
able openings.  The  water  that  during  a  dry  season  has  ceased 
to  issue  through  springs  or  pther  openings  but  has  remained  nearly 
if  not  quite  static  will  have  had  a  longer  time  to  be  attacked  by 
ores  and  gangue  minerals  with  which  it  is  in  contact.  At  depths 

rSfMM       OHrlC*30~ 


FIG.  66. — Diagram  showing  water  levels  and  zones  of  ground  water  in  a 
relatively  moist  climate.  In  part  of  the  zone  of  sluggish  circulation  acid  and 
alkaline  conditions  alternate  with  seasons. 

where  air  is  excluded  acid  will  be  neutralized,  and  as  nearly  all 
rocks  give  alkaline  reactions  with  water  the  solutions  will  tend 
to  become  alkaline.  Higher  up,  at  and  above  the  water  level, 
where  air  has  access,  the  solutions  will  be  acid.  But  after  a 
season  of  heavy  rains  the  acid  water  of  the  higher  zones  will 
rapidly  encroach  upon,  mingle  with,  and  tend  to  crowd  out  the 
alkaline  waters  below,  which,  of  course,  issues  at  the  surface  where 
openings  are  available.  Thus  we  may  with  good  reason  assume 
that  certain  parts  of  zones  of  alteration  are  alternately  in  alkaline 
and  in  acid  environments  (Fig.  66). 

Physical  Conditions  that  Influence  Enrichment. — Enrichment 
is  influenced  by  many  factors,  among  them  permeability,  lati- 
tude, altitude,  and  relief. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  135 

Permeability  is  essential  for  sulphide  enrichment.  If  the  pri- 
mary deposits  are  not  permeable  the  solutions  that  pass  down- 
ward through  the  oxidized  zone  will  move  laterally  along  the  con- 
tacts between  oxidized  and  sulphide  ores  and  ultimately  will 
escape  into  fractures  in  the  wall  rocks  or  reissue  as  springs  at 
some  level  below  the  points  of  entry.  If  they  do  not  encounter 
a  reducing  environment  the  metals  may  be  scattered.  In  de- 
posits that  have  been  shattered  by  strong  movements  there  is 
generally  more  extensive  and  deeper  enrichment  than  in  deposits 
that  have  been  but  slightly  fractured.  Brittle  •  minerals  like 
quartz  and  chert  fracture  readily,  and  deposits  composed  largely 
of  the  brittle  minerals  are  generally  more  deeply  enriched  than 
deposits  of  tough  or  elastic  minerals.  Many  of  the  heavy  silicate- 
sulphide  ores  of  contact-metamorphic  origin  that  carry  a  gangue 
of  abundant  fibrous  amphibole,  mica,  chlorite,  or  like  minerals 
do  not  show  sulphide  enrichment  to  great  depths. 

A  warm  climate,  which  favors  chemical  action,  is  favorable 
to  weathering  and  enrichment.  Deposits  in  cold  regions  are 
not  so  likely  to  show  extensive  concentration:  low  temperature 
decreases  chemical  activity,  and  freezing  prevents  solution. 
Where  the  ground  is  frozen  to  considerable  depths  during  the 
winter  and  thaws  out  only  a  short  distance  below  the  surface 
during  the  summer  thorough  weathering  can  not  extend  to 
great  depths. 

As  a  rule  the  relief  is  great  in  areas  of  high  altitudes,  and  ero- 
sion is  consequently  more  rapid.  Moreover,  in  such  areas  tem- 
peratures are  lower  and  conditions  are  less  favorable  to  solution. 
Deposits  located  at  very  high  altitudes,  where  rocks  are  disinte- 
grated by  frost  and  carried  away  un weathered  as  talus  and 
boulders,  are  not  so  likely  to  be  extensively  enriched  as  deposits 
that  lie  at  lower  altitudes.  On  the  other  hand,  under  some  con- 
ditions the  processes  of  enrichment  are  effective  at  considerable 
altitudes.  Many  deposits  in  Colorado  and  Montana  that  crop 
out  at  more  than  8,000  feet  above  sea  level  contain  extensive 
zones  of  secondary  ores. 

Strong  relief  supplies  head  and  is  therefore  favorable  to  deep 
and  rapid  circulation  of  underground  water,  and  it  is  likewise 
favorable  to  relatively  deep  enrichment.  In  base-leveled  regions 
underground  circulation  is  sluggish,  and  the  nearly  stagnant 
waters  can  not  descend  far  into  the  zone  of  primary  sulphides 
without  losing  the  valuable  metals  which  they  dissolve  higher  up. 


136      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Under  such  conditions  enrichment  may  be  practically  halted, 
because  the  ground-water  level  becomes  nearly  stationary,  and 
after  the  soluble  metals  have  all  been  dissolved  from  the  oxidized 
zone  solution  may  practically  cease. 

As  strong  relief  is  favorable  to  rapid  erosion  it  is  unfavorable 
to  thorough  leaching.  Where  erosion  is  slow  the  outcrops  and 
upper  portions  of  deposits  are  exposed  to  processes  of  weathering 
for  periods  long  enough  to  favor  thorough  leaching  and,  if  the 
metals  are  reprecipitated  at  lower  depths,  to  favor  enrichment. 
On  the  other  hand,  erosion  may  be  delayed  to  a  point  beyond 
which  it  is  unfavorable  to  solution  and  precipitation;  the  down- 
ward migration  of  the  zone  of  oxidation  exposes  new  surfaces 
to  solution,  making  masses  of  fresh  sulphides  available  for  recon- 
centration.  Consequently  where  metals  dissolve  readily,  com- 
paratively rapid  erosion  may  favor  rapid  concentration.  The 
metallic  contents  of  many  deposits  of  secondary  ores  represent 
not  only  what  has  been  leached  from  the  gossan  now  exposed  but 
also  what  has  been  dissolved  from  portions  of  the  deposits  that 
have  been  carried  away  by  erosion. 

Other  conditions  being  similar,  the  amount  of  enrichment  must 
depend  on  the  length  of  time  the  deposits  have  been  exposed  to 
weathering  and  erosion.  In  general,  weathering  has  acted  for  a 
shorter  time  on  late  Tertiary  deposits  than  on  middle  Tertiary, 
early  Tertiary,  or  Cretaceous  deposits.  The  age  of  the  deposit  is 
not,  however,  invariably  the  most  important  factor  in  determin- 
ing the  extent  of  its  enrichment,  for  some  of  the  middle  or  late 
Tertiary  deposits,  such  as  those  in  the  southwestern  part  of  the 
United  States,  show  more  extensive  migration  of  the  metals  than 
is  shown  by  some  older  deposits  that  have  been  exposed  to 
weathering  for  a  much  longer  time. 

Inasmuch  as  sulphide  enrichment  depends  on  the  action  of 
surface  agencies,  it  is  important  to  know  as  far  as  possible  the 
details  of  the  history  of  any  deposit  considered,  the  length  of  time 
it  has  been  exposed  to  weathering,  and  whether  faulting  or  fold- 
ing or  a  second  episode  of  primary  ore  formation  has  taken  place 
since  it  was  first  formed.  In  short,  any  geologic  or  physiographic 
data  might  have  a  bearing  on  the  problem  of  enrichment. 

If  the  present  topography  is  like  that  which  existed  when  pri- 
mary deposition  took  place — and  it  may  be  if  the  deposits  were 
formed  in  comparatively  late  geologic  time — then  the  richer  ore 
of  the  primary  deposits  may  have  an  obvious  relation  to  the 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  137 

present  surface.  In  some  deposits  of  gold  and  silver  ore  the 
maximum  precipitation  of  the  metals  appears  to  have  taken  place 
at  relatively  short  distances  below  the  surface  that  existed  at  the 
time  of  deposition.  Thus  the  primary  ore  may  show  a  compara- 
tively constant  change  in  value,  which  may  decrease  with  in- 
creasing depth.  In  general,  the  more  remote  the  period  of 
primary  deposition  the  less  the  probability  that  the  main  fea- 
tures of  the  present  topography  are  similar  to  those  which  existed 
during  that  period  and  the  less  the  probability  of  error  in  attri- 
buting the  deposition  of  a  rich  body  of  primary  ore  to  secondary 
processes. 

Glaciation. — In  comparatively  late  geologic  time  a  consider- 
able portion  of  North  America  was  capped  by  a  continental  ice 
sheet,  which  removed  by  erosion  the  loose  debris  and  the  surface 
rock  over  great  areas.  Glaciation  was  most  extensive  in  northern 
latitudes,  but  the  continental  glacier  extended  southward  as 
far  as  Ohio  and  Missouri  rivers,  and  smaller  glaciers  accumu- 
lated in  the  more  lofty  mountain  ranges  of  the  American  Cor- 
dillera. Many  of  the  ore  deposits  that  lay  in  the  paths  of  the 
glaciers  were  planed  off,  and  the  ores  in  their  upper  portions 
were  scattered  in  the  rocky  material  that  was  left  when  the  ice 
had  melted.  Erratic  fragments  of  such  deposits  have  been  car- 
ried far  from  their  sources  and  have  been  the  cause  or  much  fruit- 
less prospecting.  Weathering  does  not  attend  erosion  by  ice, 
and  at  low  temperatures  chemical  action  is  retarded ;  consequently 
the  metals  present  in  the  portions  of  rocks  or  ore  deposits  that 
are  removed  are  likely  to  be  scattered  by  the  ice  rather  than' 
concentrated.  The  amount  of  rock  removed  by  the  continental 
ice  sheet  is  known  to  be  considerable,  for  the  drift  which  it  de- 
posited is  in  many  places  more  than  200  feet  thick.  It  is  prob- 
able that  glacial  erosion  was  in  places  equally  great  or  greater. 
Whatever  the  amount  of  ice  erosion,  it  appears  to  have  been 
sufficient  to  remove  the  highly  altered  zones  of  sulphide  deposits 
in  most  parts  of  northern  North  America. 

The  processes  of  solu'tion  are  retarded  in  regions  of  low  tem- 
perature. The  areas  in  which  ice  erosion  has  been  most  vigorous 
are  those  in  which  the  lower  temperatures  prevail  today,  and 
there  is  reason  to  suppose  that  the  deposits  in  these  areas  were 
not  so  deeply  altered  before  the  glacial  epoch  as  similar  deposits 
in  warmer  latitudes.  In  Canada  and  in  Alaska  there  are  few 
large  ore  deposits  that  are  clearly  of  secondary  origin. 


138      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Glaciers  do  not  erode  their  beds  equally  at  all  places.  In  their 
higher  portions,  where  the  ice  is  accumulating,  pressures  are 
greater,  the  ice  is  more  rigid,  and  erosion  is  more  vigorous.  Near 
the  margins,  where  the  ice  is  melting,  deposition  exceeds  erosion 
and  the  deposit  of  drift  protects  the  surface  from  wear.  These 
differences  are  very  conspicuous  in  some  mountainous  sections  of 
the  West,  where  the  glaciers  covered  only  portions  of  the  country 
and  the  processes  are  more  clearly  shown.  In  some  of  the  ranges 
of  Montana,  Colorado,  and  Utah,  where  ore  deposits  are  numer- 
ous and  varied,  the  evidences  of  mountain  glaciation  are  con- 
spicuously preserved.  At  some  places  the  mountain  glaciers 
seem  to  have  removed  very  little  of  the  altered  ore,  for  the  sec- 
ondary sulphide  zones  and  even  the  oxidized  ores  are  intact,  and 
some  of  these  appear  to  be  too  extensive  to  have  been  formed 
since  the  glacial  epoch.  In  general,  erosion  by  mountain  glaciers 
has  been  localized,  the  maximum  wear  taking  place  near  the  heads 
of  the  glaciers. 

Erosion  by  the  continental  glaciers  also  has  been  somewhat 
erratic,  for  great  differences  in  the  effect  of  the  action  of  ice  may 
be  seen  in  a  comparatively  small  area.  In  the  Mesabi  range  of 
Minnesota  the  hard,  fresh  country  rock  is  polished  clean  in  places, 
whereas  a  few  rods  away  and  at  but  slightly  lower  altitudes 
thick  bodies  of  cellular,  powdery  iron-oxide  ore  remain  intact. 

General  Character  of  Outcrops  above  Sulphide  Deposits. — 
Sulphide  ores  almost  invariably  carry  iron  sulphides.  Al- 
though iron  is  readily  dissolved  in  sulphate  waters  it  is  pre- 
cipitated as  hydroxide  in  the  presence  of  oxygen,  and  the  super- 
ficial zone  and  outcrop  of  a  sulphide  ore  body  are  generally 
stained  with  iron.  Prospectors  looking  for  metalliferous  deposits 
generally  seek  iron-stained  rocks.  The  old  Cornish  term 
"gossan,"  the  Spanish  "colorados,"  and  the  German  "eiserner 
Hut,"  applied  to  the  outcrop,  indicate  iron-stained  rock  and 
generally  are  assumed  to  imply  metalliferous  deposits  below.1 
If  the  sulphide  ore  bodies  carry  large  proportions  of  iron  sulphide, 
the  outcrops  will  invariably  carry  iron.  In  some  deposits,  how- 
ever, copper  sulphides  replace  iron  sulphides  in  the  secondary 
sulphide  zone,  and  the  oxidation  product  of  a  chalcocite  zone  may 

1  PENROSE,  R.  A.  F.,  JR.  :  The  Superficial  Alteration  of  Ore  Deposits. 
Jour.  Geol,  vol.  2,  pp.  298-317,  1894. 

EMMONS,  W.  H.;  Outcrops  of  Ore  Bodies,  in  BAIN,  H.  F.,  and  others: 
"Types  of  Ore  Deposits,"  pp.  299-323,  San  Francisco,  1911. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  139 

show  very  little  iron  oxide.  Some  valuable  bodies  of  copper  ore 
have  outcrops  of  light-colored  kaolinized  rocks  that  are  not 
highly  ferruginous.  Some  outcrops  are  light  yellow,  not  rusty 
brown. 

In  recent  years  the  discovery  of  rich  ore  bodies  below  brown 
gossan  has  stimulated  the  prospecting  of  iron-stained  rocks  in 
many  regions.  Some  of  the  prospecting  has  been  disappointing, 
for  obviously  a  pyritic  material  that  carries  neither  gold,  silver, 
nor  copper  will  oxidize  to  iron-stained  rock  that  is  chemically 
and  mineralogically  like  one  that  may  cap  a  deposit  of  copper  or 
of  precious  metals. 

As  a  general  rule,  the  outcrop  of  a  valuable  silver  deposit 
carries  silver.  The  outcrops  of  many  silver  deposits  are  richer 
than  the  ore  below,  especially  where  horn  silver  (cerargyrite-) 
forms  in  the  gossan  (see  page  443).  Outcrops  of  gold  deposits 
also  generally  carry  gold.  If  the  ore  is  free  from  manganese 
the  outcrops  are  usually  as  rich  as  or  richer  than  the  ore  below 
the  oxidized  zone.  Manganiferous  gold  deposits  may  be  leached 
at  the  outcrop,  but  many  manganiferous  gold  ores  with  a  car- 
bonate gangue  are  as  rich  near  the  surface  as  below,  if  not  richer 
(see  page  407).  Rich  copper  ores  may  be  capped  by  a  barren 
gossan,  but  as  a  rule  the  gossan  of  a  copper  ore  carries  a  little 
copper  as  carbonate,  silicate,  or  oxide.  Gossans  of  sphalerite 
ores  may  be  completely  leached  of  zinc  sulphide,  but  calamine  or 
smithsonite  are  commonly  formed  at  the  surface  or  not  far  below. 
Lead  ores  are  oxidized  slowly,  and  during  oxidation  insoluble 
products  are  formed,  such  as  cerusite  and  anglesite.  As  a  rule 
the  gossan  of  a  deposit  that  is  rich  in  galena  carries  lead  at  the 
surface  or  not  far  below  it.  These  relations  depend  on  the 
chemical  behavior  of  the  several  metals,  which  are  discussed  on 
pages  that  follow. 

Conditions  in  the  Oxidized  Zone. — The  oxidized  zone  is  in 
the  main  the  zone  of  solution.  Precipitation  also  takes  place  in 
this  zone,  especially  the  precipitation  of  the  oxides  and  hydrous 
oxides  of  iron,  aluminum,  manganese,  and  silicon.  By  redeposi- 
tion  ores  of  the  more  valuable  metals  also  are  formed  in  this 
zone.  Solution  generally  exceeds  precipitation,  however,  and  by 
solution  the  mass  is  reduced  and  open  spaces  are  enlarged. 
The  increase  in  the  size  and  volume  of  the  openings  renders  the 
downward  circulation  comparatively  free  in  the  zone  of  oxidation. 

Some  of  the  metals — for  example,  gold — dissolve  very  slowly 


140      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

in  the  zone  of  oxidation.  If  the  other  materials  in  an  ore  deposit 
are  taken  away,  however,  the  ore  may  be  enriched  by  decrease 
in  volume.1 

The  oxidized  zone  is  generally  above  a  secondary  sulphide 
zone.  As  the  sulphide  zone  is  the  richest  part  of  many  deposits 
and  as  the  zone  of  oxidation  is  gradually  descending  the  processes 
of  oxidation  may  attack  materials  that  are  comparatively  rich. 
In  many  deposits  the  first  effect  of  oxidation  is  to  convert  the 
richer  sulphides  to  rich  oxides;  consequently  the  lower  part  of 
the  oxidized  zone  may  be  as  rich  as  or  even  richer  than  the 
secondary  sulphide  zone. 

Commonly  the  metals  are  separated  in  the  zone  of  oxidation. 
Copper  will  separate  from  iron  in  cupriferous  iron  sulphide  ore. 
Lead  and  zinc  or  lead  and  silver  likewise  tend  to  separate. 

Depth  of  the  Oxidized  Zone. — The  depth  of  the  zone  of  oxida- 
tion and  the  extent  of  oxidation  within  that  zone  depend  upon 
the  permeability,  character,  and  composition  of  the  ore.  Con- 
ditions differ  greatly  in  different  districts  and  even  in  different 
deposits  in  the  same  district.  The  depth  of  thorough  oxidation 
is  generally  less  than  the  depth  of  the  vadose  circulation,  for  the 
lowering  of  the  zone  of  oxidation  follows  the  depression  of  the 
water  level.  Where  the  ground-water  level  has  been  depressed 
by  relatively  rapid  climatic  change  rather  than  by  the  gradual 
downward  migration  of  ground  water  that  attends  the  normal  de- 
gradation of  a  country,  the  rate  of  its  depression  may  be  more 
rapid  than  that  of  the  zone  of  oxidation,  and  in  consequence  the 
sulphide  ores  may  be  marooned  in  the  vadose  zone.  In  an  arid 
country  oxidation  is  probably  slow,  for  it  depends  in  a  measure  on 
the  supply  of  oxygen-bearing  waters.  Thus  in  some  districts  in 
the  Southwest  the  lower  limit  of  oxidation  has  lagged  far  behind 
the  downward-migrating  water  level.  The  depth  of  oxidation 
may  vary  from  a  few  feet  to  2,000  feet  or  more,  but  the  greater 
depths  are  exceptional.  The  deepest  oxidation  takes  place  in 
ores  containing  carbonates.  As  a  rule  deposits  are  not  thoroughly 
oxidized  below  the  water  level  except  where  the  water  level  has 
been  elevated  after  oxidation. 

Position  and  Extent  of  the  Secondary  Sulphide  Zone. — The 
secondary  sulphide  zone  is  generally  below  a  zone  of  oxidation. 

1  RICKARD,  T.  A. :  The  Formation  of  Bonanzas  in  the  Upper  Portions  of 
Gold  Veins,  in  POSEPNY,  FRANZ:  "The  Genesis  of  Ore  Deposits,"  pp.  734- 
?65,  1902. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  141 


It  is  not  everywhere  developed,  not  even  in  copper  ores  that  are 
capped  with  gossan.  In  many  deposits  the  transition  between 
the  oxidized  and  secondary  sulphide  zone  is  sharp,  being  essen- 
tially at  the  ground-water  level.  In  such  deposits  the  secondary 
ores  extend  downward  to  various  distances  below  the  water  level. 
The  vertical  extent  of  the  secondary  zone  differs  widely  in  dif- 
ferent districts.  In  some  of  the  copper  districts  of  the  southern 
Appalachian  region  the  chalcocite  zones  occupy  only  a  few  feet 
vertically.  At  Ducktown,  Tenn.,  in  all  except  one  mine  the 
average  thickness  of  the  secondary  chalcocite  zone  is  probably 
between  3  and  8  feet,  but  some  secondary  chalcopyrite  is  de- 
veloped far  below  this  zone.  In  the  Encampment  district, 
Wyoming,  the  vertical  extent  of  chalcocite  is  at  least  200  feet. 


\/  / 


xM  N,/^l/^r 

x        '     '  '-_  i  -   i 

^  _   I  x  -      —  /        >.  N     /       -^^^-Sgricitii'ed-v 
.  -v    ..  Iv   /      \    /..'.  /  ,.  I        \     7<^and^Mqnz'bni.te':PoilJh-y* 

I  ;Sericitized  Monzonite  and  Monzonite_|_     _~^  c-ncito  Co,\  ellite  :  Ghali 

,  P6rph-y.ry- with1  PyrLte  avnd^ Chalcopyrite  N       ^<^sonVe'P>  rite/u^E 
I        x     I  as  D.ots  and  Vemlets^     '    ^    —  ~  7^<ife- y  Ve i n le,t 5 


FIG.  67. — Section  through  disseminated  copper  deposit  Bingham,  Utah.     Based 
on  maps  published  by  Utah  Copper  Company. 

In  the  disseminated  deposits  in  porphyry  at  Bingham,  Utah,  the 
zone  of  workable  sulphides,  mainly  chalcocite,  covellite,  and 
chalcopyrite,  has  an  average  vertical  extent  of  421  feet1  (see 
Fig.  67).  At  Globe,  Ariz.,  in  the  Old  Dominion  mine,  chalco- 
cite has  been  found  more  than  1,200  feet  below  the  surface  and 
has  a  vertical  range  of  at  least  800  feet. 

Deposits  of  enrichment  of  silver  sulphide  ore,  at  depths 
of  1,000  to  1,200  feet  below  the  surface  are  well  authenticated. 
Deposits  of  gold  precipitated  by  descending  solutions  at  depths 
more  than  1,000  feet  below  the  surface,  however,  are  rare.  In 
general,  gold  that  is  dissolved  by  surface  waters  is  precipitated 
at  relatively  shallow  depths. 

No  definite  depth  can  be  fixed  below  which  processes  of  enrich- 

lUtah  Copper  Co.  Seventh  Ann.  Rept.,  Dec.  31,  1911. 


142      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

ment  are  not  effective.  The  maximum  precipitation  occurs  at 
comparatively  shallow  depths,  however,  and  there  is  little  reason 
to  suppose  that  these  processes  are  effectively  operative  in  the 
deeper  part  of  the  zone  of  fracture.  The  depths  to  which  .the 
metals  are  carried  depend  on  local  climatic  conditions  and  on  the 
permeability  and  mineral  composition  of  the  primary  ore  and  wall 
rock.1 

Relation  of  the  Secondary  Sulphide  Zone  to  Water  Level. — 
The  ground-water  level  has  frequently  been  regarded  as  indicat- 
ing the  top  of  the  secondary  sulphide  zone  at  the  time  that  zone 
was  formed.  The  water  level  is  not  stationary  but  oscillates, 
although  it  tends  to  move  downward  as  the  major  drainage 
channels  approach  grade.  If  the  water  level  is  comparatively 
high  or  if  the  lode  is  much  fractured,  and  particularly  if  it  con- 
tains large  spaces,  there  is  probably  but  little  precipitation  of 
"the  secondary  sulphides  above  the  water  level.  Even  if  precipi- 
tation should  take  place  the  sulphides  precipitated  would  later 
be  again  exposed  to  oxidation  and  solution. 

The  sulphides  below  the  water  level  are  protected  from  the 
oxygen  of  the  air,  however,  and  solution  of  some  metals  is  re- 
tarded if  not  prevented.  The  solution  of  copper  sulphide,  silver 
compounds,  and  gold  requires  an  oxidizing  agent.  Ferrous  sul- 
phate, which  is  present  in  the  reducing  zone  below  the  water  level, 
will  drive  gold  and  silver  from  solution.  Copper  sulphide,  which 
dissolves  readily  in  sulphuric  acid  in  the  presence  of  air,  is  not 
dissolved  in  its  absence.  Hydrogen  sulphide  is  generated  by  the 
action  of  acid  on  sulphides,  and  in  the  presence  of  the  faintest 
trace  of  hydrogen  sulphide  copper  sulphide  is  not  dissolved. 
The  solution  of  copper,  silver,  and  gold  would  be  inhibited  at 
ground-water  level  or  a  short  distance  below  it.  Although  oxygen 
is  required  for  the  solution  of  gold,  silver,  and  copper,  the  sul- 
phides of  zinc  and  iron  are  dissolved  at  depths  considerably  below 
water  level,  for  they  are  attacked  by  acid  in  the  absence  of  an 
oxidizing  agent. 

Many  deposits  of  secondary  sulphide  ore  in  the  arid  South- 
west are  well  above  the  present  ground-waiter  level.  The  lower 
limits  of  some  of  these  deposits  have  been  reached  by  mining,  and 
below  some  of  them  lie  considerable  bodies  of  pyrite  and  chalco- 

1  EMMONS,  W.  H. :  The  Mineral  Composition  of  the  Primary  Ore  as  a 
Factor  Determining  the  Vertical  Range  of  Metals  Deposited  by  Secondary 
Processes.  Cong.  geol.  internat.,  12th  sess.,  Compt.  rend.,  pp.  261-269,  1913. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  143 

pyrite  ore  which  have  been  penetrated  by  mine  workings  that 
encountered  no  standing  underground  water.  The  climate  in 
this  area  has  changed  from  humid  or  subhumid  to  arid  in  com- 
paratively recent  time,  and  it  is  possible  that  some  bodies  of  sec- 
ondary sulphide  ore  have  been  marooned  by  the  rapid  descent 
of  the  ground-water  level  that  attended  the  change. 

Precipitation  of  Sulphides  above  the  Water  Level.' — In  arid 
districts,  where  the  water  level  lies  very  deep,  the  descending 
metal-bearing  solutions  may  encounter  a  reducing  environment 
in  the  vadose  zone,  especially  in  rocks  that  contain  only  minute 
openings,  through  which  the  water  soaks  downward  from  the 
surface,  excluding  the  admission  of  any  considerable  amount  of 
air.  It  would  be  supposed  that  the  oxygen  present  in  such 
waters  and  in  the  imprisoned  air  would  be  used  up  before  the 
descending  solutions  encountered  any  zone  of  permanent  satu- 
ration, or  that  the  solutions  might  become  reduced  before  reach- 
ing ground-water  level,  so  that  the  metals  could  be  precipitated.1 

Textures  of  Secondary  Ores. — Oxidized  ore  is  generally  spongy 
and  contains  numerous  cavities  due  to  solution.  The  iron  ores 
formed  by  the  superficial  weathering  of  ferruginous  igneous  or 
sedimentary  rocks  are  almost  invariably  open-textured,  although 
in  some  deposits  the  pore  space  formed  by  the  removal  of  value- 
less material  is  eliminated  by  slumping  near  the  surface  or  by 
cementation  with  iron  oxide  at  lower  depths.  Gossans  of  sul- 
phide ores  contain  many  openings  ranging  in  size  from  minute 
pores  to  enormous  caves.  Stalactites,  stalagmites,  organ  pipes, 
botryoidal  masses,  and  reniform  bodies  are  characteristic.  These 
forms  consist  principally  of  hydrous  iron  oxides,  subordinately 
of  carbonates  and  other  compounds.  Similar  forms  are  practi- 
cally unknown  in  sulphide  ores  deposited  by  hot  solutions. 

The  solution  of  primary  sulphides  and  the  precipitation  of  sec- 
ondary sulphides  may  go  on  simultaneously,  the  secondary  min- 
erals replacing  the  primary.  Pseudomorphs  of  chalcocite  or 
covellite  after  pyrite,  chalcopyrite,  or  zinc  blende,  in  which  the 
later  minerals  have  assumed  the  forms  of  the  earlier  minerals,  are 
common.  Fig.  68  shows  chalcocite  replacing  pyrite.  Chalco- 
cite, argentite,  and  other  dark  copper  and  silver  minerals  are 
frequently  found  as  sooty  amorphous  powder  coating  firmer  and 
more  distinctly  crystallized  minerals.  Commonly  the  sooty 

1  FINCH,  J.  W. :  The  Circulation  of  Underground  Aqueous  Solutions  and 
Deposition  of  Lode  Ores.  Colo.  Sci.  Soc.  Proc.,  vol.  7,  p.  238,  1904. 


144      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

sulphide  ores  are  secondary,  but  not  invariably.  Under  some 
conditions  the  primary  sulphides  break  down  and  form  dark 
powdery  material  in  an  early  stage  of  oxidation. 

When  some  minerals  have  been  dissolved,  leaving  other  minerals 
intact,  the  empty  spaces  may  be  bounded  by  surfaces  that  repre- 
sent former  surfaces  of  dissolved  crystals.  Such  spaces  are 
commonly  developed  in  the  gossan  of  quartz-pyrite  deposits, 
especially  where  quartz  predominates  and  surrounds  the  crystals 
of  pyrite.  Galena,  tungstates,  and  many  other  minerals  in 


FIG.  68. — Chalcocite  ore  from  lower  limit  of  chalcocite  zone  of  Ryerson  mine, 
Morenci,  Arizona.  Chalcocite  (dark  gray)  is  developing  by  replacement  in 
pyrite  (light  gray).  The  chalcocite  is  accompanied  by  small  amounts  of  quartz, 
shreds  of  sericite,  and  kaolin.  Black  areas  are  open  field.  (After  Lindgren, 
U.  S.  Geol.  Survey.) 

quartz  will  likewise  be  dissolved  and  leave  their  negative  pseudo- 
morphs.  Of  nearly  related  genesis  is  the  texture  shown  by  im- 
bricating blades  of  quartz  which  join  at  angles  that  represent  the 
cleavage  of  calcite.  Ore  of  this  character  is  shown  in  the  oxidized 
zone  of  deposits  at  Bullfrog  and  Manhattan,  Nev.;  De  Lamar, 
Idaho;  Marysville,  Mont.;  and  many  other  calcite-bearing 
deposits  (see  page  72).  If  the  undissolved  calcite  is  examined 
in  thin  section  the  genesis  of  such  texture  is  obvious.  The 
cleavage  cracks  of  the  calcite  are  filled  with  numerous  thin  plates 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  145 


of  quartz,  and  after  the  calcite  has  been  removed  the  quartz  plates 
remain.  If  the  carbonate  carries  manganese,  as  much  of  it  does, 
the  quartz  septa  after  weathering  are  heavily  stained  with 
black  or  chocolate-colored  manganese  oxide.  Material  con- 
taining solution  cavities  may  in  itself  be  wholly  primary,  but  as 
the  minerals  removed  have  been  dissolved  chiefly  in  the  oxidized 
zone,  the  negative  pseudomorphs  generally  indicate  superficial 
alteration. 

Secondary  sulphides  may  be  developed  in  cleavage  cracks  of 
pyrite,  galena,  and  other  minerals,  and  they  may  show  on  pol- 
ished surfaces  a  kind  of  indistinct  network  like  the  quartz  in  cal- 
cite mentioned  above,  the  position  of  the  thin  blades  being  con- 
trolled by  the  cleavage  of  the  older  mineral.1 

Oolitic  rocks  are  those  whose  texture  resembles  that  of  fish  roe. 
In  pisolitic  rocks  the  spheres  are  as  large  as  peas,  and  in  nodular 


FIG.  69. — A  nodule  and  a  band  of  pyrite  altering  to  limonite. 

rocks  they  are  commonly  larger,  some  of  them  much  larger. 
The  term  nodular,  however,  is  applied  to  spheres  of  all  sizes. 
Oolitic,  pisolitic,  and  nodular  textures  are  formed  under  widely 
varying  conditions.  Marine  ferruginous  beds  of  oxides,  carbon- 
ates, and  silicates  of  iron  are  commonly  oolitic.  Weathered  iron- 
bearing  rocks  and  weathered  aluminous  rocks  are  commonly 
oolitic  or  pisolitic.  Oolites  are  also  formed  by  hot  springs. 
Primary  sulphide  lode  ores  rarely  if  ever  show  oolitic  texture, 
except  perhaps  some  formed  almost  at  the  surface,  where  con- 
ditions were  probably  like  those  at  the  orifices  of  hot  springs. 
Superficial  alteration,  however,  very  commonly  develops  nodules 
of  various  sizes  (Fig.  69). 

1  GRATON,  L.  C.,  and  MURDOCH,  J. :  The  Sulphide  Ores  of  Copper;  Some 
Results  of  Microscopic  Study.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  45,  pp.  26- 
81,  1914. 

BASTIN,  E.  S.:  Metasomatism  in  Downward  Sulphide  Enrichment. 
Econ.  Geol,  voV  8,  p.  60,  1913. 

10 


146      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Much  of  the  material  that  is  generally  termed  secondary  sul- 
phide ore  consists  essentially  of  shattered  and  fractured  primary 
sulphide  ore,  the  cracks  in  which  are  filled  with  later  sulphides 
(Fig.  70)  or  with  angular  fragments  of  the  earlier  sulphides  crusted 
over  with  those  that  were  introduced  later.  Such  textures  do  not 
invariably  indicate  sulphide  enrichment  by  descending  solutions. 
Many  examples  show  that  in  the  course  of  primary  mineraliza- 


FIG.  70. — Banded  ore  from  the  South  vein  of  the  Granite-Bimetallic  lode, 
Philipsburg,  Montana.  Secondary  ruby  silver  is  deposited  in  cross  veinlets 
and  vugs. 

tion  the  ore  first  deposited  has  been  fractured  and  that  solutions 
from  below  have  deposited  later  sulphides  in  the  fractures. 

Pseudomorphous  replacement  indicates  a  change  of  physical 
conditions  or  of  chemical  environment.  Minerals  that  were 
stable  under  certain  conditions  have  been  dissolved,  and  other 
minerals  have  simultaneously  been  deposited.  On  the  other 
hand,  fractured  ore  cemented  by  later  minerals  may  be  a  result 
of  normal  and  perhaps  continued  deposition  from  below.  If, 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  147 


however,  the  minerals  that  fill  the  later  cracks  are  those  that  are 
commonly  formed  by  descending  solutions,  and  if  they  do  not 
persist  in  depth,  the  assumption  that  they 'are  secondary  may 
with  .considerable  confidence  be  regarded  as  confirmed. 

In  some  deposits  the  sulphides  are  intimately  intergrown  so 
that  under  the  microscope  their  structure  resembles  that  of 
graphic  granite.  Laney1  first  recognized  such  intergrowths  in 
copper  ores  of  the  Virgilina  district  in  Virginia  and  North  Caro- 
lina (Fig.  71).  Later  similar  intergrowths  in  copper  ores  were 


FIG.  71. — Graphic  intergrowth  (pri- 
mary) of  chalcocite  (light)  and  bor- 
nite  (dark),  Wall  mine,  Virgilina  dis- 
trict, Virginia  and  North  Carolina. 

(After  Laney.) 


Fia.  72. — Graphic  intergrowth  (pri- 
mary) of  chalcocite  (light)  and  bornite 
(dark),  Mount  Lyell  copper  mine, 
Tasmania.  (After  Gilbert  and  Pogue.) 


recognized  in  ores  of  Engel's  mine,  Plumas  County,  California;2 
at  Butte,  Mont.;3  in  the  Bevelheimer  mine,  Washoe  County, 
Nevada;4  in  the  Tintic  district,  Utah;8  at  Mount  Lyell,  Tasmania 

1  LANEY,  F.  B. :  The  Relation  of  Bornite  and  Chalcocite  in  Copper  Ores 
of  the  Virgilina  District  of  North  Carolina  and  Virginia.     Econ.  Geol.,  vol. 
6,  pp.  399-411,  1911. 

2  ROGERS,  A.  F. :  Secondary  Sulphide  Enrichment  of  Copper  Ores,  with 
Special  Reference  to  Microscopic  Study.     Min.  and  Sri.  Press.,  vol.   109, 
p.  680,  1914. 

3  GRATON,  L.  C.,  and  MURDOCH,  JOSEPH:  The  Sulphide  Ores  of  Copper; 
Some  Results  of  Microscopic  Study.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  54, 
pp.   26-81,    1914. 

4  SEGALL,  JULIUS:  The  Origin  of  Certain  Crystallographic  Intergrowths. 
Econ.  Geol.,  vol.  10,  pp.  462-470,  1915. 

6  WHITEHEAD,  W.  L. :  The  Paragenesis  of  Certain  Sulphide  Intergrowths. 
Econ.  Geol.,  vol.  11,  pp.  1-13,  1916. 


148      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

(Fig.  72)  j1  in  the  Mountain  Lake  mine,  Big  Cottonwood  Canyon, 
Utah;2  and  at  other  localities.  In  general  such  structure  has 
been  regarded  as  characteristic  of  primary  ores.  Some  geologists 
maintain,  however,  that  such  intergrowths  may  form  by  the 
replacement  of  one  mineral  by  another  along  cleavage  cracks. 
Segall,  Whitehead,  and  Rogers  have  expressed  this  view. 

Primary  and  Secondary  Minerals. — In  the  list  below  are  in- 
cluded the  common  ore  and  gangue  minerals  of  mineral  de- 
posits and  minerals  that  may  be  found  in  their  wall  rocks.  Those 
that  are  primary  are  marked  "p."  These  include  minerals  that 
may  be  formed  by  the  replacement  of  other  minerals  at  consid- 
erable depths  by  ascending  hot  waters.  Minerals  marked  "s" 
may  form  by  processes  of  superficial  alteration  and  enrichment. 
Some  minerals  so  marked  may  be  formed  also  near  the  surface 
and  at  the  orifices  of  hot  springs  under  conditions  of  oxidation. 
Minerals  that  are  both  primary  and  secondary  are  marked 
"p  s."  Minerals  that  are  mainly  secondary  but  also  primary 
are  marked  "s  P." 

MINERALS  OP  EPIGENETIC  DEPOSITS 

acmite     p  apophyllite     p  brucite     P  s? 

actinolite     p  aragonite     P  s  calamine     s 

adularia     p  arfvedsonite     p  calaverite     p 

aegirite    p  argentite     s   p  calcite     P  s 

albite     p  arsenic     s  caledonite     s 

alum     s  arsenopyrite     p  calomel     s 

alunite     p  s  atacamite     s  cancrinite     p 

allanite     p  augite     p  cassiterite     p 

amalgam     s  aurichalcite     s  celestite     P  s? 

amphibole     p  azurite     s  cerusite     s 

analcite     p  barite     P  s  cerargyrite     s 

andalusite     p  bastite     s  chalcanthite     s 

andradite     p  bauxite     s  P  chalcedony     s  p 

anglesite     s  beryl     p  chalcocite     s   p 

anhydrite     p  biotite     p  chalcopyrite     p  s 

ankerite     p  s  bismuth     p  s  chert     P  s 

anorthite     p  bismuthinite     p  chlorite     P  s 

anthophyllite     p  bornite     p  s  chromite     p 

antimony     s  bort     P  chrysocolla     s 

apatite     p  bromyrite     s  cinnabar     p  s 

aquamarine     p  brookite     p  cobaltite     p 

1  GILBERT,  C.  G.,  and  POGUE,  J.  E.:  The  Mount  Lyell  Copper  District  of 
Tasmania.     U.  S.  Nat.  Mus.  Proc.,  vol.  45,  pp.  609-625,  1913. 

2  ROGERS,  A.  F. :  The  So-called  Graphic  Intergrowth  of  Bornite  and  Chal- 
cite.     Econ.  Geol,  vol.  11,  pp.  582-593,  1916. 


SUPERFICIAL  ALTERA  TION  AND  ENRICHMENT  149 


MINERALS  OF 

EPIGENETIC  DEPOSITS, 

,  —  (Continued) 

copper    s  P 

magnesite     p  s 

sericite     p 

corundum     p 

magnetite     p  s 

serpentine     s  p? 

cordierite     p 

malachite     s 

siderite     p  s 

covellite     s  p 

manganite     s 

sillimanite     p 

cryolite     p 

marcasite     p  s 

silver  (native)     s  P 

cuprite     s 

melaconite     s 

smithsonite     s 

diallage     p 

melilite     p 

sodalite     p 

diamond     p 

mercury     s 

specularite     P  s? 

diopside     p 

microcline.   p 

spinel     P 

dolomite     p  s 

millerite     s  p? 

spodumene     P 

elseolite     p 

molybdenite     p 

staurolite     P 

emerald     p 

molybdite     s 

steatite,  talc     s 

emery     p 

monazite     p 

stephanite     s  P? 

enargite     p   s? 

muscovite     P 

stibnite     p 

epidote     p 

nepheline     p 

stilbite     P 

fayalite     p 

niccolite     P 

stromeyerite     s   p? 

fluorite     p 

noselite     p 

sulphur     s 

forsterite     p 

octahedrite     p 

sylvanite     p 

franklinite     p 

olivine     p 

talc     s 

gahnite     p 

opal     PS? 

tellurides     P 

galena     p  s 

orpiment     p   s 

tennantite     P  s 

garnet     P 

orthoclase     p 

tenorite     s   • 

gaylussite     p 

ottrelite     P 

tetradymite     p 

gibbsite     p  s 

pentlandite     p 

tetrahedrite     P  s 

glauconite     p 

perofskite     p 

tin  (native)     s 

glaucophane     p 

petzite     P 

titan  ite     P 

gold  (native)     p   s 

picotite     P 

topaz     P 

goslarite     s 

platinum     p   s? 

tourmaline     P 

greenockite     s   p? 

polybasite     s   p? 

tremolite     P 

graphite     p 

proustite     s   P? 

tridymite     P 

gypsum     p  s 

psilomelane     s 

turgite    and    amor- 

halite    p  s 

pyrargyrite    s  P 

phous  hematite    s  P 

haiiynite     p 

pyrite     p   s 

turquoise     s 

hematite     p   s 

pyrolusite     s 

uralite     P  s 

hornblende     p 

pyromorphite     s 

valencianite     p 

humite  group     p 

pyrophyllite     p 

vesuvianite     P 

hydrozincite     s 

pyroxenes     F 

willemite     P  s 

ilmenite     p 

pyrrhotite     p 

witherite     s 

ilvaite     p 

quartz     p   s 

wolframite     P 

iron  (native)     p 

realgar     p  s 

wollastonite     P 

jadeite     p 

rhodochrosite     P   s? 

wurtzite     s 

kalgoorlite     p 

rhodonite     p 

xenotime     p 

kaolin     s   p 

riebeckite     p 

yttrialite     p 

kyanite     p 

ruby     P 

zeolites     P  s 

lead  (native)     s 

rutile     P 

zinc  blende     P  s 

leadhillite     s 

sapphire     p 

zincite     P 

lepidolite     p 

scapolite     P 

zircon     p 

leucite     P 

scheelite     P  s? 

zoisite     p 

limonite    s    p 

selenite    s  p 

150      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Summary  of  Criteria  for  Determination  of  Secondary  Ores. — 
As  secondary  ores  are  superficial  they  will  give  out  in  depth; 
thus  the  problems  of  enrichment  are  vital  to  intelligent  exploita- 
tion. Every  metal  behaves  differently  under  the  conditions  that 
produce  such  ores.  The  migration  of  metals  depends  also  upon 
the  composition  of  the  ore  and  gangue  and  upon  many  other 
factors.  Yet  there  are  certain  general  principles  that  may  be 
applied  to  the  investigation  of  all  such  deposits.  Secondary  ores 
may  be  divided  into  two  classes — those  enriched  by  subtraction 
of  valueless  material  and  consequent  reduction  of  mass,  and  those 
enriched  by  precipitation  or  substitution  of  certain  metals. 

1.  Ores  that  are  formed  by  the  removal  of  valueless  material 
without  much  addition  of  valuable  material  are  generally  porous, 
the  pores  representing  the  material  that  has  been  removed ;  slump- 
ing near  the  surface  and  cementation  below  operate  to  eliminate 
pore  space.     Ores  formed  in  this  manner  generally  merge  downward 
into  low-grade  protore.     They  usually  show  an  evident  relation 
to  the  surface  and  commonly  also  to  the  ground-water  level;  the 
deposits  are  likely  to  be  thicker  under  a  hilltop,  where  the  out- 
crop is  high  and  the  water  level  farther  below  the  outcrop,  than 
in  a  valley.     Ores  of  this  character  are  developed  on  many  base- 
leveled  surfaces  and  may  be  found  on  plateaus  or  elevated  regions, 
on  surfaces  that  have  been  raised  after  the  ores  were  extensively 
weathered.     Circulation  may  have  been  controlled  by  structural 
features,  and  the  deposits  enriched  by  the  removal  of  valueless 
material  may  be  found  fractured  areas-  or  in  structural  basins 
or  in  pitching  troughs    because    circulation    was    active    there. 
Ores   of  this   class    have  been  concentrated  at  the  surface  be- 
cause the  metals  they  contain  are  not  readily  removed  by  solu- 
tion; in  many  places  such  metals  are   removed  by  mechanical 
erosion  from  the  outcrops,  and  commonly  they  are  found  along 
streams  in  beds  that  have  been  formed  by  mechanical  disintegra- 
tion and  deposition  near  by.     Placer  deposits  and   detrital  sedi- 
ments  are   frequently  formed  near  older   deposits    containing 
metals.     The  minerals  are  those  that  are  stable  under  surface 
conditions,    .the     so-called    end    products  of    weathering — iron 
.oxides,  kaolin,  bauxite,  manganese  oxides — and  the  stable  residuals, 
such  as  magnetite,  chromite,  and  in  some  deposits  gold,  platinum, 
cassiterite,  and  monazite. 

2.  Ores  enriched  by  precipitation  or  substitution  of  certain 
metals  may  be  porous  and  open  textured,  especially  in  the  zone 
of  oxidation;  hi  depth  cementation  tends  to  eliminate  openings. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  151 

The  ores  commonly  show  characteristic  changes  downward:  a 
leached  zone  at  and  near  the  surface  may  grade  into  a  zone  of 
high-grade  oxidized  ore  and  this  into  high-grade  sulphide  ore, 
which  in  turn  changes  into  low-grade  sulphide  ore  at  greater 
depth.  Although  these  zones  are  developed  in  a  majority  of  the 
deposits  of  this  group,  they  are  not  everywhere  developed  and 
rapid  erosion  may  remove  one  or  all  of  them.  The  secondary 
zones  are  related  to  the  topography  of  the  time  when  they  were 
formed,  their  tops  generally  lying  farther  below  the  surface,  yet 
at  higher  altitudes,  below  hilltops  than  below  valleys  (Fig.  67). 
The  top  of  the  secondary  sulphide  zone  is  generally  related  also 
to  the  water  level  of  the  time  when  it  was  formed,  but  subsequent 
changes  of  the  water  level  may  obscure  this  relation,  and  in  arid 
countries,  where  the  water  level  may  be  very  deep,  secondary 
sulphide  ores  appear  to  have  formed  above  and  independent  of 
the  water  level.  Many  primary  ores  that  were  formed  near  the 
surface  show  a  relation  to  the  topography  existing  when  they 
were  formed,  because  the  maximum  precipitation  of  the  metals 
in  the  original  ore  body  was  accomplished  by  mingling  of  the 
hot  ascending  solutions  with  cold  and  perhaps  oxidized  meteoric 
waters,  or  by  the  escape  of  gases  that  had  held  the  metals  in 
solution.  Thus  a  relation  of  the  rich  zone  to  the  present  topog- 
raphy is  a  more  certain  criterion  as  applied  to  older  and  more 
deeply  eroded  deposits  than  to  younger  deposits  that  may  have 
been  formed  under  topographic  conditions  which  were  perhaps 
similar  in  essential  features  to  those  now  prevailing. 

The  metals  that  are  concentrated  by  transportation  and  pre- 
cipitation are  those  that  go  into  solution  and  consequently  those 
that  are  commonly  leached  from  outcrops.  Their  deposits  gen- 
erally carry  iron  sulphides.  A  porous  leached  outcrop  stained 
with  iron  suggests  secondary  ores  below.  Erosion  may  be  so 
rapid,  however,  that  the  gossan  is  not  completely  leached;  the 
valuable  metals  may  not  all  be  removed.  Thus  placers  may  be 
formed  from  a  deposit  whose  gossan  is  only  partly  leached,  or  by 
rapid  erosion  the  gossan  or  even  the  secondary  sulphide  zone 
itself  may  be  removed,  exposing  the  primary  ore  at  the  outcrop. 
Altered  zones  may  have  been  tilted  or  faulted  or  covered  with 
later  igneous  rocks  or  sedimentary  beds. 

The  groups  of  minerals  of  the  several  zones  are  characteristic. 
Few  minerals  are  exclusively  primary  or  exclusively  secondary, 
yet  some  are  essentially  one  or  the  other,  and  an  adequate  study 


152      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

of  the  minerals  and  their  relations  will  serve  to  determine  the 
genesis  of  nearly  all  deposits.  Because  the  heavy  silicates  and 
several  other  minerals  characteristic  of  deposits  formed  at  con- 
siderable depths  are  not  formed  by  secondary  processes,  the 
mineralogic  criteria  are  more  satisfactorily  applied  to  these 
deposits.  Kaolin  and  gypsum  are  formed  in  some  secondary 
ores  by  acid  solutions  reacting  on  gangue  minerals  or  on  wall 
rock.  Other  secondary  ores,  however,  contain  neither  kaolin  nor 
gypsum.  The  value  of  each  common  mineral  as  indicating 
genesis  is  discussed  elsewhere. 

The  texture  of  an  ore  and  its  paragenesis  may  indicate  its  ori- 
gin. The  primary  minerals  are  by  definition  earlier  than  the 
secondary  minerals,  and  the  latter  will  be  found  in  cracks  cut- 
ting the  earlier  ore,  or  replacing  it  along  cracks.  The  mere  fact 
that  one  association  of  minerals  is  found  in  cracks  in  an  older 
association  of  minerals  or  that  one  mineral  has  been  formed  by 
metasomatic  replacement  of  another  is  not  an  indication  of  de- 
position by  downward-moving  waters.  Veins  are  frequently 
fractured  after  they  are  formed,  and  ascending  waters  may  de- 
posit minerals  of  different  composition  in  the  cracks.  Therefore 
this  criterion  may  not  lead  to  final  decision,  and  in  considering 
it  proper  weight  should  be  given  to  the  mineral  composition  of 
the  older  and  younger  groups  of  minerals  and  to  the  relation  of 
the  ore  body  to  other  zones.  From  the  study  of  a  large  number 
of  deposits  it  is  found  that  there  are  certain  downward  changes 
that  may  be  considered  standard.  These  standard  changes, 
which  are  different  for  each  of  the  metals  and  for  each  metal  in 
different  mineral  associations,  are  discussed  on  pages  that  follow. 

The  vertical  extent  of  a  secondary  sulphide  zone,  though  de- 
pending somewhat  on  the  topography,  past  and  present,  the 
duration  of  the  period  of  erosion,  the  geologic  history,  and  the 
environment,  should  show,  nevertheless,  a  relationship  to  the 
permeability  of  the  primary  ores  and  to  their  mineral  composition. 
In  permeable  primary  deposits  the  valuable  metals  may  be  car- 
ried farther  before  they  are  precipitated  than  in  deposits  that  are 
less  permeable.  But  in  equally  permeable  ores  the  rates  at 
which  the  valuable  metals  are  precipitated  differ  greatly.  The 
secondary  sulphide  zone  will  have  smaller  vertical  range  in  de- 
posits whose  primary  ores  act  readily  to  precipitate  the  metals. 

Finally,  secondary  ores  do  not  contain  elements  that  were  not 
in  the  primary  ores  or  in  the  country  rock  or  in  air  or  surface  waters. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  153 

Estimates  of  Portions  of  Lodes  Eroded.  —  Where  there  is  reason 
to  suppose  that  the  primary  ore  was  of  approximately  uniform 
composition  before  secondary  alteration  took  place,  it  is  possible 
to  estimate  the  vertical  extent  of  the  portion  of  the  deposit  that 
has  been  eroded.  The  ore  in  the  secondary  zone  generally  con- 
tains valuable  metals  that  were  present  in  the  primary  ore,  those 
that  were  leached  from  the  oxidized  zone  above  the  secondary 
sulphide  ore,  and  those  that  were  leached  from  the  portion  of  the 
deposit  that  has  been  removed  —  the  metals  that  were  carried 
downward  by  solutions  in  advance  of  erosion.  In  some  deposits 
the  material  dissolved  is  carried  away  in  the  general  circulation 
or  is  carried  into  fractures  in  the  country  rock  and  redeposited 
outside  of  the  original  deposits,  and  the  loss  of  the  original 
deposits  should  be  taken  into  consideration. 

If  x  equals  the  vertical  extent  in  feet  of  the  part  of  the  lode 
which  has  been  removed  from  above  the  present  apex,  a  equals 
the  vertical  extent  in  feet  of  the  leached  zone,  6  equals  the  vertical 
extent  in  feet  of  the  enriched  zone,  I  equals  the  valuable  metals 
(stated  in  any  convenient  unit,  such  as  assay  content  per  ton) 
remaining  in  the  leached  zone,  e  equals  the  valuable  metals  in 
the  enriched  zone,  and  p  equals  the  valuable  metals  in  the  primary 
ore,  the  following  formula  may  be  applied  to  ascertain  the  num- 
ber of  feet  removed: 


P 

This  formula  does  not  take  into  account  the  changes  in  mass1 
in  the  ore  itself  nor  the  pore  space  formed,  and  it  is  recognized,  of 
course,  that  many  other  factors  may  modify  the  results;  for  the 
valuable  metals  in  the  material  dissolved  may  not  all  be  redepos- 
ited in  the  ore  body,  and  the  primary  ore,  before  alteration  and 
enrichment,  may  not  have  been  of  equal  richness  throughout  the 
deposit.  The  estimates  therefore  give  only  a  rude  approxima- 
tion, but  one  which  may  be  used,  in  connection  with  other  geologic 
data,  as  a  check  on  conclusions  regarding  the  minimum  amount 
of  erosion  that  has  taken  place  since  the  primary  ore  was 
deposited. 

1  If  value  can  be  expressed  as  assay  content  per  unit  volume  instead  of 
per  ton,  changes  in  mass  are  accounted  for. 


154       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Experimental  Data  on  Solution  and  Precipitation  of  Metals.— 

The  oxidation  of  pyrite  proceeds  as  follows:1 

FeS2  +  7O  +  H2O  =  FeSO4  +  H2SO4. 

Ferrous  sulphate  in  the  presence  of  air  is  oxidized  to  ferric  sul- 
phate or  to  ferric  sulphate  and  ferric  hydroxide: 

6FeS04  +  30  +  3H2O  =  2Fe2(SO4)3  +  2Fe(OH)3. 

The  hydrolysis  of  ferric  sulphate  may  first  give  a  basic  ferric 
sulphate,2  but  this  subsequently  breaks  down,  forming  ferric  hy- 
droxide and  sulphuric  acid,  as  indicated  below: 

Fe2(SO4)3  +  6H2O  =  2Fe(OH)3  +  3H2S04. 
Ferric  hydroxide  on  partial  dehydration  would  give  limonite: 
4Fe(OH)3  =  2Fe203  +  6H2O  =  2Fe2O3.3H2O  +  3H2O. 

A.  N.  Winchell3  treated  powdered  pyrite  to  dripping  aerated 
water.  At  the  end  of  ten  months  300  grams  of  pyrite  had  lost 
0.2  gram  and  the  solution  circulating  through  the  pyrite  had 
gained  0.332  gram  of  ferric  sulphate  and  0.068  gram  of  sulphuric 
acid. 

Grout4  also  performed  these  experiments,  subjecting  powdered 
pyrite  to  repeated  drying.  In  a  year  the  specimens  had  lost  from 
0.015  to  0.057  per  cent.  These  losses  are  of  the  same  order  as 
those  obtained  by  Winchell,  namely,  0.067  per  cent. 

Buehler  and  Gottschalk5  treated  finely  powdered  sulphides  to 
dripping  water.  Each  sulphide  when  treated  alone  was  dissolved 
slowly,  but  when  two  were  treated  together  the  action  on 
one  of  them  was  accelerated  and  that  on  the  other  retarded. 

1  ALLEN,  E.  T.:  Sulphides  of  Iron  and  Their  Genesis.  Min.  and  Sci. 
Press,  vol.  103,  pp.  413-414,  1911. 

GOTTSCHALK,  V.  H.,  and  BUEHLER,  H.  A.,  Oxidation  of  Sulphides.  Econ. 
Geol,  vol.  7,  pp.  15-34,  1912. 

"PENROSE,  R.  A.  F.,  JR.:  The  Superficial  Alteration  of  Ore  Deposits. 
Jour.  Geol,  vol.  2,  p.  293,  1894. 

BRAUNS,  REINHARD:  "Chemische  Mineralogie,"  p.  368,  Leipzig,  1896. 

3  WINCHELL,  A.  N.:  The  Oxidation  of  Pyrite.     Econ.  Geol.,  vol.  2,  pp. 
290-294,  1907. 

4  GROUT,  F.  F.:  The  Oxidation  of  Pyrite.     Econ.  Geol.,  vol.  3,  pp.  532- 
534,  1908. 

6  BUEHLER,  H.  A.,  and  GOTTSCHALK,  V.  H. :  Oxidation  of  Sulphides. 
Econ.  Geol,  vol.  5,  pp.  28-35,  1910;  vol.  7,  pp.  15-34,  1912. 

WELLS,  R.  C.:  Electric  Activity  in  Ore  Deposits.  U.  S.  Geol.  Survey 
Bull  548,  pp.  1-78,  1914. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  155 

Sphalerite  dissolved  more  readily  in  the  presence  of  pyrite.  The 
acid  generated  by  the  oxidation  of  iron  sulphide  was  not  the  only 
factor  that  favored  solution,  for  when  the  iron  sulphide  was  placed 
on  a  niter  above  zinc  sulphide  the  rate  of  solution  of  zinc  sulphide 
was  not  greatly  increased.  Buehler  and  Gottschalk  concluded 
that  the  acceleration  of  the  reactions  is  due  to  electric  currents 
generated  by  the  contact  of  minerals  of  different  potential.  The 
current  flows  from  the  mineral  having  the  higher  potential,  and 
so  the  one  of  lower  potential  will  dissolve  more  rapidly,  the  one 
of  higher  potential  being  protected  from  oxidation  and  solution. 
The  same  investigators  arranged  several  minerals  in  a  series  anal- 
ogous to  the  electromotive  series  of  the  metals,  having  determined 
their  potentials  by  the  use  of  the  potentiometer.  The  results  of 
the  experiments  are  shown  in  part  below.  The  force  generated 
while  any  two  of  these  minerals  are  immersed  in  a  solution  tends 
to  accelerate  the  oxidation  and  dissolution  of  that  sulphide  which 
stands  lower  in  the  series  and  to  retard  the  oxidation  of  the  one 
that  stands  higher.  It  is  noteworthy  that  the  experiments  were 
made  with  distilled  water.  The  potential  varies  with  the  solu- 
tion, and  for  some  salts  the  relations  are  not  those  indicated  in 
the  table. 

POTENTIAL,  IN  VOLTS,  OP  SEVERAL  SULPHIDES  MEASURED  IN  DISTILLED 
WATER  AGAINST  COPPER  WIRE 

Marcasite +0.37  Galena +0. 15 

Argentite +0.27  Chalcocite +0.14 

Chalcopyrite +0 . 18  to  0 . 30  Metallic  copper 0 . 00 

Covellite +0.20  Stibnite -0.17  to  0.60 

Pyrite.. +0.18  Sphalerite -0.20  to  0.40 

Bornite +0.17 

H.  V.  Winchell1  placed  in  a  sealed  jar  crystals  of  pyrite  in  a 
slightly  acid  solution,  containing  sulphur  dioxide  and  a  dilute 
solution  of  copper  sulphate.  At  the  end  of  3  months  films  of 
chalcocite  were  deposited  on  the  pyrite,  and  its  copper  content, 
which  at  the  beginning  of  the  experiment  was  1.50  per  cent.,  had 
increased  to  3.60  per  cent.  The  chalcocite  coated  the  iron  sul- 
phide completely  with  glistening  surfaces.  In  another  jar,  with 
similar  reagents  except  sulphur  dioxide,  no  chalcocite  was  depos- 
ited. Winchell  suggests  that  sulphur  dioxide  is  necessary  for  the 

1  WINCHELL,  H.  V. :  Synthesis  of  Chalcocite  and  Its  Genesis  at  Butte, 
Mont.  Geol.  Soc.  America  Bull.,  vol.  14,  pp.  272-275,  1903. 


156      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

deposition  of  cuprous  sulphide.  He  infers  that  the  cupric  sul- 
phate in  solution  with  sulphur  dioxide  supplies  cuprous  ions  for  the 
reaction  with  the  pyrite  that  supplies  the  sulphur  for  the  chalco- 
cite,  the  iron  going  into  solution  as  ferrous  salt.  He  found  that 
iron  was  dissolved  and  subsequently  precipitated  as  ferric 
hydroxide. 

Recently  Spencer1  has  discussed  the  possible  reactions  in- 
volved in  chalcocite  deposition  and  has  shown  that  chalcocitiza- 
tion  may  result  from  a  long  series  of  oxidizing  reactions. 

In  the  table  below  the  solubilities  of  several  anhydrous  sul- 
phates are  compared.  Of  these  only  the  least  soluble — barium, 
lead,  and  calcium  sulphates — form  stable  minerals  in  ore  deposits. 


ANHYDROUS  SULPHATES  HELD  IN  SOLUTION  IN  A  LITER  OF  WATER  AT  18°C. 

[After  Kohlrausch] 
Grams  Grams 

BaSO4 0.0023  K2SO4 111.1000 

PbSO4 0.0410  Na2SO4 168.3000 

CaSO4 2.0000  MgSO4 354.3000 

Ag2SO4 5.5000  ZnSO4 531.2000 

Some  metals  are  precipitated  from  sulphate  solution  by  alka- 
line silicates.  E.  C.  Sullivan2  placed  finely  ground  materials — 
such  as  orthoclase,  albite,  amphibole,  and  clay  gouge— in  flasks 
with  copper  sulphate,  silver  sulphate,  and  other  salts.  The  solu- 
tions were  dilute,  and  the  precipitation  of  copper  or  silver  in 
many  of  the  flasks  was  nearly  complete.  Reactions  of  this  char- 
acter are  probably  at  some  places  effective  processes  in  the  en- 
richment of  ore  bodies,  for  the  essential  conditions  of  the  experi- 
ments commonly  exist  in  nature.  The  neutralization  of  the  acid 
solutions  by  the  alkaline  silicates  permits  such  reactions  as  are 
favored  by  a  neutral  environment. 

According  to  Weigel3  the  solubilities  of  the  freshly  precipitated 
sulphides  are  as  follows: 

1  SPENCER,  A.  C. :  Chalcocite  Deposition,  Wash.  Acad.  Sci.  Jour.,  vol. 
3,  pp.  70-75,  1913;  Econ.  Geol,  vol.  8,  pp.  621-652,  1913. 

2  SULLIVAN,  E.  C.:  The  Interaction  between  Minerals  and  Water  Solu- 
tions, with  Special  Reference  to  Geologic  Phenomena.     U.  S.  Geol.  Survey 
Butt.  312,  pp.  37-64,  1907. 

3  WEIGEL,  OSKAR:  Die  Loslichkeit  von  Schwermetallsulfiden  in  reinem 
Wasser.     Zeitschr.  phys.  Chemie,  vol.  58,  pp.  293-300,  1907. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  157 

SOLUBILITIES  OF  SEVERAL  SULPHIDES  EXPRESSED  AS  MOLS  PER  LITER  X  10~6 

MnS 71.600  PbS 3.600 

ZnS 70.600  CuS 3.510 

FeS 70.100  As2S3 2.100 

CoS 41.620  Ag2S 0.552 

NiS 39.870  Bi2S3 0.350 

CdS 9.000  HgS 0.054 

Sb2S3 5.200 

Schuermann1  brought  several  metallic  sulphates  into  contact 
with  different  metallic  sulphides  and  established  a  series,  the  sul- 
phides of  any  one  of  the  metals  of  which  will  be  precipitated  at 
the  expense  of  any  sulphide  lower  in  the  series.  This  series, 
which  he  regards  as  showing  the  "relative  affinity"  of  the  heavy 
metals  for  sulphur,  is  mercury,  silver,  copper,  bismuth,  cad- 
mium, antimony,  tin,  lead,  zinc,  nickel,  cobalt,  iron,  arsenic, 
thallium,  and  manganese.  Those  last  named  have  the  least 
affinity.  As  pointed  out  by  Wells,  the  principal  results  of  Schuer- 
mann's  experiments  may  be  regarded  as  reactions  establishing 
chemical  equilibria,  the  salts  being  fixed  in  the  order  of  their  solu- 
bilities under  the  conditions  of  the  experiments.  The  positions 
of  the  commoner  metals  in  the  Schuermann  series  agree  fairly 
well  with  the  solubilities  of  the  sulphides  determined  by  Weigel, 
and  the  metals  that  replace  those  lower  in  the  series  generally 
have  lower  solubilities  than  those  which  they  replace. 

Hydrogen  sulphide  is  made  in  the  chemical  laboratory  by  treat- 
ing artificial  ferrous  sulphide  with  acid.  The  generation  of  hy- 
drogen sulphide  is  almost  instantaneous.  As  shown  by  Allen, 
pyrrhotite  is  a  solid  solution  of  ferrous  sulphide  (FeS)  and  sul- 
phur. Both  pyrrhotite  and  zinc  blende  dissolve  in  acid  with 
noticeable  evolution  of  hydrogen  sulphide,  whereas  pyrite  and 

1  SCHUERMANN,  ERNST:  Ueber  die  Verwandtschaft  der  Schwermetalle 
zum  Schwefel.  Liebig's  Annalen,  vol.  249,  p.  326,  1888. 

EMMONS,  S.  F. :  The  Secondary  Enrichment  of  Ore  Deposits.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  30,  p.  212,  1901. 

WEED,  W.  H.:  The  Enrichment  of  Gold  and  Silver  Veins.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  30,  p.  428,  1901.  The  Affinity  of  Metals  for 
Sulphur.  Eng.  and  Min.  Jour.,  vol.  50,  p.  484,  1890.  Review  of  lecture  by 
Watson  Smith,  on  Schuermann's  reactions.  Soc.  Chem.  Ind.  Jour.,  vol. 
11,  pp.  869-871,  1892. 

ZIES,  E.  G.,  ALLEN,  E.  T.,  and  MERWIN,  H.  E.:  Some  Reactions  In- 
volved in  Secondary  Sulphide  Enrichment.  Econ.  Geol,  vol.  11,  pp.  407- 
503,  1916. 


158      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

chalcopyrite  are  almost  insoluble  in  sulphuric  or  hydrochloric 
acid.  They  are  readily  decomposed,  however,  in  nitric,  an  oxi- 
dizing acid. 

Several  experiments  were  made  by  R.  C.  Wells  to  ascertain  the 
rates  at  which  hydrogen  sulphide  is  generated  by  the  action  of 
cold  dilute  acid  waters  on  sulphides  of  the  metals.  He  exposed 
over  night  five  minerals  to  0.057  normal  sulphuric  acid.  The  re- 
sulting solutions  were  titrated  with  iodine  solution  to  ascertain 
the  amount  of  hydrogen  sulphide  generated,  the  iodine  solution 
required  being,  for  pyrrhotite,  28.5  cubic  centimeters;  for  sphal- 
erite, 1.05  cubic  centimeters;  for  galena,  0.40  cubic  centimeter; 
for  chalcopyrite,  0.29  cubic  centimeter;  for  pyrite,  0.28  cubic 
centimeter. 

With  equal  surfaces  exposed,  cold  dilute  acid  solutions  set 
hydrogen  sulphide  free  at  least  four  times  as  rapidly  from  some 
zinc  blende  as  from  pyrite  or  chalcopyrite,  and  about  twenty-five 
times  as  rapidly  from  pyrrhotite  as  from  zinc  blende.  Zinc 
blende  containing  considerable  iron  sulphide  will  react  more 
readily  with  acid  than  pure  zinc  sulphide. 

Grout1  showed  that  alkaline  solutions  react  with  many  sulphides 
and  give  alkali  sulphides  that  precipitate  the  metals  from  sulphate 
solutions.  Pyrrhotite  acts  much  more  rapidly  than  other  iron 
sulphides.  Whether  the  solutions  are  acid  or  alkaline  the  metals 
will  be  precipitated  more  rapidly  with  pyrrhotite  than  with  other 
iron  sulphides. 

Composition  of  Mine  Waters. — The  following  tables  show  the 
results  of  analyses  of  samples  of  water  taken  from  mines  contain- 
ing deposits  of  sulphide  ores : 

1  GROUT,  F.  F. :  On  the  Behavior  of  Cold  Acid  Sulphate  Solutions  of 
Copper,  Silver,  and  Gold  with  Alkaline  Extracts  of  Metallic  Sulphides. 
Earn.  Geol.,  vol.  8,  pp.  407-433,  1913. 

NISHIHARA,  GEORGE  S.:  The  Rate  of  Reduction  of  Acidity  of  Descend- 
ing Waters  by  Certain  Ore  and  Gangue  Minerals  and  Its  Bearing  on  Second- 
ary Sulphide  Enrichment.  Econ.  Geol,  vol.  9,  pp.  743-757,  1914. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  159 


ANALYSES  OF  MINE  WATERS  IN  SULPHIDE  DEPOSITS 

(Parts  per  million) 
WATERS  OP  COPPER  MINES 


1 

2 

3 

4 

5 

SO4  
Cl  
CO3  
PO4 

71,053.3 
17.7 

1  5 

2,672.0 
13.0 

Trace 

415.8 
0.7 

476.8 
0.4 

5,064 
Undet. 

B«O7 

Trace 

Br 

Trace 

F 

Trace 

SiO2 

67  4 

47  7 

37  0 

49  9 

76 

K 

6  8 

13  1 

2  7 

2  2 

\ 

Na 

41.7 

39.6 

5.2 

5  5 

\  Undet. 

Li  
Ca  
Me 

Trace 
307.7 
149  2 

132.5 
61  6 

19.7 
5  2 

30.4 
6  2 

436 
61 

Al 

85  2 

83  5 

14  5 

19  1 

Undet 

Mn 

13  2 

12  0 

0  2 

0  1 

236 

Ni 

3.5 

\ 

Co 

4  6 

>        0.5 

Cu  
Zn  
Cd  
Fe"  

45,633.2 
411.2 

\ 

59  1 
852.0 
41.1 

28.1 
2.4 

71.4 

11.0 
2.9 

89.2 

1,659 
Undet. 

305 

Fe'" 

>      49.8 

159.8 

20  3 

55  9 

Acidity:  HzSCh  

210.2 

97.5 

970 

1.  Mountain  View  mine,  Butte,   Montana,  second  level.     W.  F.  Hillebrand,  analyst. 
CLARKE,  F.  W.:  The   Data  of  Geochemistry,  3d  ed.  U.  S.  Geol.  Survey  Bull.  616,  p.  633, 
1916.     WEED,  W.  H.:  Geology  and  and  Ore  Deposits  of  the  Butte  District,  Montana.    *U.  8. 
Geol.  Survey  Prof.  Paper  74,  p.  101,  1912. 

2.  St.  Lawrence  mine,  Butte,  Montana.     W.  F.  Hillebrand,  analyst.     CLARKE,  F.  W.: 
Op.  cit.,  p.  632.     WEED,  W.  H.,  Op.  cit.,  p.  101. 

3.  Callaway  shaft,  Ducktown,  Tenn.,  at  water  level.     R.  C.  Wells,  analyst.     EMMONS, 
W.  H.  and  LANEY,  F.  B.:  Preliminary  Report  on  the  Mineral  Deposits  of  Ducktown,  Tenn., 
U.  S.  Geol.  Survey  Bull.  470,  pp.  171-172,  1911. 

4.  Callaway  shaft,  Ducktown,  Tenn.,  37  feet  below  water  level.     R.  C.  Wells,  analyst. 
EMMONS  and  LANEY:  Idem. 

5.  Capote  mine,  Cananea,  Mex.,  300-foot  level.     G.  W.  Hawley  (chief  chemist  Cananea 
Consolidated  Copper  Company),  analyst. 


160      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 
WATERS  OF  GOLD  AND  SILVER  MINES 


6 

7 

8 

9 

SO4  
Cl      

474.00 
19.00 

209,100.00 
127  .  60 

60.12 
Trace 

2,039.51 
8  16 

CO3  

20.45 

SiO2 

133  40 

616  00 

18  00 

43  80 

K 

53  40 

2  41 

70  00 

Na 

132  00 

535  00 

28  52 

106  27 

Li  
Ca 

100  10 

1  286  00 

17  36 

187  15 

Mg  

5.88 

6,590.00 

1.30 

93.50 

Al  

1.37 

9,670.00 

3.12 

Mn 

885  10 

0  25 

155  58 

Cu 

147  50 

77  05 

Zn  

49  66 

Pb  

3.44 

Fe"  

1.47 

164  .  82 

Fe'"  
H  (acids) 

6.33 

5,025.00 
2,575  00 

Alk 

CO2 

61  53 

6.  C.  &  C.  shaft   (Comstock  lode),   Storey  County,  Nevada;  2,250-foot  level.     N.   E. 
Wilson,  analyst.     REID,  J.  A.:  The  Structure  and  Genesis  of  the  Comstock  Lode.     Cal. 
Univ.  Dept.  Geology  Bull.  vol.  4,  p.  189,  1906. 

7.  Central   tunnel    (Comstock   lode),   Storey   County,    Nevada;   vadose   water.     N.    E. 
Wilson,  analyst.      REID,  J.  A.,  Op.  cil.,  p.  192. 

8.  Bachelor  mine,  Creede,  Colorado.     Iron  includes  some  aluminum.     Water  is  alkaline. 
Chase  Palmer,  analyst. 

9.  Stanley  mine,  Idaho  Springs,  Colorado.    L.  J.  W.  Jones,  analyst.    JONES,  L.  J.  W.:  Ferric 
Sulphate  in  Mine  Waters  and  Its  Action  on  Metals.     Col.  Sci.  Soc.  Proc.,  vol.  6,  for  1897- 
1900,  p.  48. 

General  Character  of  Mine  Waters. — In  the  table  above  are 
stated  analyses  of  several  waters  from  mines  yielding  sulphide 
ores  of  copper,  gold,  and  silver.1  The  opening  of  a  mine  increases 
circulation  and  probably  makes  the  waters  that  permeate  the 
deposits  more  dilute;  mine  workings  also  permit  access  of  air  at 
lower  levels,  and  this  makes  the  waters  more  acid  and  more  active 
solvents.  It  is  not  certain  that  the  solutions  that  have  enriched 
sulphide  ores  are  like  the  waters  that  now  traverse  the  depos- 
its, but  it  is  highly  probable  that  they  are  of  the  same  general 
character. 

1  Many  other  analyses  may  be  found  in  my  paper,  The  Enrichment  of 
Ore  Deposits,  (U.  S.  Geol.  Survey  Bull.  625,  pp.  86-89,  1917),  and  in  a  paper 
by  E.  T.  HODGE:  (The  Composition  of  Waters  in  Mines  of  Sulphide  Ores). 
Econ.  Geol,  vol.  10,  pp.  123-139,  1915. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  161 

Sulphides  exposed  to  air  and  water  are  changed  to  sulphates 
and  to  sulphuric  acid.  The  iron  minerals  are  the  chief  sources 
of  sulphuric  acid  because  some  of  them  contain  more  sulphur  than 
that  necessary  to  balance  iron  when  iron  sulphate  forms  and  be- 
cause iron  sulphate  in  the  presence  of  oxygen  forms  the  ferric 
salt,  which  hydrolyzes  readily,  giving  basic  ferric  sulphate  and 
ultimately  limonite.  Galena  and  zinc  blende  may  oxidize  to  sul- 
phates without  liberating  acid.  The  following  equations,  which 
represent  certain  stages  in  the  reactions,  illustrate  this  principle: 

FeS2  +  H20  +  70  =  FeSO4  +  H2SO4. 
CuFeS2  +  80  =  FeSO4  +  CuS04. 
PbS  +  40  =  PbSO4. 
ZnS  +  4O  =  ZnSO4. 

The  oxidation  of  ferrous  sulphate  to  ferric  salt  and  the  hydrol- 
ysis of  ferric  sulphate  take  place  very  readily  in  the  presence  of 
oxygen. 

2FeS04  +  H2S04  +  O  =  Fe2(S04)3  +  H20. 
6FeSO4  +  3O  +  3H2O  =  2Fe2(SO4)3  +  Fe2(OH)6. 
Fe2(S04)3  +  6H20  =  2Fe(OH)s  +  3H2S04. 

All  mine  waters  from  sulphide  deposits  are  sulphate  solutions. 
They  generally  carry  some  chlorides,  however,  which  are  spar- 
ingly present  in  most  rain  water  and  in  some  wall  rocks  and  ores. 
Penrose,1  discussing  the  distribution  of  chloride  ores,  pointed  out 
that  the  chlorides  form  most  abundantly  in  undrained  areas. 
Carbonates  are  present  in  many  mine  waters,  especially  in  those 
that  are  alkaline.  Nitrates  are  practically  unknown.  Hydrox- 
ides are  probably  present  in  some  waters.  They  have  not  been 
determined  in  the  analyses,  but  in  several  analyses  their  presence 
is  suggested  by  the  fact  that  the  positive  radicles  exceed  the  nega- 
tive radicles.  Nearly  all  mine  waters  carry  a  little  silica.  Phos- 
phates are  sparingly  represented  in  a  few  samples,  also  arsenic 
and  antimony.  Potassium  and  sodium  are  nearly  always  pres- 
ent, but  as  a  rule  sodium  is  the  more  abundant.  Calcium,  iron 
and  magnesium  are  generally  present  in  considerable  amounts. 
Acid  waters  commonly  contain  aluminum  and  manganese.  The 
alkalies,  alkaline  earths,  aluminum,  and  manganese  are  dissolved 
by  sulphuric  acid,  which  attacks  the  ore  and  wall  rock.  Near  the 
surface  iron  is  in  the  ferric  state.  In  mines  of  copper  and  zinc 

1  PENROSE,  R.  A.  F.,  JR.  :  The  Superficial  Alteration  of  Ore  Deposits. 
Jour.  Geol,  vol.  2,  p.  314,  1894. 
11 


162      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

sulphides  copper  and  zinc  are  generally  in  solution.  Small 
amounts  of  gold  and  silver  have  been  determined  in  some  waters. 

Changes  of  Mine  Waters  in  Depth. — Waters  descending  from 
the  surface  through  sulphide  ore  deposits  pass  through  a  chang- 
ing chemical  environment  and  are  continually  changing  in  com- 
position.1 In  the  oxidized  zone  they  are  oxidized  waters  and 
acid  waters;  when  they  pass  below  the  region  where  oxygen  is  in 
excess  their  acidity  and  their  state  of  oxidation  change.  Their 
reaction  on  sulphides  produces  hydrogen  sulphide,  and  their  re- 
action on  silicates,  carbonates,  and  other  minerals  decreases 
acidity.  Iron  is  generally  abundant  in  waters  of  pyritic  ore 
bodies,  and  the  degree  of  the  oxidation  of  the  iron  is  an  index  to 
the  state  of  oxidation  of  the  water. 

The  deeper  waters  are  all  ferrous  sulphate  waters,  and  many  of 
them  are  alkaline.  Samples  3  and  4  of  the  table  on  page  159 
were  taken  from  the  same  column  of  water  standing  in  a  shaft — 
one  from  the  top  and  one  37  feet  lower.  The  acidity  of  the  deeper 
water  is  less  than  half  that  of  the  water  at  the  water  level.  Sev- 
eral waters  from  the  Capote  mine,  Cananea,  Mexico,  analyzed  by 
George  Hawley,2  show  a  marked  decrease  of  acidity  with  increase 
of  depth. 

Precipitates  from  Mine  Waters  under  Superficial  Conditions. — 
Many  waters,  after  issuing  from  mineralized  areas,  deposit 
copious  precipitates.  In  some  mining  districts  the  streams  that 
carry  away  the  underground  drainage  from  the  adits  have  stained 
their  beds  for  great  distances  from  the  points  of  issue.  In  cer- 
tain of  these  districts,  as  at  Cananea,  Mexico,  and  Bingham, 
Utah,  the  gravels  above  the  present  streams  are  cemented  by 
iron  oxides,  showing  that  the  processes  of  precipitation  at  the 
surface  were  operative  before  the  mines  were  opened.  These 
deposits,  formed  under  atmospheric  pressure  and  in  the  presence 
of  oxygen,  are  very  different  from  the  deposits  of  secondary  ore 
that  are  formed  at  depths  where  sulphide  enrichment  takes  place. 
Few  if  any  of  them  are  workable  for  the  more  valuable  metals. 

1  NISHIHARA,  G.  S. :  The  Rate  of  Reduction  of  Acidity  of  Descending 
Waters  by  Certain  Ore  and  Gangue  Minerals  and  Its  Bearing  on  Secondary 
Sulphide  Enrichment.     Econ.  Geol.,  vol.  9,  pp.  743-757,   1914. 

HODGE,  E.  T.:  The  Composition  of  Waters  in  Mines  of  Sulphide  Ores. 
Econ.  Geol,  vol.  10,  pp.  123-140,  1915. 

2  See  EMMONS,  W.  H. :  The  Enrichment  of  Ore  Deposits.    U.  S.  Geol.  Survey 
Bull.  625,  p.  87,  1917. 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  163 


Nearly  all  of  them  consist  largely  of  hydrous  iron  oxides, 
hydrous  aluminum  compounds,  and  hydrous  silica. 

Oxidation  and  Solution  of  Metallic  Sulphides. — In  the  pres- 
ence of  air  the  oxidation  and  the  solution  of  the  metallic  sulphides 
take  place  simultaneously,  and  it  is  difficult  to  consider  the  two 
processes  separately.  The  rate  of  solution  depends  on  many  fac- 
tors, among  them  (1)  the  solubility  of  the  material  in  water,  (2) 
the  molecular  and  physical  structure  of  the  material,  (3)  the  solu- 
bility of  the  salts  formed  by  oxidation,  hydration,  and  related 
processes,  (4)  the  composition,  concentration,  temperature,  and 
pressure  of  the  solutions,  (5)  the  mineral  and  chemical  environ- 
ment, (6)  the  rapidity  of  circulation,  and  (7)  the  potential  or 
electromotive  force  of  the  mineral  compared  with  the  electro.- 
motive  force  of  the  mineral  or  minerals  with  which  it  is  in  contact. 

The  following  table  shows  the  order  of  oxidation  of  some  of  the 
sulphides,  according  to  the  views  of  several  investigators.  The 
sulphides  that  are  most  readily  attacked  are  placed  highest  in 
each  series.  *  . 

ORDER  OF  OXIDATION  OF  SULPHIDES 


1 

2 

3 

4 

5 

Pyrrhotite 

Sphalerite 

Sphalerite 

Chalcocite 

Chalcocite 

Chalcocite 

Chalcocite 

Galena 

Galena 

Bornite 

Pyrrhotite 

Pyrrhotite 

Pyrite 

Chalcopyrite 

Chalcopyrite 

f  Chalcopyrite 

{     and  pyrite 

Pyrite 

Pyrite 

Pyrite 

Argentite 

1.  LINDGREN,  WALDEMAR:  The  Copper  Deposits  of -the  Clifton-Morenci  District,  Arizona. 
U.  S.  Geol.  Survey  Prof.  Paper  43,  p.  180,  1905.     At  some  places  in  Clifton  district. 

2.  EMMONS,  W.  H.,  and  LANET,  F.  B.:  Preliminary  Report  on  the  Mineral  Deposits  of 
Ducktown,  Tenn.     U.  S.  Geol.  Survey  Bull.  470,  pp.   151-172,   1911.     Above  the  water 
level. 

3.  VOGT,  J.  H.  L.:  Problems  in  the  Geology  of  Ore  Deposits,  in  POSEPNY,  FRANZ:  "The 
Genesis  of  Ore  Deposits,"  pp.  676-677,  1902.     Order  of  attack  of  sulphides  by  ferric  chloride. 

4.  GOTTBCHALK,   V.   H.,   and   BUEHLER,   H.   A.:   Oxidation  of   Sulphides.     Econ.  Geol., 
vol.  7,  p.  31,  1912.     Table  shows  relative  potential  of  several  sulphides. 

5.  WELLS,  R.  C.:  Table  shows  rate  of  attack  of  0.057  normal  solution  H~2SO4  on  several 
sulphides  (see  page  158). 

Chemical  Changes  During  Oxidation  of  Certain  Ores. — The 

results  of  the  chemical  changes  that  take  place  during  the  oxi- 


164      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

dation  of  sulphide  deposits  depend  largely  on  the  minerals  that 
form  the  deposits.  Some  minerals  disappear  completely,  others 
are  partly  dissolved,  and  some  elements  of  still  other  minerals 
remain  in  the  mass  in  new  combinations.  The  deposits  of  Duck- 
town,  Tenn.  (see  Fig.  73),  illustrate  extreme  weathering.  The 
gossans  of  the  copper  deposits  have  been  smelted  for  iron,  and 
average  analyses  of  thousands  of  tons  are  available  from  furnace 
records.  Yearly  averages  from  the  smelters  of  sulphide  ore  from 
the  same  deposits  are  also  available.  On  the  assumption  that 
the  average  of  several  specific-gravity  determinations  of  the  gos- 
san (2.2)  applies  to  the  entire  mass  and  that  the  average  specific 
gravity  of  the  sulphide  ore  is  4.05,  the  following  table  has  been 


Gossan  iron  ore     Horizon  of    low-grade  iron  and 
chalcocite       copper  sulphides 

FIG.  73. — Side  elevation  of  a  part  of  the  old  Tennessee  Cherokee  lode,  Duck- 
town,  Tennessee,  showing  position  of  gossan  iron  ore,  chalcocite  zone,  and 
low-grade  primary  copper  ore. 

prepared  to  indicate  the  nature  of  the  change  by  which  primary 
ore  becomes  gossan.  Column  la  shows  the  percentage  weight 
of  the  constituents  of  the  primary  sulphide  ore  of  the  Mary  mine 
(average  of  all  ore  smelted  in  1906).  Column  16  shows  its 
percentage  weight  times  its  specific  gravity  (4.05)  and  may  be 
regarded  as  expressing  the  number  of  grams  in  100  cubic 
centimeters  of  the  primary  ore.  Column  2a  shows  the  chemical 
composition  of  the  gossan  (average  of  two  large  shipments).  Col- 
umn 25  gives  its  percentage  weight  times  its  specific  gravity  (2.2, 
corresponding  to  39  per  cent,  porosity).  Column  3  shows  the 
gain  and  the  loss  of  several  constituents.  Losses  are  shown  for 
sulphur,  silica,  alumina,  lime,  iron,  zinc,  copper;  gains  for  oxygen 
and  water.  Carbon  dioxide  and  magnesia  were  not  determined 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  165 


in  the  analyses  of  gossan  but  were  probably  almost  entirely  car- 
ried away. 

CHEMICAL  CHANGES  BY  OXIDATION  PROCESSES  AT  DUCKTOWN,  TENN. 


la 

Ib 

2« 

26 

3 

SiO2  
A12O3   

22.44 
2.93 

90.880 
11.870 

9.95 
1.57 

21.89 
3  45 

-  69.00 
-     8.40 

Fe  

33.43 

135.390 

49.90 

109.78 

-  25  60 

MgO 

3  15 

12  760 

(?) 

(?) 

—   12  70 

CaO 

8  28 

33  534 

0  35 

0  77 

—  32  70 

CO2 

2  85 

11  540 

—   11  50 

s 

21  23 

85  980 

0  65 

1  43 

—  84  50 

MnO  
Cu  
Zn  
F2O 

0.44 
2.45 

2.79 

1.780 
9.920 
11.300 

0.86 
a!5  40 

1.89 
33  88 

-     8.00 
-   11.30 

+  33  88 

0  

a21  .  38 

47.04 

+47.04 

99.99 

404.954 

100.06 

220.13 

-182.78 

a  HsO  and  O  are  estimated,  on  the  assumption  that  the  Fe  is  in  hmonite. 

The    following    is    a    close    approximation    to    the    mineral 
composition  of  the  unoxidized  ore: 


6.0 
3.0 


25. 


Pyrrhotite  

38  .  5 

Calcite                            .... 

Pyrite 

5  1 

Garnet  

Chalcopyrite  
Sphalerite  

7.1 

'.  .     4.2 

Amphiboles,    pyroxene,    zoi 
site,  etc  

Quartz  

10.3 

100.0 

Oxidation  has  changed  this  ore  into  a  gossan  consisting  essen- 
tially of  limonite  with  a  little  silica  and  kaolin,  carrying  a  fraction 
of  1  per  cent,  of  copper  and  sulphur. 

Metasomatic  Replacement  of  Primary  Sulphides. — The  series 
of  Schuermann  does  not  agree  exactly  with  the  solubilities  of  all 
the  sulphides  involved;  but  if  comparatively  rare  metals,  such  as 
arsenic,  antimony,  cobalt,  and  bismuth,  are  eliminated  the  series 
is  mercury,  silver,  copper,  lead,  zinc,  iron — almost  the  same  as 
that  indicated  by  the  molar  solubilities  of  the  sulphides  as  de- 
termined by  Weigel. 

In  the  processes  of  sulphide  enrichment  the  primary  sulphides 
are  commonly  replaced  pseudomorphously  by  the  secondary  sul- 


160      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


phides.  In  the  diagram  below  the  metals  are  placed  in  order  of 
increasing  solubilities  of  the  sulphides  of  the  metals  in  water, 
mercury  sulphide  being  the  least  and  iron  sulphide  the  most 
soluble. 

METASOMATIC  REPLACEMENT  OF  SEVERAL  SULPHIDES 
[In  the  order  of  Schuermann's  series] 


Mercury 

Silver 

Copper 

Lead 

Zinc 

Iron 

Mercury 

Silver 

?' 

On  PbS 

On  ZnS 

On  FeS2 

Copper 

Metaso- 
matic 

Pseudo- 
morphie 

Pseudo- 
morphic 

Lead 

On  ZnS 

On  FeS2 

Zinc 

"Drives 
out  iron" 

Iron 

According  to  Schuermann's  series  it  might  be  supposed  that 
mercury  sulphide  would  replace  the  sulphides  of  silver,  copper, 
lead,  zinc,  and  iron;  that  silver  sulphide  would  replace  the  sul- 
phides of  copper,  lead,  zinc,  and  iron;  that  copper  sulphide  would 
replace  the  sulphides  of  lead,  zinc,  and  iron;  and  so  on.  It  is 
improbable,  however,  that  mercury  sulphide  would  replace  ex- 
tensively the  more  soluble  sulphides  of  other  metals,  for  the  solu- 
tions that  transport  the  metals  are  sulphate  solutions,  and  mer- 
cury sulphate  has  a  very  low  solubility.  Nevertheless,  secondary 
cinnabar  is  not  unknown.  Silver  dissolves  in  dilute  sulphuric 
acid  solutions  in  the  presence  of  ferric  sulphate,  and  at  depth  its 
sulphide  is  precipitated  on  those  of  lead,  zinc,  and  iron. 

The  occurrences  of  secondary  argentite  are  generally  described 
as  incrustations  on  the  primary  sulphides  or  as  veinlets  cutting 
them.  In  many  places  secondary  chalcocite  contains  silver 
which  is  disseminated  through  and  doubtless  contemporaneous 
with  the  copper  sulphide.  In  view  of  the  fact  that  copper  sul- 
phide is  highly  stable  in  acid  solutions  in  the  absence  of  an  oxi- 
dizing agent,  it  is  not  remarkable  that  the  replacement  of  copper 
sulphides  by  silver  sulphides  is  not  common  in  the  deeper  zones. 

Copper  is  much  more  abundant  in  its  deposits  than  silver,  and 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  167 

the  nature  of  its  changes  is  more  easily  recognized.  Copper 
sulphide  replaces  galena  in  deposits  at  Ducktown,  Tenn.,  and 
covellite  replaces  galena  in  the  San  Francisco  district,  Utah. 
Copper  sulphide  replaces  sphalerite  at  Morenci,  Ariz.;  in  the 
San  Francisco  district,  Utah;  and  at  Butte,  Mont.  Without 
much  doubt  similar  replacement  is  common  in  many  mineral  de- 


FIG.  74. — Diagram  showing  approximately,  by  width  of  line,  an  estimate  of 
the  relative  importance  of  superficial  enrichment  in  ore  deposits  and  protores 
of  several  metals.  Placers  and  all  other  deposits  formed  at  places  where  no 
ore  body  or  protore  was  present  before  are  grouped  in  the  first  column  with 
primary  deposits.  Broken  line  indicates  fewer  or  smaller  deposits  than  solid 
line;  short  dashes  indicate  fewer  or  smaller  deposits  than  line  with  long  dashes. 


posits.  Examples  of  the  pseudomorphous  replacement  of  chal- 
copyrite  and  pyrite  by  copper  sulphide  are  well  established  and 
numerous,  this  method  being  the  most  common  mode  of  origin 
of  secondary  copper  sulphide  ores.  Although  there  are  well- 
authenticated  examples  of  sulphide  enrichment  of  lead  deposits, 
and  statements  are  made  that  lead  has  driven  iron  or  zinc  out  of 


168      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

its  sulphide  combinations,  lead  veins  afford  few  examples  of  the 
pseudomorphous  replacement  of  sphalerite  or  pyrite.  It  has  fre- 
quently been  stated  that  zinc  sulphide  has  been  precipitated  at 
the  expense  of  iron  sulphide  and  that  zinc  has  driven  iron  out  of 
its  sulphide  combination,  but  no  examples  of  the  pseudomorphous 
replacement  of  pyrite  or  marcasite  by  zinc  blende  are  available. 
Under  the  usual  conditions  of  concentration  in  the  secondary 
sulphide  zone  a  sulphide  that  appears  to  the  right  of  the  heavy 
line  in  the  table  on  page  166  would  not  replace  one  to  the  left  of 
it  on  the  same  line.  Thus  the  copper  sulphides  would  not  be 
replaced  by  lead  sulphide  or  zinc  sulphide,  and  so  on.  Some  ex- 
amples, however,  do  not  agree  with  .the  relations  indicated  in  this 
series,  for  there  are  pseudomorphs  of  pyrite  after  chalcopyrite 
and  tetrahedrite,  and  pseudomorphs  of  marcasite  after  galena, 
argentite,  chalcopyrite,  zinc  blende,  and  other  minerals.  Prob- 
ably these  pseudomorphs  were  formed  in  alkaline  solutions.  The 
replacements  of  great  economic  significance  are  in  the  order  of 
the  Schuermann  series.  An  estimate  of  the  relative  importance 
of  enrichment  in  ores  of  several  metals  is  shown  in  figure  74. 

Influence  of  Primary  Ores  on  the  Extent  of  the  Secondary 
Sulphide  Zones. — The  vertical  extent  of  the  secondary  sulphide 
zone  depends  partly  on  the  amount  of  fracturing  of  the  primary 
ore  body  and  the  size,  continuity,  and  character  of  the  fractures. 
The  fractures  determine  the  course  of  descending  waters  and  the 
rates  at  which  the  solutions  descend.  In  their  descent  the  metal- 
bearing  solutions  react  on  the  walls  of  the  watercourses,  and 
these  reactions  produce  changes  of  chemical  equilibria  and  de- 
position of  certain  metals.  These  changes  depend  not  only  on 
the  rate  at  which  the  solutions  descend  but  also  on  the  chemical 
environment  through  which  they  pass.  In  limestone  or  in  cal- 
cite  or  siderite  gangue  the  downward  migration  of  copper  would 
be  delayed,  at  least  temporarily,  by  the  formation  of  carbonates, 
and  calcite  would  quickly  drive  gold  from  acid  solutions  in  which 
it  was  held  dissolved  as  chloride. 

Dilute  acid  waters  dissolve  pyrrhotite  rapidly  and  set  free  hy- 
drogen sulphide.  Under  similar  conditions  the  zones  of  second- 
ary ores  formed  from  primary  ores  that  carry  abundant  pyrrho- 
tite, though  generally  richer,  should  be  of  smaller  vertical  extent 
than  those  of  secondary  ores  formed  from  primary  ores  of  pyrite 
and  chalcopyrite  that  contain  no  pyrrhotite,  for  the  reaction  is 
brought  near  to  completion  more  quickly.  Briefly  stated,  the 


SUPERFICIAL  ALTERATION  AND  ENRICHMENT  169 

vertical  extent  of  the  secondary  sulphide  zones1  should  vary  in- 
versely with  the  rate  at  which  the  descending  sulphate  solutions 
attack  the  ore  and  gangue  minerals  through  which  they  pass. 
In  superficial  alteration  each  metal  behaves  differently,  its  action 
depending  on  its  chemical  relations.  These  are  discussed  in  sec- 
tions treating  separately  the  deposits  of  the  metals. 

1  EMMONS,  W.  H. :  The  Enrichment  of  Ore  Deposits.     U.  S.  Geol.  Survey 
Bull.  625,  pp.  152-154,  1917. 


CHAPTER  XVI 
OPENINGS  IN  ROCKS 

Size  of  Openings. — Epigenetic  deposits — that  is,  deposits  that 
have  been  introduced  by  solutions  into  the  rocks  that  contain 
them — are  formed  both  by  filling  and  by  replacement.  Briefly, 
they  include  veins  of  all  zones,  contact-metamorphic  deposits, 
and  some  pegmatites.  Except  deposits  of  contact-metamorphic 
origin  and  some  formed  by  replacement  of  limestone,  at  mod- 
erate depths,  replacement  deposits  are  generally  related  to  easily 
recognized  openings. 

Openings  may  be  classified  with  respect  to  their  size  and  with 
respect  to  their  origin.  With  respect  to  size,  they  may  be  placed 
in  three  groups — supercapillary,  capillary,  and  subcapillary. l 

Supercapillary  openings  are  those  in  which  water  obeys  the 
ordinary  laws  of  hydrostatics.  For  water  at  ordinary  tempera- 
tures, tubes  with  holes  more  than  0.508  millimeter  in  diameter 
or  sheet  openings  more  than  0.254  millimeter  wide  are  super- 
capillary. 

Capillary  openings  are  tubes  with  holes  less  than  0.508  and 
greater  than  0.0002  millimeter  in  diameter,  or  sheet  openings 
between  0.254  and  0.0001  millimeter  wide.  In  these  water 
does  not  obey  the  ordinary  laws  of  hydrostatics  but  is  affected 
by  capillary  attraction.  Water  will  not  circulate  so  freely  in 
such  openings  because  of  the  greater  friction  along  the  walls. 
Hot  water  may  move  through  such  openings  more  readily  than 
cold,  however,  and  under  pressure  either  hot  or  cold  solutions 
may  be  forced  through  capillary  openings. 

Subcapillary  openings  include  tubes  with  holes  less  than  0.0002 
millimeter  diameter  and  sheet  openings  less  than  0.0001  milli- 
meter wide.  In  these  the  attraction  of  the  molecules  of  the  solid 
extends  across  the  open  space.  Water  may  enter  such  openings, 
but  it  tends  to  remain  as  if  fixed  to  the  walls,  prohibiting  further 

1  DANIEL,  ALFRED:  "A  Textbook  of  the  Principles  of  Physics,"  p.  315, 
1895. 

VAN  HISE,  C.  R.:  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey 
Mon.  47,  p.  135,  1904. 

170 


OPENINGS  IN  ROCKS  171 

entrance  of  solutions.  Circulation  of  solutions  at  ordinary  tem- 
peratures through  such  openings  is  therefore  very  slow. 

If  two  rocks  have  equal  amounts  of  pore  space — supercapillary 
in  one  and  subcapillary  in  the  other — the  one  with  the  larger 
openings  will  afford  more  favorable  conditions  for  the  movement 
of  solutions.  Muds,  clays,  shales,  and  rock  powders,  which  con- 
tain exceedingly  minute  openings,  are  the  great  natural  barriers 
to  circulating  waters,  whether  the  waters  are  hot  or  cold.  In 
the  following  genetic  classification  of  openings  the  supercapillary 
openings  and  the  larger  capillary  openings  are  considered  chiefly, 
for  most  epigenetic  deposits  are  related  to  such  openings.  It  is 
noteworthy,  however,  that  rocks  containing  subcapillary  open- 
ings may  be  replaced  by  solutions  that  are  under  sufficient  pres- 
sure at  high  temperature;  but  under  most  conditions  such  rocks 
tend  to  impede  the  circulation  and  thus  to  limit  the  size  of  ore 
bodies.  Solutions  that  deposit  ore  in  the  minute  openings  in 
rocks  generally  move  outward  from  the  larger  openings  or 
"master  fractures." 

Origin  of  Openings. — With  respect  to  their  origin,  openings  in 
rocks  are  classified  as  follows : 

PRIMARY  OPENINGS: 

Intergranular  spaces. 
Bedding  planes. 
Vesicular  spaces. 
Openings  in  pumice. 
Miarolitic  cavities. 
Submicroscopic  spaces. 

SECONDARY  OPENINGS: 
Formed  by  solution: 

Caves. 

Geodes. 

Solution  cavities  in  veins,  etc. 
Formed  by  movement: 

Shrinkage  cracks  due  to  dehydration,  cooling,  loss  of  fluids,  etc. 

Openings  due  to  force  of  crystallization. 

Openings  due  to  the  thrust  of  solutions. 
•  Openings  due  to  the  greater  earth  stresses. 

PRIMARY  OPENINGS 

Intergranular  Spaces  in  Sedimentary  Rocks.— The  pore  spaces 
in  sedimentary  rocks  constitute  a  percentage  of  the  volume  of 
the  rock  ranging  from  less  than  1  up  to  20  or  even  more.  Ac- 


172      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

cording  to  Buckley,1  the  Dunnville  sandstone  of  Wisconsin  has  a 
pore  space  of  28.28  per  cent.  Many  sandstones  have  10  per  cent, 
or  more.  As  shown  by  Slichter,2  the  size  of  the  grains  does  not 
determine  the  amount  of  pore  space;  a  fine-grained  rock  may  be 
as  porous  as  a  coarse  conglomerate.  Fig.  75  represents  a  section 
of  several  balls  piled  so  as  to  represent  rounded  sand  grains.  It 
is  obvious  that  if  this  figure  were  to  be  increased  or  decreased  in 
size  the  changes  would  affect  similarly  the  solid  balls  and  the 
spaces  between  them.  The  amount  of  space  depends  principally 
upon  the  assortment  of  grains  and  the  system  of  packing.  If 
small  grains  fill  in  the  spaces  between  large  grains,  the  porosity 


FIG.  75. — Section  of  balls  with  triangular  pore  spaces.     (After  Slichter,    U.  S. 
Geol.  Survey.) 

is  obviously  diminished.  This  principle  is  illustrated  in  sizing 
and  desliming  pulp  for  leaching  in  cyanide  practice.  The  con- 
ditions for  leaching  are  most  favorable  when  the  material  is  of 
nearly  uniform  size  and  evenly  distributed.  The  fact  that  very 
fine  material  will  not  permit  the  free  movement  of  water  is  not 
due  to  the  absence  of  openings  but  to  the  small  size  of  the  open- 
ings. In  rock  with  subcapillary  openings  water  tends  to  remain 
fixed  to  the  rock  particles.  The  pore  spaces  of  the  more  coarsely 
granular  rocks  like  sandstone  are  more  likely  to  serve  as  seats 
of  ore  deposition  than  those  in  fine-grained  rocks  like  shales. 
Colloidal  matter  in  shale  also  tends  to  decrease  its  permeability. 

1  BUCKLEY,  E.  R.:  Building  and  Ornamental  Stones  of  Wisconsin.  Wis. 
Geol.  and  Nat.  Hist.  Survey  Bull.  4,  pp.  225,  403,  1898. 

2 SLIGHTER,  C.  S.:  Theoretical  Investigation  of  the  Motion  of  Ground 
Waters.  U.  S.  Geol.  Survey  Nineteenth  Ann.  Rept.,  part  2,  p.  305,  1899. 


OPENINGS  IN  ROCKS  173 

In  the  Keweenaw  copper  district  of  the  Lake  Superior  region,  con- 
glomerates carry  valuable  ore  bodies.  Sandstones  in  many  dis- 
tricts are  impregnated  with  metallic  sulphides  or  with  carbonates. 

Bedding  Planes. — Bedding  planes  are  due  to  the  assortment 
or  sizing  of  material  during  transportation  and  deposition.  On 
account  of  the  assortment  of  grains  there  may  be  also  different 
arrangements  of  the  pore  spaces  in  the  different  beds.1  Conse- 
quently, even  in  a  nearly  homogeneous  rock  the  different  beds 
commonly  have  different  degrees  of  permeability.  Water  mov- 
ing along  the  beds  may  follow  the  most  permeable  layer,  but 
water  moving  across  them  must  traverse  also  the  most  imper- 
meable layers.  Solutions  will  therefore  pass  along  beds  more 
readily  than  across  them.  Because  of  the  greater  permeability 
of  certain  layers,  many  ore  deposits  that  obviously  have  been 
introduced  after  the  beds  containing  them  were  laid  down  are 
found  below  beds  of  shale  or  beds  of  material  somewhat  more 
aluminous  than  the  associated  beds.  Deposition  may  take  place 
along  bedding  planes  also  because  certain  layers  are  more  readily 
replaced  than  others.  A  pure  limestone  will  commonly  be  more 
readily  replaced  than  a  clayey  limestone,  because  it  is  more 
soluble  and  also  more  permeable.  Bedding  planes,  especially  in 
tilted  rocks,  are  likely  to  be  fissured  and  faulted  because  they  are 
planes  of  easy  separation.  The  terms  "bed  vein"  and  "bedded 
vein"  are  sometimes  used  synonymously  with  bedding-plane  de- 
posit, but  the  use  of  this  term  as  applied  to  deposits  that  are 
later  than  the  containing  rocks  is  not  to  be  encouraged.  Ex- 
amples of  bedding-plane  deposits  are  cited  on  page  197. 

Vesicular  Spaces. — Magmas  generally  contain  included  fluids. 
When  the  magmas  are  erupted  and  flow  out  upon  the  surface, 
pressure  is  relieved  and  the  fluids  expand  and  escape  as  gases. 
If  they  expand  when  the  lavas  are  in  a  sticky  or  viscous  condi- 
tion, and  near  the  point  of  solidification,  the  openings  due  to  ex- 
pansion are  preserved.  In  the  diabases  of  the  Keweenaw  region 
of  Michigan  (see  page  395)  such  openings  contain  ore  and  gangue 
minerals  in  large  quantities,  but  in  many  regions  vesicular 
spaces  in  lavas  are  not  especially  favorable  places  for  ore  deposits. 
The  openings  due  to  expanded  gases  in  lavas,  unlike  the  pores  in 
sandstone,  are  generally  not  connected  and  therefore  do  not  offer 
continuous  passages  to  solutions.  That  waters  penetrate  them 

1  KING,  F.  H. :  Principles  and  Conditions  of  the  Movements  of  Ground 
Water.  U.  S.  Geol  Survey  Nineteenth  Ann.  Rept.,  part  2,  p.  135,  1899. 


174      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

in  some  places,  however,  is  shown  by  the  abundance  of  amygda- 
loidal  matter  which  is  found  in  the  vesicles. 

Openings  in  Pumice. — Pumices  are  lavas  that  contain  very  large 
amounts  of  pore  space.  The  openings  are  formed  in  the  same 
manner  as  vesicles  but  are  generally  smaller  and  much  more 
numerous.  Siliceous  lavas,  such  as  rhyolites  and  other  acidic 
rocks,  are  more  viscous  than  basic  lavas,  such  as  basalt  or  diabase. 
Rock  froth  is  more  generally  formed  with  siliceous  material,  al- 
though both  acidic  and  basic  lavas  are  commonly  vesicular. 
Open  spaces  in  a  pumice  are  not  continuous,  and  the  water  cir- 
culation in  them  is  slow.  Fragments  of  pumice  placed  in  water 
may  float  for  days  before  the  spaces  are  filled  and  the  water- 
logged fragments  sink.  Metalliferous  deposits  in  pumice  are 
rare. 

Miarolitic  Cavities. — Some  igneous  rocks  and  some  pegmatites 
contain  small  openings  which  are  believed  to  be  spaces  formerly 
occupied  by  fluids  of  the  rock  magma  that  were  unable  to  escape 
during  the  solidification  of  the  rock.  Such  openings,  called  miaro- 
litic,  are  present  in  rocks  that  solidified  under  pressure  and  are 
unlike  vesicles,  although  both  are  due  to  imprisoned  fluids.  The 
miarolitic  cavities  are  in  general  not  so  nearly  spherical  as  vesicular 
openings,  although  their  dimensions  are  more  nearly  equal  than 
those  of  spaces  formed  by  movement.  The  walls  of  miarolitic 
cavities  are  usually  rough,  because  they  are  lined  by  crystals  of 
the  rock.  Miarolitic  cavities  are  not  known  to  be  seats  of  de- 
position of  metalliferous  ores  of  economic  importance.  In  many 
pegmatites  they  contain  gems  and  other  minerals,  especially  min- 
erals which,  because  they  contain  elements  of  the  so-called  miner- 
alizers,  boron,  fluorine,  lithium,  etc.,  and  because  they  generally 
line  the  cavities,  are  presumed  to  have  been  formed  at  a  late 
stage  of  crystallization. 

Submicroscopic  Spaces. — The  denser  rocks,  which  appear  solid, 
contain  nevertheless  small  amounts  of  pore  space.  A  granite, 
which  under  the  microscope  has  no  visible  openings,  will  absorb 
a  small  amount  of  water  in  the  cold.  At  high  temperatures  the 
speed  of  absorption  is  increased,  and  under  pressure  hot  water 
may  be  forced  through  the  denser  rocks.  Many  minerals  and 
rocks  when  boiled  in  dye  are  colored.  The  relations  of  hydro- 
thermally  altered  rocks  to  fissures  suggest,  however,  that  the 
rate  of  transfer  of  material  by  solutions  in  such  dense  rocks  is 
very  slow,  for  as  a  rule  the  hydrothermal  alteration  does  not  ex- 


OPENINGS  IN  ROCKS  175 

tend  far  away  from  visible  openings.  The  restriction  of  such 
alteration  to  the  walls  near  the  major  channels  is  more  noticeable 
in  rocks  that  have  been  altered  at  greater  depths,  where  the 
rocks  were  fractured  under  greater  pressure. 

SECONDARY  OPENINGS 

Openings  Formed  by  Solution. — In  soluble  rocks  like  lime- 
stones and  dolomites  large  openings  may  be  formed  by  solution 
and  by  removal  of  rock  matter.  Solution  usually  proceeds  by 
enlarging  smaller  openings,  such  as  joints,  bedding  planes,  or 
fissures.  These  openings  may  become  the  principal  drainage 
channels  of  the  country,  and  the  solution  cavities  along  them  may 
be  developed  on  an  enormous  scale.  As  a  rule  solution  is  more 
active  above  the  water  level,  but  large  cavities  have  been  found 
considerably  below  the  present  water  level,  especially  where  sub- 
sequent to  solution  there  have  been  changes  in  the  drainage  or 
climate  and  in  the  position  of  the  water  level. 

Before  the  development  of  the  theory  of  replacement  large 
solution  cavities,  such  as  limestone  caves,  were  assumed  to  play 
an  important  part  in  the  genesis  of  many  ore  deposits,  but  most 
investigators  at  present  are  inclined  to  minimize  their  importance. 
It  is  believed,  however,  that  many  of  the  zinc  deposits  of  south- 
western Missouri1  fill  ancient  limestone  caverns  that  were  formed 
during  a  period  of  erosion  between  Mississippian  and  Pennsyl- 
vanian  time. 

The  ore  of  veins  is  frequently  dissolved  out  along  fractures. 
Deposits  which  are  assumed  to  have  replaced  solid  rock  may 
contain  solution  cavities  of  considerable  size,  especially  in  soluble 
material.  Geodes  in  limestone  and  some  open  spaces  in  veins 
are  clearly  due  to  solution. 

Openings  Due  to  Shrinkage. — Shrinkage  may  be  caused  by 
dolomitization,  dehydration,  cooling,  and  other  processes.  If 
a  fairly  pure  limestone  is  changed  to  dolomite  without  addition 
of  carbon  dioxide  a  shrinkage  of  about  12  per  cent,  takes  place. 

1  SIEBENTHAL,  C.  E. :  Origin  of  the  Zinc  and  Lead  Deposits  of  the  Joplin 
Region.  U.  S.  Geol.  Survey  Bull.  606,  p.  28,  1915. 

SMITH,  W.  S.  T.,  and  SIEBENTHAL,  C.  E.:  U.  S.  Geol.  Survey  Geol.  Atlas, 
Joplin  district  folio  (No.  148),  p.  11,  1907. 

BAIN,  H.  F.,  and  VAN  HISE,  C.  R.:  Preliminary  Report  on  the  Lead  and 
Zinc  Deposits  of  the  Ozark  Region.  U.  S.  Geol.  Survey  Twenty-second 
Ann.  Rept.,  part  2,  pp.  23-228,  1901. 


176      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  porosity  of  some  dolomites  is  assumed  to  be  due  to  shrinkage. 
Cracks  due  to  shrinkage  in  drying  are  common.  Cooling  cracks 
are  formed  soon  after  the  solidification  of  igneous  rocks,  before 
they  have  cooled  to  the  temperature  of  the  surrounding  rocks. 
The  planes  separating  the  columns  of  some  basalt  dikes  and  the 
irregular  fractures  in  some  granites  and  other  rocks  are  commonly 
attributed  to  cooling.  In  observed  examples  where  such  partings 
are  clearly  due  to  cooling  the  individual  openings  are  not  con- 
tinuous, but  they  either  form  small  patterns  like  the  hexagonally 
arranged  cracks  in  some  basalts  or  are  irregular. 

Magmas  are  known  to  be  intruded  into  the  upper  part  of  the 
earth's  crust  at  high  temperatures,  but  where  they  are  heavily 
charged  with  water  and  other  fluids  they  remain  liquid  until 
they  have  lost  a  considerable  part  of  their  heat.  The  tempera- 
ture of  solidification  may  reasonably  be  estimated  as  high  as 
600°C.  or  higher,  and  in  cooling  to  the  temperature  of  the 
surrounding  rocks  they  probably  lose  heat  equivalent  to  500°C. 
or  more.  Some  experiments  indicate  that  for  every  degree 
which  a  rock  cools  it  will  shrink  about  0.002  per  cent,  of  its 
volume.1  At  that  rate,  in  cooling  500°C.  it  would  shrink  1.0 
per  cent.  If  all  spaces  due  to  such  shrinkage  were  expressed  as 
parallel  fissures,  then  an  area  a  mile  wide  should  have  a  sum  of 
open  spaces  equivalent  to  1  per  cent,  of  a  mile,  or  about  52  feet 
measured  at  right  angles  to  such  fissures.  It  thus  appears  that 
shrinkage  during  cooling  is  competent  to  form  openings  the  sum 
total  of  which  would  be  comparable  in  size  to  the  fissures  filled  in 
an  area  where  ore  veins  are  rather  closely  spaced.  It  is  uncertain, 
however,  whether  many  metallized  fractures  of  common  types 
have  formed  by  shrinkage.  In  most  districts,  although  great 
bodies  of  rock  have  shrunk  from  cooling,  the  mineralized  frac- 
tures are  found  only  in  restricted  areas.  Fractures  may  be 
formed  by  the  settling  of  large  bodies  of  rock  due  to  cooling 
and  shrinkage  of  rocks  below  them. 

Openings  Due  to  the  Force  of  Crystallization. — The  force 
which  crystallizing  matter  exerts  on  the  containing  walls  has 
been  assumed  to  be  sufficient  to  push  the  walls  apart.  If  this 
force  so  operates  it  would  be  supposed  that  a  metalliferous  solu- 
tion, having  once  gained  entrance  to  a  fissure,  however  narrow, 
could  enlarge  the  fissure  while  it  was  being  filled.  This  process 

1  CHAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:  "Geology,"  vol.  1,  p.  573, 
1909. 


OPENINGS  IN  ROCKS  177 

does  not  produce  open  spaces  but  is  assumed  to  widen  those  al- 
ready formed.  Becker  and  Day1  performed  experiments  to 
ascertain  the  strength  of  such  a  force  and  found  it  to  be  of  the 
same  order  as  the  crushing  strength  of  crystals.  They  say:  " It 
is  manifest  that  we  here  have  to  deal  with  a  force  of  great  geo- 
logical importance.  If  quartz,  during  crystallization,  exerts  a 
pressure  on  the  sides  of  a  vein  which  is  of  the  same  order  of 
magnitude  which  it  offers  to  crushing,  then  this  force  is  also 
of  the  same  order  of  magnitude  as  the  resistance  of  the  wall 
rocks,  and  it  thus  becomes  possible  that  *  *  *  veins  have 
actually  been  widened  to  an  important  extent,  perhaps  as  much 
as  100  per  cent,  or  even  more,  by  pressure  due  to  this  cause." 
Laney,2  recognizes  this  force  as  a  possible  factor  in  the  formation 
of  the  spaces  that  are  occupied  by  the  gold  veins  of  Gold  Hill, 
N.  C.,  and  the  cracks  in  pyrite  in  which  chalcopyrite  forms  thin 
seams.  Dunn3  considers  the  force  of  crystallization  as  an  agent 
that  has  operated  in  expanding  the  openings  of  quartz-filled 
reefs  of  Bendigo,  Victoria.  The  hypothesis  appears,  however,  to 
be  of  limited  application,  if  not  untenable  for  many  deposits. 
Very  commonly  fissure  veins  show  numerous  vugs,  and  it  is  im- 
probable that  crystals,  where  they  could  grow  freely  into  open 
spaces,  would  thrust  aside  great  masses  of  rock.  Moreover,  the 
crystals  themselves  are  generally  not  distorted.  They  do  not 
show  that  their  own  growth  was  affected  by  such  enormous  pres- 
sures as  are  demanded  by  this  hypothesis. 

Openings  Due  to  Pressure  of  Solutions. — In  his  discussion  of 
the  origin  of  certain  small  lenticular  bodies  of  quartz  ore  in  the 
schists  of  the  southern  Appalachians,  Graton4  has  suggested 
that  the  metalliferous  solutions  themselves  were  under  heavy 
pressure,  sufficient  to  push  the  rocks  apart  along  their  cleavage 
planes,  making  the  openings  while  they  filled  them,  after  the 
manner  of  igneous  injections  (see  page  54). 

Openings  Due  to  the  Greater  Stresses. — 'The  earth  is  losing 
heat  and  shrinking.  The  exterior  or  shell  receives  heat  from  the 

1  BECKER,  G.  F.,  and  DAY,  A.  L. :  The  Linear  Force  of  Growing  Crystals. 
Wash.  Acad.  Sci.  Proc.,  vol.  7,  pp.  282-288,  1905. 

2  LANEY,  F.  B.:  The  Gold  Hill  Mining  District.     North  Carolina  Geol. 
and  Econ.  Survey  Bull.  21,  p.  91,  1910. 

3  DUNN,  E.  J. :  Report  on  the  Bendigo  Gold  Field,  Victoria,  p.  25,  Dept. 
of  Mines,  1896. 

4  GRATON,  L.  C. :  Reconnaissance  of  Some  Gold  and  Tin  Deposits  of  the 
Southern  Appalachians.     U.  S.  Geol.  Survey  Bull.  293,  p.  60,  1906. 


178       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

interior  and  radiates  it  but  probably  remains  at  approximately 
the  same  average  temperature  through  long  periods.  The  loss 
of  heat  causes  the  interior  to  shrink  more  rapidly  than  the  shell, 
which  is  then  too  large  to  fit  the  core.  The  sheh1  is  not  strong 
enough  to  bear  its  own  weight.  Consequently  it  is  a  failing 
structure  which  is  pulled  downward  by  gravity  and  is  wrinkled 
here  and  there,  thus  becoming  smaller.  There  are  probably  also 
causes  other  than  the  loss  of  heat  for  the  shrinkage  of  the  interior.1 

As  a  result  of  this  failure  of  the  shell,  rocks  are  folded  and  moun- 
tain chains  are  formed.  These  are  commonly  ascribed  to  com- 
pressive  stresses,  because  they  are  formed  in  the  main  by  com- 
pression and  shortening  of  the  earth's  crust.  Obviously  they 
are  due  to  the  pull  or  tension  of  gravity  acting  along  the  radii  of 
the  earth,  but  this  force  is  resolved  into  various  forces  which  act 
in  many  directions,  and  those  along  which  movement  takes 
place  most  readily  are  mainly  at  right  angles  to  the  radii,  or 
approximately  along  the  great  circles  of  the  earth,  which  are 
shortened  or  compressed.  When  magmas  are  thrust  into  the 
crust  or  extravasated  upon  the  earth,  the  rocks  above  the  places 
they  previously  occupied  tend  to  settle,  and  such  readjustment 
sets  up  stresses  of  various  kinds.  Relief  of  stress  may  be  ac- 
complished by  fracturing. 

The  larger  earth  fractures  are  commonly  referred  to  three 
classes — compressional  fractures,  tensional  fractures,  and  tor- 
sional  fractures. 

Compressional  Fractures. — Many  fractures  filled  with  ore 
have  been  formed  by  the  relief  of  compressive  stresses.  .  In  the 
classic  experiment  of  Daubre*e2  a  brittle  block  was  subjected  to 
pressure  applied  at  its  end.  The  block  when  viewed  from  the 
side  shows  two  sets  of  fissures  approximately  at  right  angles  to 
each  other,  making  angles  of  about  45°  with  the  direction  of  pres- 
sure (see  Fig.  76).  In  some  mining  districts  there  are  two  groups 
of  nearly  parallel  intersecting  fissures,  and  from  analogy  with 
DaubreVs  experiment  these  are  assumed  to  have  resulted  from 
compressive  stresses. 

In  this  experiment  it  was  possible  for  movement  outward  to 

1  CHAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:   "Geology,"  vol.  1,  p.  548, 
1909. 

2  DAUBREE,  A. :   "Etudes  synthetiques  de  geologic  experimental, "  p.  318, 
1879. 

BECKER,  G.  F. :  Finite  Homogeneous  Strain,  Flow,  and  Rupture  of  Rocks. 
Geol.  Soc.  America  Bull,  vol.  4,  p.  13,  1892. 


OPENINGS  IN  ROCKS 


179 


take  place  in  all  directions  except  vertically.  Consequently 
several  coordinate  systems  of  fractures  are  shown  on  the  four 
free  sides.  But  if  the  block  had  been  buttressed  on  one,  two,  or 
three  sides,  then  movement  would  have  been  restricted  and  there 
would  be  a  smaller  number  of  systems. 

When  a  block  of  rock  near  the  surface  of  the  earth  is  deformed 
by  compression,  it  is  free  to  move  upward.  In  that  direction 
only  gravity  opposes  movement  due  to  lateral  thrust;  in  other 
directions  it  is  buttressed  by  the  surrounding  rocks.  If  the  mass 
can  move  in  one  direction  only,  the 
number  of  fracture  systems  is  ob- 
viously diminished  and  stresses  may 
be  relieved  by  one  system  only.  In  a 
large  number  of  mining  districts  there 
are  many  veins  that  are  rudely  parallel 
both  on  strike  and  dip.  In  general, 
however,  the  surrounding  rocks  and 
the  compressed  mass  itself  are  hetero- 
geneous and  of  different  strengths. 
The  fracture  systems  are  not  all  de- 
veloped with  regular  patterns  of  any 
kind,  although  a  tendency  to  parallel- 
ism or  coordination  is  common  in 
many  districts. 

Under  compressive  stress  fractures 
will  develop  along  planes  of  maximum 
shear1  which  are  inclined  to  the  direc- 
tion of  maximum  stress.  InDaubree's 
experiment  the  shear  planes  are  inclined  at  about  45°  to  the 
direction  of  stress.  This  experiment  illustrates  nonrotational 
strain.2  Rotational  strains  are  those  in  which  tha  planes  of 
strain  are  rotated  during  deformation.  The  planes  of  shear  are 
not  grouped  symmetrically  with  reference  to  the  direction  of 
stress  but  are  rotated  and  may  intersect  it  at  very  small  angles. 

The  experiment  of  Daubre"e  was  made  on  a  body  of  homogene- 
ous material.  Rocks  as  a  rule  are  not  homogeneous  but  are 
bedded,  sheeted,  or  jointed  in  varying  degrees.  Experiments 
made  by  W.  B.  Lang  show  the  effect  of  the  grain  in  deformation  of 

'LEITH,  C.  K:  "Structural  Geology,"  p.  16,  1913. 

2HosKiNs,  L.  M.:  Flow  and  Fracture  of  Rocks  as  Related  to  Structure. 
U.  S.  Geol.  Survey  Sixteenth  Ann.  Rept.,  part  1,  p.  845,  1896. 


FIG.  76. — Block  of  material 
deformed  by  pressure  applied 
at  end.  (After  Daubree.) 


180      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


a  block  of  wood  that  had  been  soaked  in  hot  water.  Small  open- 
ings developed  along  planes  of  maximum  shear  (see  Fig.  77). 
Changes  of  this  character  may  illustrate  those  resulting  from  the 
deformation  of  tough  rocks,  such  as  some  schists  and  some  slates. 
Fig.  78  shows  the  effect  of  pressure  on  a  column  of  dry  oak. 
Fractures  are  developed  in  rudely  parallel  systems,  and  the  longer 
axes  of  each  system  lie  at  about  45°  to  the  direction  of  pressure. 
Single  systems  made  up  of  many  short  parallel  fractures  like 
the  two  systems  shown  in  Fig.  78  are  commonly  developed  in  a 
testing  machine  where  wooden  blocks  deformed  by  pressure  are 


Fro.  77. — Block  of  oak  deformed  FIG.  78. — Column  of  oak  deformed 
by  applying  pressure  in  direction  of  by  applying  pressure  in  direction  of 
arrows.  (After  Lang.)  arrows.  (After  Lang.) 

arranged  so  that  one  end  of  the  block  is  free  to  move,  as  is  the 
case  during  rotational  strains. 

Tensional  Fractures. — Rupture  by  tension  in  homogeneous 
bodies  takes  place  in  planes  that  lie  approximately  at  right 
angles  to  the  direction  of  the  force.  Of  the  greater  stresses 
acting  parallel  to  the  earth's  surface,  or  along  the  great  circles 
of  the  earth,  the  dominant  ones  are  compressional  rather  than 
tensional,  and  the  larger  number  of  ore  veins  are  probably  related 
to  the  fractures  resulting  from  compression.  Some  fissures, 
however,  are  clearly  due  to  tension  that  results  from  compres- 
sional stress.  Thus  when  brittle  rocks  are  compressed  into  folds, 
fractures  may  be  formed  across  the  bedding,  especially  near  the 


OPENINGS  IN  ROCKS 


181 


axes  of  the  folds  (see  Fig.  79).  The  crest  of  an  anticline  may  be 
stretched  as  a  result  of  compression  of  its  limbs.  At  the  crests 
of  folds  rocks  may  separate  also  along  the  beds,  leaving  open 
spaces  which  may  be  filled  with  ore  (see  Fig.  80).  Some  bedding- 
plane  deposits  are  either  confined  to  or  greatly  enlarged  at  the 
crests  of  anticlines.  Such  places  are  favorable  for  ore  deposition, 
doubtless  because  the  openings  formed  during  deformation 
supply  passages  for  solutions.  Many  anticlinal  deposits  and 
saddles  (see  page  201)  are  formed  by  the  replacement  of  soluble 
calcareous  rocks,  and  it  is  probable  that  the  fracturing  during 
deformation  has  favored  the  entrance  of  the  replacing  solutions 
at  the  crests  of  the  folds.  Tensional  fractures  may  be  formed 


FIG.  79.  —  Diagram  showing  brittle  FIG.  80.  —  Diagram  showing  spaces 
beds  fractured  at  and  near  the  axis  of  an  developed  at  and  near  axis  of  an 
anticline.  anticline. 

also  by  the  drying  out  of  sedimentary  rocks,  by  the  shrinkage  of 
cooling  igneous  bodies  (page  175),  and  by  the  settling  of  blocks 
that  may  attend  igneous  activities.1  Tensional  stresses  may 
operate  when  rocks  are  deformed  near  the  surface,  where  pres- 
sures are  comparatively  low. 

A  study  of  the  thickness  of  the  earth's  shell  involved  in  moun- 
tain folds2  and  in  faulted  regions  shows  that  the  deformed  mass 
probably  extends  only  a  few  miles  below  the  surface.  T.  C. 
Chamberlin3  suggests  that  there  is  probably  a  "horizon  of 
shear"  a  few  miles  below  the  surface  which  plays  a  very  important 
part  in  deformation.  This  shear  zone  he  considers  the  basal 


,  J.  E.:  The  Relation  of  Ore  Deposition  to  Faults.     Econ.  GeoL, 
vol.  11,  pp.  601-622,  1916. 

2  CHAMBERLIN,    R.    T.:  Appalachian    Folds    of    Central    Pennsylvania. 
Jour.  Geol.,  vol.  18,  pp.  228-251,  1910. 

3  CHAMBERLIN,  T.  C.:  The  Fault  Problem.     Econ.  Geol.,  vol.  2,  p.  598, 
1907. 


182      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

plane  of  the  greater  fault  blocks — a  plane  in  which  the  shell 
shears  over  the  subshell.  The  huge  thrust  faults  like  those 
recognized  in  Alberta  and  Montana  may  be  the  outcropping 
edges  of  the  great  shear  plane  that  exists  at  the  base  of  the  shell 
and  that  is  exposed  only  in  a  few  regions. 

Continents,  plateaus,  or  mountain  ranges  may  be  elevated 
by  compression  of  the  earth's  shell.  They  are  surrounded  by 
lower  regions:  the  ocean  basins,  for  example,  are  several  miles 
below  the  continental  platforms.  At  the  bases  of  continental 
bodies  there  are  unbalanced  pressures  of  10,000  to  30,000  pounds 
to  the  square  inch.  Chamberlin  postulates  slow  movements 
of  rocks  to  lower  regions  along  the  great  shear  zone.  This 
continental  creep  or  creep  of  any  elevated  region  toward  lower 
regions  he  compares  to  the  movement  of  a  glacier  over  the  earth's 
surface.  Thus  tension  cracks  may  be  formed  comparable  to 
the  crevasses  that  are  formed  in  glaciers.1 

Thrust  faulting  and  folding,  which  usually  result  in  shortening 
the  earth's  crust,  are  followed  by  a  period  of  creep  faulting  or 
normal  faulting,  which  usually  results  in  extending  the  earth's 
crust.  This  hypohesis  suggests  an  explanation  for  the  conclu- 
sion reached  by  many  observers  that  normal  faults  are  more 
numerous  than  other  faults. 

Torsional  Fractures. — To  illustrate  the  effect  of  torsion,  experi- 
ments have  been  made2  in  which  a  plate  of  glass  covered  with 
a  brittle  wax  was  held  firmly  at  one  end  and  twisted.  Figure  81 
shows  the  character  of  the  fractures  resulting  from  such  stresses. 
They  follow  two  general  directions,  cross  at  nearly  the  same 
angles,  and  are  inclined  about  45°  with  the  axis  of  torsion.  Some 
cracks  are  very  short;  others  extend  across  the  plate.  In  general, 
the  short  cracks  stop  at  longer  cracks.  They  are  not  displaced 
by  the  longer  cracks,  but  are  obviously  contemporaneous  with 
them.  Some  of  the  longer  cracks  radiate  from  points,  making 
fan-like  patterns. 

Becker,  Leith3  and  Lindgren4  regard  torsional  cracks  as  due  to 

1. CHAMBERLIN,  T.  C.:  Op.  tit.,  p.  712. 

2  DAUB  REE,  A. :  "-Etudes  synthStiques  de  geologic  experimental, "  p.  310, 
1879. 

BECKER,  G.  F. :  The  Torsional  Theory  of  Joints.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  24,  p.  130,  1894.  Finite  Homogeneous  Strain,  Flow,  and  Rup- 
ture of  Rocks.  Geol.  Soc.  America  Bull,  vol.  4,  p.  48,  1893. 

3  LEITH,  C.  K.:   "Structural  Geology,"  p.  15,  1913. 

4  LINDGREN,  WALDEMAR:  "Mineral  Deposits,"  p.  135,  19ia 


OPENINGS  IN  ROCKS  183 

tension  rather  than  compression.  In  general  they  are  not  con- 
sidered important  seats  of  ore  deposition.  In  some  districts, 
however,  the  master  fractures  are  joined  at  small  angles  by 


FIG.  81. — Sketch  showing  effect  of  torsion  on  glass  plate.     Lower  end  was 
held  fast  to  block  and  upper  end  was  twisted.     (After  Daubree.) 

many    branching    fractures.     Possibly    torsional    stresses    have 
modified  some  vein  patterns  formed  principally  by  compressional 


CHAPTER  XVII 

STRUCTURAL  FEATURES  OF  OPENINGS  IN  ROCKS  AND 
OF  EPIGENETIC  DEPOSITS 

Ore  deposits,  both  syngenetic  and  epigenetic,  are  commonly 
classified  according  to  form.  Syngenetic  deposits  are  generally 
of  simple  form  except  in  folded  rocks,  where  they  have  partici- 
pated in  all  the  deformation  affecting  the  contemporaneous  or 
later  rocks  with  which  they  are  associated.  Of  the  terms  de- 
scribing forms  that  are  defined  below,  all  are  applied  to  epigenetic 
deposits;  a  few,  such  as  "saddle"  and  "trough,"  may  be  applied 
also  to  syngenetic  deposits;  some  irregular,  rudely  equidimen- 
sional  deposits  formed  by  magmatic  segregation  may  be  termed 
"chambers."  Some  writers  use  the  term  "lode"  to  describe  a 
sedimentary  ore  bed  that  is  rudely  tabular  and  not  horizontal, 
but  this  usage  is  to  be  discouraged.  "Ore  bed"  is  a  better 
term;  "seam"  is  applied  to  the  sedimentary  iron-ore  beds  of 
Birmingham,  Ala.,  and  similar  deposits. 

The  origin  of  openings  is  briefly  discussed  on  preceding  pages. 
Certain  definitions  that  are  convenient  for  description  are  given 
below. 

Fissure. — A  fissure  is  an  opening  or  parting  in  the  earth's  crust 
that  is  due  to  movement.  It  may  be  a  mere  crack  with  no 
visible  space,  like  a  crack  in  a  pane  of  glass  that  does  not  extend 
across  it,  or  it  may  present  a  wide  open  space.  The  movement 
may  all  be  across  or  nearly  at  right  angles  to  the  crack,  and  the 
elasticity  of  the  broken  material  may  practically  close  the  open- 
ing. If  there  has  been  movement  parallel  to  the  plane  of  the 
fissure  it  is  termed  a  fault  fissure.1  Joints  are  essentially  small 
fissures. 

1  The  use  of  the  term  "fault  fissure"  is  not  uniform.  Many  apply  it  to 
fissures  which  contain  gouge  or  fragments  rounded  by  abrasion  or  whose 
walls  are  striated  or  grooved.  On  the  other  hand,  some  investigators  apply 
the  term  only  to  those  fissures  along  which  beds,  veins,  or  other  bodies  are 
cut  off.  In  surface  mapping  this  practice  is  common;  in  underground  map- 
ping most  investigators  use  the  term  "fault"  to  describe  fissures  along  which 
there  is  any  evidence  of  tangential  movement,  such  as  slickensides  or  gouge, 
even  where  no  geologic  bodies  are  observed  to  be  displaced. 

184 


FEATURES  OF  OPENINGS  IN  ROCKS  185 

Fissures  range  in  length  from  inches  to  miles.  Some  faults 
have  been  traced  for  many  miles,  and  some  fault  systems  for 
scores  or  hundreds  of  miles;  but  few  single  mineralized  fissures 
are  known  to  be  more  than  5  or  6  miles  long.  The  Comstock 
lode  is  about  3  miles  long;  some  of  the  veins  near  Telluride,  Colo., 
are  3  or  4  miles  long;  the  Amethyst  vein,  at  Creede,  Colo.,  is  work- 
able for  about  2  miles.  Many  fissures  are  more  than  a  mile 
long.  In  actual  practice  the  exact  points  at  which  the  larger 
fissures  end  are  seldom  found  except  where  they  join  other  fissures 
or  faults;  but  it  is  common  to  find  the  terminations  of  ore  shoots 
in  fissures,  and  also  of  the  mineralized  parts  of  fissures  that  are 
of  too  low  grade  to  work. 

Many  fissure  veins  have  been  explored  to  depths  of  half  a  mile 
or  more,  and  there  is  much  evidence  that  some  of  them  when 
formed  extended  more  than  a  mile  below  the  surface.. 


b 

FIG.  82. — a,  Undulating  fissure;  b,  undulating  fissure  after  movement  at  right 
angles  to  its  plane;  c,  undulating  fissure  showing  lenticular  openings  developed 
by  movement  along  plane. 

Many  fissures  are  narrow.  There  is  evidence  that  many  veins 
have  been  formed  mainly  by  replacement;  the  original  spaces 
along  the  fissures  of  such  deposits  were  narrower  than  the  veins, 
and  perhaps  some  were  only  thin  openings.  Many  wide  veins 
were  formed,  at  least  in  part,  by  replacement.  Long,  wide  spaces 
are  probably  rare,  especially  at  great  depths,  yet  in  some  veins 
there  must  have  been  much  open  space,  for  the  vugs  or  unfilled 
portions  that  remain  in  them  are  numerous  and  widely  separated. 

Movements  along  an  undulating  fissure  may  yield  open  spaces 
(see  Fig.  82).  By  mapping  both  walls  of  the  original  opening 
the  amount  of  movement  may  sometimes  be  determined.  The 
fact  that  few  veins  are  of  even  approximately  uniform  width 
indicates  irregularities  of  open  spaces  in  zones  of  fracturing. 
A  great  many  veins  occupy  zones  of  shattering,  sheeting,  crush- 


186      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

ing,  or  horse-tail  structure,  along  which  openings  have  been 
filled  and  shattered  rock  replaced. 

The  importance  of  original  fractures,  however  narrow,  is  ob- 
vious: replacement  is  generally  confined  to  the  wall  rock  near 
the  openings;  the  force  of  crystallization  acts  where  there  are 
original  openings;  the  thrust  of  solutions  is  in  cracks  already 
formed. 

Vein. — A  vein1  is  a  tabular  mineral  mass  occupying  or  closely 
associated  with  a  fracture  or  set  of  fractures  formed  by  deposition 
from  solutions  either  by  filling  fissures  and  pores  in  the  wall 
rock,  or  by  replacement  of  the  wall  rock,  or  by  both  filling  and 
replacement.  The  term  "true  fissure  vein,"  as  generally  used, 
is  intended  to  imply  persistence  in  depth.  It  is  a  favorite  term 
with  some  promoters  of  mining  companies.  As  any  vein  is 
obviously  ."  true "  the  term  has  little  real  significance  and  is 
redundant. 

Fault  Fissure  Vein. — A  fault  fissure  vein  is  a  vein  that  occupies 
a  fault  fissure.  Many  fault  fissures  with  indeterminate  throw 
show  evidence  of  movement  of  the  rocks  along  the  fissure,  such 
as  slickensides  or  triturated  rock.  Many  investigators,  however, 
apply  the  term  "fault  fissure"  only  to  those  veins  along  which 
it  is  possible  to  prove  displacement. 

Lode. — A  lode  is  a  tabular  ore  body,  an  ore  body  with  one  short 
and  two  long  dimensions.  The  term  may  be  applied  to  veins, 
deposits  in  sheeted  or  fracture  zones,  or  replaced  beds.  It  is 
applied  also  to  deposits  filling  a  number  of  thin,  closely  spaced, 
anastomosing  fissures.  The  term  generally  carries  the  idea  of 
a  linear  outcrop,  though  it  is  sometimes  applied  to  ore  bodies 
that  do  not  crop  out. 

Reef. — In  Australia  and  some  other  British  colonies  the  term 
"reef"  is  used  synonymously  with  "vein."  In  general  it  is 
applied  to  a  vein  that  projects  above  the  surface,  but  in  Australia 
it  is  applied  to  some  ore  bodies  that  do  not  outcrop.  The 
"saddle  reef"  is  a  deposit  at  the  crest  of  an  anticline. 

Ledge. — The  term  "ledge"  is  sometimes  used  as  a  synonym 
of  "vein."  Like  "reef,"  it  may  designate  a  lode  projecting 
above  the  surface.  As  defined  by  Ransome,2  it  is  applied  to 

1  LINDGREN,  WALDEMAR:  Metasomatic  Processes  in  Fissure  Veins.     Am. 
Inst.  Min.  Eng.  Trans.,  vol.  30,  p.  580,  1901. 

2  RANSOME,  F.  L. :  The  Geology  and  Ore  Deposits  of  Goldfield,  Nevada. 
U.  S.  Geol.  Survey  Prof.  Paper  66,  p.  150,  1909. 


FEATURES  OF  OPENINGS  IN  ROCKS 


187 


irregular  masses  of  altered  and  mineralized  rock,  traversed  by 
multitudes  of  small,  irregularly  intersecting  fractures  that 
pass  locally  into  areas  of  thorough  brecciation.  Although  the 
word  is  suggested  by  the  outcrop 
of  such  material,  it  is  applied  to 
mineralized  rocks  at  all  depths. 
Ladder  Vein. — A  ladder  vein 
is  a  fractured  zone  in  which  there 
are  cross  fractures  more  or  less 
regularly  spaced.  The  type  is 
not  common  and  generally  is 
found  in  dikes  or  earlier  veins 
that  are  fractured.  Quartz 
stringers  with  a  ladder-like  ar- 
rangement locally  cut  the  Stand- 
ard-Mammoth lode  of  the  Coeur 
d'Alene  district,  Idaho1  (see  Fig. 
83).  The  Morning  Star  dike, 
Woods  Point,  Victoria,2  which  is 
mineralized  by  cross  fracture,  is 
a  ladder  vein. 


Quartz 
Quartzite 

Sulphides,  chiefly  galena 
Siderite 

'  Quart2'  te.  partly  replaced 
by  siderite 

0          1         2  feet 


FIG.    83. — Generalized     sketch     of 
part     of     Standard-Mammoth     lode, 
Coeur  d'Alene  district,    Idaho,  show- 
FractUied  Zone.— A  fractured     "Jg  tode  cross  barred  with  small  veins 

of   barren    quartz.     (After    Ransome, 

zone  is  a  mass  of  rock  cut  by  a    u.  s.  Geol.  Survey.) 

large  number  of  small  irregular 

fractures,  the  mass  as  a  whole  being  more  or  less  tabular  (see 

Fig.  84).     The  fissures  ordinarily  are  filled  with  veinlets  very 


FIG.  84. — Fractured  zone. 


FIG.  85. — Reticulated  vein. 


Such 


closely  spaced,  and  the  country  rock  is  replaced  with  ore. 
a  mass,  if  workable,  is  commonly  mined  as  a  unit. 

1  RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Coeur  d'Alene  District,  Idaho.  U.  S.  Geol.  Survey  Prof.  Paper  62,  p.  128, 
1908. 

2LiNDGREN,  WALDEMAB:  "Mineral  Deposits,"  p.  134,  1913. 


188      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Reticulated  Vein. — A  reticulated  vein  (Fig.  85)  is  a  fractured 
zone  in  which  the  fissures  are  rudely  coordinate,  forming  a  net-like 
pattern. 

Disseminated  Deposit. — In  some  ore  bodies  the  metallic 
minerals  occur  as  thin,  closely  spaced  seams  or  veinlets,  and  the 
rock  between  carries  also  numerous  "shots"  and  seams  of  ore, 
the  whole  mass  being  workable  where  mining  and  milling  costs 
are  sufficiently  low.  These  are  commonly  termed  disseminated 
deposits  (Fig.  86).  Examples  include  the  "porphyry"  copper 
deposits  of  Bingham,  Utah;  Ely,  Nev.;  Morenci,  Silverbell,  and 
Ajo,  Ariz.;  Santa  Rita  and  the  Burro  Mountains,  N.  Mex.; 
Cananea,  Mex.,  and  many  others.  The  ores  of  Miami  and  Ray, 
Ariz.,  are  disseminated  in  schist. 


FIG.  86. — a,  Disseminated  deposit  in  igneous 
rock;  b,  disseminated  deposit  in  limestone.  (After 
Buckley.) 


FIG.  87. — Breccia  vein. 


Stockwork. — A  stockwork  is  a  mass  of  rock  cut  by  a  large 
number  of  intersecting  reticulated  or  irregular  veins  or  veinlets. 
The  country  rock  is  generally  impregnated  with  or  replaced  by 
"shots"  of  ore,  so  that  the  whole  deposit  may  be  workable.  The 
stockwork  differs  from  the  re'ticulated  vein  or  fractured  zone  in 
that  the  mass  as  a  whole  is  generally  less  definitely  tabular. 
The  ore  in  a  stockwork  is  usually  disseminated  ore. 

Breccia  Vein. — In  a  breccia  vein  the  vein  matter  fills  the  spaces 
around  numerous  fragments  of  wall  .rock  inclosed  within  the 
walls  of  the  fissure  (see  Fig.  87) .  The  proportion  of  filling  to 
inclosed  country  rock  is  generally  greater  in  a  breccia  vein  than 
in  a  disseminated  deposit,  sheeted  zone,  fractured  zone,  or  reticu- 
lated vein.  Some  of  the  veins  of  Telluride,  Colo., 1  are  breccia  veins. 

Sheeted  Zone. — A  sheeted  zone  (Fig.  88)  is  made  up  of  a  num- 
ber of  closely  spaced  parallel  fissures.  These  may  be  filled  with 

1  PURINGTON,  C.  W. :  A  Preliminary  Report  on  the  Mining  Industries  of 
the  Telluride  Quadrangle,  Colorado.  U.  S.  Geol.  Survey  Eighteenth  Ann. 
Rept.,  part  3,  p.  771,  1898. 


FEATURES  OF  OPENINGS  IN  ROCKS 


189 


ore,  and  the  country  rock  between  them  may  be  partly  replaced. 
If  the  veins  are  of  fair  size  and  the  intervening  bodies  of  country 
rock  are  relatively  small,  the  sheeted  zone  is  commonly  termed  a 
compound  vein. 


FIG.  88. — Sheeted  zone. 

Shear  Zone. — A  shear  zone  (Fig.  89)  is  a  zone  of  crushed  rock 
formed  by  compressive  stresses,  in  which  the  openings  or  slips 
are  generally  small,  tabular,  and  closely  spaced.  The  individual 
fissures  may  be  smaller  than  those  of  sheeted  zones,  and  in  general 
there  is  clearer  evidence  of  compression. 
Ore  bodies  occupying  shear  zones  are  gener- 
ally deposited  in  whatever  open  spaces  are 
available,  and  the  country  rock  between 
these  spaces  may  be  replaced. 

Gash  Vein. — Gash  veins1  occupy  fissures 
of  moderate  extent,  usually  restricted  to 
one  formation  and  not  connected  with  any 
very  profound  earth  movement.  In  south- 
western Wisconsin,  where  they  are  typically  developed,  they 
occupy  joint  cracks.  Usually  several  small  parallel  fissures 
overlap;  these  are  known  collectively  as  a  "range."  As  a  rule 
replacement  has  occurred  along  the  fissures.2 

1  WHITNEY,  J.  D.:   "Metallic  Wealth  of  the  United  States,"  p.  48,  1854. 

2  GRANT,  U.  S.:  Report  on  the  Lead  and  Zinc  Deposits  of  Wisconsin,  with 
an  Atlas  and  Detailed  Map.      Wis.  Geol.  Survey  Bull.  14,  1906. 

BAIN,  H.  F.:  Zinc  and  Lead  Deposits  of  the  Upper  Mississippi  Valley. 
U.  S.  Geol.  Survey  Bull.  294,  1906. 


FIG.  89. — Shear  zone. 


190      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Run. — A  ribbon-like,  irregular  ore  body,  lying  flat  or  nearly 
flat,  following  the  stratification,  is  called  a  run.  Many  are 
formed  at  the  intersections  of  ore  horizons  with  vertical  fissures. l 
Flats  and  Pitches. — Flats  follow  nearly  horizontal  bedding 
planes,  and  pitches  follow  dipping  joint  planes  (see  pages  81, 
481).  These  deposits,  which  occupy  openings  formed  by  the 
settling  of  limestone  over  a  shrinking  bed,  are  structurally  unique. 
They  have  been  found  only  in  southwestern  Wisconsin.2 

Lens. — A  lens  is  a  rudely  tabular  body  that  thins  out  at  the 
edges.  Most  tabular  bodies,  as  they  have  not  infinite  extent, 
are  strictly  lenses,  but  the  term  is  commonly  used  to  define  a 
relatively  small  body  inclosed  in  schist  and  lying  parallel  to  the 
schistosity  (see  page  117).  Overlapping  lenses  are  shown  in 
Fig.  90.  Some  of  these  are  older  veins  deformed  during  dynamic 

metamorphism;  others  are  de- 
posited in  openings  along  the 
schistosity. 

Pod. — A  pod  is  a  rudely  cylin- 
drical ore  body  that  decreases  at 
the  ends  like  a  cigar.  The  term 
was  formerly  used  to  describe 
certain  bodies  long  in  one  and 

FIG.  90.-0verlapping  lenses.          ^^  ^  twQ  dimensionSj  inclosed 

in  schist,  the  long  axis  lying  parallel  to  the  schistosity.  It  is 
not  much  used  at  present. 

Fahlband. — The  term  "fahlband"  was  first  applied  to  belts  in 
which  disseminated  deposits  of  pyrite,  pyrrhotite,  and  chalcopy- 
rite  appear  in  dark  micaceous  schists.  The  original  fahlbands  at 
Kongsberg,  Norway,  are  more  than  100  feet  wide.  They  are 
themselves  of  too  low  grade  to  work,  but  the  silver  veins  near 
them  are  enriched.  When  the  sulphides  weather  and  dark 
minerals  such  as  biotite  turn  rusty  brown,  the  bands  become  of 
lighter  color  compared  with  the  dark  schist  between  the  bands, 
hence  the  name  (gray  bands). 

Reopened  -Veins. — Many  veins  are  fractured  and  recemented 
by  ore  deposited  later,  thus  showing  mineralization  of  two 
periods.  They  are  called  brecciated  veins  and  should  be  dis- 

1  JENNEY,  W.  P. :  The  Lead  and  Zinc  Deposits  of  the  Mississippi  Valley. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  22,  p.  189,  1894. 

2  CHAMBERLIN,  T.  C. :  The    Ore    Deposits    of    Southwestern    Wisconsin. 
Geology  of  Wisconsin,  vol.  4,  p.  469,  1882. 


FEATURES  OF  OPENINGS  IN  ROCKS 


191 


tinguished  from  breccia  veins,  which  may  be  formed  by  filling 
spaces  around  rock  fragments.     There  is  much  evidence  that  in 


FIG.  91. — Brecciated  vein,   showing  pyrite  cut  by  quartz,   Milan  mine,   New 
Hampshire,     a,  Quartz;  b,  pyrite. 


FIG.  92. — Brecciated  vein  in  Mendota  mine,  near  Georgetown,  Colorado. 
Structure  due  to  postmineral  movement,  a,  Granite  wall  rocks  of  vein;  b, 
comb  quartz  bordering  vein;  c,  pyrite;  d,  coarse  zinc  blende;  e,  uniformly  ground- 
up  granite,  cemented  hard  by  silica  and  having  a  close  resemblance  to  the 
unbroken  granite  of  the  walls.  (After  Spurr  and  Garrey.) 

some  places  ore  deposition  and  earth  movement  have  gone  on 
during  the  .same  period.  Thus  in  a  comparatively  brief  time 
ores  may  be  formed  and  then  fractured  and  the  fractures  in  them 


192       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

recemented  by  ore  or  gangue.  Fig.  91  shows  a  body  of  pyrite 
cut  by  quartz,  the  quartz  having  a  rudely  reticulated  pattern. 
Fig.  92  shows  a  fractured  vein  in  granite,  which  has  also  been 
finely  fractured  and  compacted  so  that  it  seems  now  to  include 
fragments  of  the  vein.  Some  of  the  northwest  veins  in  the  Butte 
district,  Montana,  show  such  features. 

Fault  Fissures  as  Seats  of  Deposition.  —  Faults  are  fissures 
along  which  there  has  been  movement  (see  pages  102-112). 
If  the  plane  of  the  fault  is  undulating,  long  wedge-shaped  open- 
ings may  remain  here  and  there  along  the  fissures  (see  Fig.  82). 
Along  reverse  faults,  which  in  the  main  are  formed  deep  in 
the  zone  of  fracture,  the  larger  openings  will  tend  to  be  closed  by 
the  pressure  of  the  overlying  load.  Faults  may  remain  open 
longer  at  moderate  depths.  For  this  reason  faults  which  are 
formed  within  1^  miles  of  the  surface  and  faults  which  are  of  the 
normal  type  or  along  which  there  has  been  but  slight  movement 
are  more  favorable  places  for  ore  deposition  than  deep-seated 
faults.  Not  many  large  thrust  faults  contain  veins. 

Owing  to  the  fact  that  they  bring  different  rocks  into  juxtaposi- 
tion faults  with  great  displacement  are  in  general  more  easily 
mapped  than  fissures  along  which  no  displacement  is  shown. 
As  they  are  also  likely  to  be  longer  and  deeper,  it  might  appear  to 
follow  that  they  would  contain  the  larger,  more  persistent  ore 
deposits.  This  is  not  generally  the  case,  however,  as  was  pointed 
out  by  S.  F.  Emmons.1  In  a  great  many  districts  where  large 
faults  are  present,  the  principal  ore  deposits  are  in  or  along 
fractures  that  show  but  little  displacement  or  none.  As  a  result 
of  the  movement  along  the  faults  the  rock  may  be  ground  so 
finely  that  no  supercapillary  openings  remain.  The  fissure  may 
be  no  more  permeable  to  solutions  than  the  country  rock  on 
either  side.  Where  many  small  fractures  have  been  developed 
on  one  side  of  the  fault  plane,  the  fractured  wall  may  be  more 
permeable  than  the  fault,  which  acts  as  a  barrier  to  solutions 
rather  than  as  a  passageway  for  them.  Many  valuable  ore 
deposits,  however,  are  in  and  along  fault  fissures. 

Because  it  is  nearer  the  surface  and  under  less  pressure  when 
deformation  takes  place,  and  because  generally  it  is  the  block 
that  settles  down,  the  hanging  wall  is  more  commonly  fractured 
than  the  foot  wall  of  a  fault.  Ore  deposits  that  fill  the  smaller 


,  S.  F.:  Structural  Relations  of  Ore  Deposits.     Am.  Inst.  Min. 
Eng.  Trans.,  vol.  16,  p.  804,  1888. 


FEATURES  OF  OPENINGS  IN  ROCKS 


193 


synchronous  fractures  related  to  the  fault  plane  are  more  likely 
to  be  above  the  fault  than  below  it,  although  they  may  be  found 
in  either  position. 

A  list  of  mining  districts  containing  veins  along  which  no  dis- 
placement can  be  measured  would  include  many  of  the  districts 
of  the  United  States  that  contain  epigenetic  deposits.  In  the 
Philipsburg  district,  Montana,  there  are  faults  of  large  displace- 


Earlier  Vein 

Hornblende  Matter 

Andesite 

FIG.  93. — Cross-section    of    Comstock  Lode  through  C  and  C  shaft.     (After 
Becker,  U.  S.  Geol.  Survey.) 

ment  known  to  be  older  than  the  veins,  but  none  of  the  principal 
veins  are  along  faults  of  proved  displacement.  In  the  Coeur 
d'Alene  district,  Idaho,  there  are  several  faults  that  have  dis- 
placements of  many  thousand  feet,  but  with  the  exception  of 
the  Bunker  Hill  and  Sullivan  lode,  none  of  the  larger  deposits  are 
along  fissures  of  measured  displacement.1  The  ore  of  this 

1  RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Coeur  de'Alene  Mining  District,  Idaho.     U.  S.  Geol.  Survey  Pro/.  Paper 
62,  p.  159,  1908. 
13 


194      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

lode  is  generally  found  in  the  hanging  wall  of  the  principal  fault 
plane. 

In  many  districts,  however,  there  is  a  close  relation  of  faulting 
to  metallization.1  The  Comstock  lode,  Nevada  (Fig.  93),  is 
developed  along  and  in  the  hanging  wall  of  a  fault  of  great  throw. 
The  Amethyst  vein  at  Creede,  Colo.,2  and  some  veins  of  Bull- 
frog, Nev.,3  occupy  faults  of  considerable  displacement.  The 
Coronado  vein  at  Morenci,  Ariz.,4  follows  a  fault  fissure. 

In  limestone  bodies  of  ore  are  commonly  developed  along 
faults.  At  Aspen,  Colo.,5  there  are  great  replacement  deposits 
in  limestone  at  intersections  of  faults.  In  the  Hornsilver  mine, 
in  the  San  Francisco  region,  Utah,6  the  ore  occurs  along  a  fault 
of  considerable  throw.  At  Globe,  Ariz.,7  some  of  the  ore  bodies 
in  limestone  are  deposited  along  the  Old  Dominion  fault,  espe- 
cially in  the  hanging  wall,  extending  irregularly  into  the  lime- 
stone. At  Bisbee,  Ariz.,8  some  of  the  great  ore  bodies  of  the 
Copper  Queen  mine  are  related  to  the  Dividend  fault.  In  the 
Eureka  district,  Nevada,9  most  of  the  largest  deposits  are  in 
zones  of  fractured  limestone  between  great  fault  planes. 

In  the  Nevada  City  and  Grass  Valley  region,  California,10  the 


J.  E.:  The  Relation  of  Ore  Deposition  to  Faulting.     Econ.  Geol, 
vol.  11,  p.  601,  1916. 

2  EMMONS,  W.  H.,  and  LARSEN,  E.  S.:  A  Preliminary  Report  on  the  Geol- 
ogy and  Ore  Deposits  of  Creede,  Colo.     U.  S.  Geol.  Survey  Bull.  530,  pp. 
42-65,  1913. 

3  RANSOME,  F.  L.,  EMMONS,  W.  H.,  and  GARREY,  G.  F.:  Geology  and  Ore 
Deposits  of  the  Bullfrog  District,  Nevada.     U.  S.  Geol.  Survey  Bull.  407, 
1910. 

4LiNDGREN,  WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.  U.  S.  Geol.  Survey  Prof.  Paper  43,  pp.  339-346,  1905. 

6  SPURR,  J.  E.:  Geology  of  the  Aspen  Mining  District,  Colorado.  U.  S. 
Geol.  Survey  Mon.  31,  1898. 

6  BUTLER,  B.  S.:  Geology  and  Ore  Deposits  of  the  San  Francisco  and 
Adjacent  Regions,  Utah.     U.  S.  Geol.  Survey  Prof.  Paper  80,  pp.  166-168, 
1913. 

7  RANSOME,  F.  L.  :  Geology  of  the  Globe  Copper  District,  Arizona.    U.  S. 
Geol.  Survey  Prof.  Paper  12,  p.  140,  1903. 

8  RANSOME,  F.  L.  :  Geology  and  Ore  Deposits  of  the  Bisbee  Quadrangle, 
Arizona.     U.  S.  Geol.  Survey  Prof.  Paper  21,  p.  152,  1904. 

9  CURTIS,  J.  S.:  Silver-lead  Deposits  of  Eureka,  Nev.,  U.  S.  Geol.  Survey 
Mon.  7,  p.  73,  1884. 

10  LINDGREN,  WALDEMAR:  The  Gold-quartz  Veins  of  the  Nevada  City  and 
Grass   Valley   District,  California.     U.  S.  Geol.  Survey  Seventeenth  Ann. 
Rept.,  part  2,  p.  259,  1896. 


FEATURES  OF  OPENINGS  IN  ROCKS  195 

fault  throws  of  the  veins  are  in  general  small.  On  the  Merry- 
field  and  Ural  veins,  however,  great  movements  have  taken 
place,  probably  over  1,000  feet  along  the  planes  of  the  faults. 
In  this  region  nearly  all  the  faults  recognized  are  overthrusts. 
In  the  Turquoise  district,  Arizona,1  ore-bearing  solutions  have 
apparently  gained  access  to  shattered  limestone  along  a  thrust 
fault.  In  the  Mother  Lode  region,  C  alif ornia, 2  a  large  vein  system 
appears  to  be  related  to  a  system  of  reverse  faults. 

Subsidiary  fractures  near  either  normal  or  reverse  faults  are 
commonly  mineralized.  Near  a  normal  fault  the  fractures  in 
its  hanging  wall  are  the  more  likely  to  be  mineralized,  as  is  illus- 
trated in  many  of  the  districts  above  noted.  In  the  Amethyst 
lode,  Creede,  Colo.,  where  the  fault  itself  is  heavily  mineralized, , 
ore  is  found  also  in  subsidiary  fractures  in  both  walls,  but  a  larger 
number  of  fractures  are  in  the  hanging  wall.  At  Butte,  Mont., 
the  older,  easterly  (Anaconda)  system  of  fissures  were  formed 
with  little  or  no  movement  parallel  to  the  fissures.  The  veins 
that  fill  these  fissures  are  regular,  nearly  uniform,  and  very  per- 
sistent, some  of  them  having  been  stoped  for  thousands  of  feet 
along  their  strike,  and  they  show  little  if  any  tendency  to  de- 
velop ore  shoots.  These  veins  are  crossed  by  fissures  of  later 
age,3  which  have  displaced  them.  The  later  faults  are  usually 
from  5  to  20  feet  wide  and  exhibit  one  or  more  planes  of  movement 
marked  by  tough  clay  a  quarter  of  an  inch  to  5  or  6  inches  thick. 
Some  of  these  faults  are  of  the  reverse  type.  They  have  been 
explored  along  the  strike  for  more  than  a  mile  and  at  intervals 
contain  valuable  ore  shoots.  The  ore  shoots  are  very  irregular 
in  form  and  range  from  mere  pockets  to  masses  1,000  feet  long 
and  2,000  feet  deep  or  more.  Their  width  ranges  from  a  knife- 
edge  to  20  feet,  or  the  entire  width  of  the  fault  zone.  Complete 
replacement  of  the  crushed  country  rock  by  ore  minerals  has 
locally  obliterated  the  evidence  that  the  original  fissure  was  due 
to  faulting.  The  barren  places  in  the  faults  are  dry,  but  the  ore 
is  wet,  and  Sales  maintains  that  the  metal-bearing  waters  trav- 
ersed only  the  places  now  mineralized  and  that  the  crushed 
granite  and  attrition  clay  have  formed  impermeable  barriers 

1  RANSOME,    F.    L. :  The    Turquoise    Copper-mining    District,    Arizona. 
U.  S.  Geol.  Survey  Bull.  530,  p.  133,  1913. 

2  RANSOME,  F.  L.:  U.  S.  Geol.  Survey  Geol.  Atlas,  Mother  Lode  District 
Folio  (No.  63),  1900. 

3  SALES,  R.  H. :  The  Localization  of  Values  in  Ore  Bodies  and  the  Occur- 
rence of  Shoots  in  Metalliferous  Deposits.     Econ.  Geol.,  vol.  3,  p.  326,  1908. 


196      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

| 

that  effectively  sealed  off  certain  parts  of  the  faults.  There  are 
many  other  known  examples  of  faults,  older  than  veins,  that  have 
influenced  the  courses  of  solutions  by  damming  them  off.1 

Influences  of  Rock  Structure  on  Fissuring. — In  some  districts 
fissures  show  a  strong  tendency  to  follow  bedding  planes,  planes 
of  schistosity,  contact  planes,  dikes,  and  other  structural  features. 
This  tendency  is  more  noticeable  where  such  planes  are  regular 
and  strongly  developed.  They  are  generally  planes  of  weakness 
and  therefore  favor  rupture.  They  do  not  localize  a  rupture 
completely,  however,  except  where  they  are  favorably  oriented 
with  respect  to  the  forces  that  are  applied.  Generally  the  fissures 
follow  such  planes  at  some  places  and  cut  across  them  at  others. 
They  may  follow  a  regular  contact  along  both  strike  and  dip,  or 
only  along  the  strike.  Many  bedding-plane  deposits  are  simply 
veins  which  follow  bedding-plane  fissures.  A  sheeted  zone, 
consisting  of  closely  spaced  parallel  fissures,  may  be  formed  where 
highly  schistose  and  somewhat  brittle  rocks  are  fractured  by 
compressive  stresses.  Many  igneous  dikes  localize  fracturing 
and  ore  deposition  because  they  are  more  brittle  than  associated 
rocks.  Planes  of  contact  between  two  rock  formations  in 
many  districts  are  likewise  planes  of  rupture.  On  the  Mother 
Lode,  in  California,2  many  of  the  veins  follow  the  planes  of 
schistosity  or  contacts  between  two  formations.  At  Cripple 
Creek,  Colo.,3 many  of  the  veins  follow  dikes  or  contacts  between 
the  dikes  and  other  rocks.  Similar  relations  are  shown  near 
Georgetown4  (Fig.  94),  and  at  Creede,  Colo.;  at  Bullfrog,  Nev.;5 
at  Morenci,  Ariz.6  (Coronado  vein),  and  elsewhere. 

Many  fissure  veins  formed  near  the  surface  and  inclosed  in 

1  RANSOME,  F.  L. :  The  Relation  of  Certain  Ore-bearing  Veins  and  Gouge- 
filled  Fissures.     Econ.  Geol,  vol.  3,  pp.  331-337,  1909. 

2  RANSOME,  F.  L.:    U.  S.  Geol.  Survey  Geol.  Atlas,  Mother  Lode  District 
Folio  (No.  63),  1900. 

"LiNDGREN,  WALDEMAR,  and  RANSOME,  F.  L.:  Geology  and  Gold  Depos- 
its of  the  Cripple  Creek  District,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper 
54,  p.  154,  1906. 

4  SPTJRB,  J.  E.,  and  GARRET,  G.  H.:  Economic  Geology  of  the  Georgetown 
Quadrangle,  Colorado,  with  General  Geology  by  S.  H.  Ball.  U.  S.  Geol. 
Survey  Prof.  Paper  63,  1908. 

8  RANSOME,  F.  L.,  EMMONS,  W.  H.,  and  GARRET,  G.  H.:  Geology  and  Ore 
Deposits  of  the  Bullfrog  District,  Nevada.  U.  S.  Geol.  Survey  Bull.  407, 
1910. 

"LiNDGREN,  WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.  U.  S.  Geol.  Survey  Prof.  Paper  43,  p.  341,  1905. 


FEATURES  OF  OPENINGS  IN  ROCKS 


197 


schists  do  not  follow  the 
schistosity.  In  the  middle 
and  upper  part  of  the  zones 
of  fracture  the  rocks  are  under 
less  load  and  the  separate 
blocks  may  move  more  freely 
than  at  greater  depths.  Thus, 
at  Silver  Plume  and  in  some 
districts  in  the  San  Juan  region 
Colorado,  the  lodes  cut  across 
the  schistosity  at  large  angles. 
Not  all  deposits  that  follow 
the  beddimg  planes  of  schists 
are  fissure  veins.  Some  of 
them  are  dynamically  meta- 
morphosed deposits  of  various 
types.  These  are  discussed 
on  page  114. 

Bedding-plane  Deposits. — 
Epigenetic  deposits  may  be 
restricted  to  certain  beds  be- 
cause those  beds  are  perme- 
able, because  they  are  chemic- 
ally more  hospitable,  or  be- 
cause they  have  been  shattered 
or  fissured  by  movement.  In 
the  Black  Hills  of  South 
Dakota,1  numerous  small  fis- 
sures or  "verticals"  traverse 
beds  of  quartzite  and  lime- 
stone. In  the  quartzite  there 
is  little  or  no  deposition,  but 
the  limestone  above  is  exten- 
sively replaced.  The  solu- 
tions, which  were  not  com- 
petent to  replace  the  quart- 
zite, when  they  reached  the 

1  IRVING,  J.  D. :  Economic  Re- 
sources of  the  Northern  Black 
Hills.  U.  S.  Geol.  Survey  Prof. 
Paper  26,  pi.  19,  1904. 


198      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

limestone  spread  through  a  large  mass  and  deposited  flat,  tabular 
bodies  of  ore.  Extensive  deposits  at  Rico,  Colo.,1  were  formed 
by  replacement  in  limestone  below  shale.  The  vertical  fissures 
that  cross  the  limestone  divide  into  many  small  fractures  in  the 
shale.  Just  below  the  shale  the  limestone  is  replaced  by  long, 
flat  ribbons  of  ore  which  cap  the  verticals  and  follow  the  stratifica- 
tion of  the  rock.  The  shape  of  a  section  of  the  deposit  may  be 
compared  with  a  T-square,  if  the  upper  bar  of  the  square  is  ex- 
aggerated in  thickness  and  the  lower  bar  greatly  exaggerated  in 
length. 

Some  bedding-plane  deposits  in  limestone  are  not  related  to 
beds  of  shale  or  quartzite  above  or  below.     They  follow  the 


FIG.  95. — Cross-section  of  bedding-plane  deposit  in  limestone. 

stratification  of  the  limestone  because  certain  beds  were  more 
permeable  to  the  metal-bearing  solutions.  This  greater  per- 
meability may  be  due  to  differences  in  composition  or  in  pore 
space  in  the  original  rock,  or  to  subsequent  fissuring  or  faulting 
along  the  bedding  planes.  Bedding-plane  deposits  are  illustrated 
by  Figs.  95  and  96. 

Unless  there  is  a  crosscutting  dike,  an  older  vein,  or  some  other 
plane  of  reference,  it  may  be  difficult  to  discover  movement  along 
the  bedding,  for  the  processes  of  replacement  may  obliterate 
evidences  of  movement,  such  as  friction  breccia  or  gouge.  Small 
movements  along  bedding  planes  are  doubtless  very  common  in 
areas  of  tilted  strata.  Many  bedding-plane  deposits  of  fairly 

1  RICKARD,  T.  A.:  The  Enterprise  Mine,  Rico,  Colorado.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  26,  p.  906,  1896. 

RANSOME,  F.  L.:  The  Ore  Deposits  of  the  Rico  Mountains,  Colorado. 
U.  S.  Geol.  Survey,  Second  Ann.  Rept.,  part  2,  p.  291,  1901. 


FEATURES  OF  OPENINGS  IN  ROCKS 


199 


uniform  width  are  inclosed  in  limestone  that  appears  to  be  homo- 
geneous above,  below,  and  in  the  plane  of  the  deposit. 

Some  of  the  criteria  by  which  bedding-plane  deposits  are  dis- 
tinguished from  sedimentary  ores  are  difficult  to  apply.  The 
origin  of  the  ores  in  beds  is  one  of  the  most  perplexing  prob- 
lems of  geology.  Concerning  the  genesis  of  the  ore  deposits  of 
the  most  productive  copper  district  in  Germany  (Mansfeld), 
and  of  the  most  productive  gold  district  in  the  world  (Witwaters- 
rand,  South  Africa),  even  after  extensive  studies  by  able  investi- 
gators, opinions  differ  widely.  The  deposits  of  each  of  these  dis- 
tricts are  regarded  by  many  as  sedimentary  bedded  ores  and  by 
many  others  as  epigenetic  bedding-plane  deposits. 

Long,  Slender  Deposits  and  Equidimensional  Deposits. — 
Where  two  thin  tabular  openings  intersect,  a  long,  slender  open- 


Quartzite  Ore 

FIG.  96. — Cross-sections  of  bedding-plane  deposits  in  quartzite. 

ing  wider  than  either  or  a  line  of  more  intense  fracturing  may  be 
formed  at  the  intersection.  If  such  openings  become  seats  of 
mineralization  the  deposits  may  be  chimneys  or  ribbons  of  ore, 
their  form  depending  upon  the  nature  and  attitude  of  the  rocks 
involved.  Intersections  of  dikes  or  of  bedding  planes  with  fis- 
sures similarly  yield  long,  thin  lines  of  fracturing  that  are  favor- 
able places  for  deposition  (see  Figs.  97,  98).  Fracturing  is 
likely  to  be  more  extensive  where  the  planes  intersect  at  small 
angles. 

Where  several  fissures  intersect  and  are  suitably  spaced,  open- 
ings or  shattered  masses  that  are  nearly  equidimensional  may  be 
formed.  Some  ore  bodies  assumed  to  be  related  to  such  openings 
or  shattered  zones  are  nearly  isodiametric,  or  rudely  spherical. 


200      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Many  chimneys,  ribbons,  and  isodiametric  masses  of  ore  are  re- 
placement deposits,  and  it  may  be  difficult  to  determine  whether 


FIG.  97. — Stereogram  showing  an  ore  shoot  formed  at  intersection  of 
dike  and  vein. 

or  not  such  deposits  are  related  to  large  open  spaces  at  inter- 
sections.    In  many  districts  the  localization  of  the  ore  at  such 


FIG.  98. — Stereogram  showing  an  ore  shoot  formed  at  intersection  of  fissure 
with  limestone  below  a  shale  bed.  The  ore  makes  out  from  the  fissure  along 
the  bottom  of  the  shale  bed.  (After  Butler,  U.  S.  Geol.  Survey.) 

places  is  due  to  more  extensive  fracturing  of  the  wall  rocks  near 
the  intersections  rather  than  to  the  widening  of  open  spaces. 


FEATURES  OF  OPENINGS  IN  ROCKS 


201 


Chimneys  of  ore  in  limestone  are  not  uncommon.  Some  slender 
ore  bodies  in  limestone  are  curved  into  sigmoidal  or  snake-like 
forms;  some  are  very  irregular:  miners  call  them  "corkscrews." 
As  a  rule  such  deposits  are  formed  at  the  intersections  of  fissures 
or  joint  planes,  or  both.  The  circulation  may  follow  a  fissure 
here,  a  bedding  plane  there,  or  a  joint  plane  at  another  place. 
Obviously  "blocking  out"  ore  in  such  deposits  requires  numer- 
ous and  closely  spaced  exposures.  A  continuous  ore  body  may 
be  formed  by  replacement  of  the  rock  locally  along  any  set  of 
planes  or  along  intersections,  and  it  is  generally  unsafe  to 
predict  the  position  of  such  an  ore  body  beyond  the  point  of 
exploration.  The  ore  body  may  end  abruptly  at  any  place;  a 
chimney  followed  upward  may 
thus  become  what  the  miner 
terms  a  "blind  chimney,"  or 
one  that  does  not  extend  to 
the  surface. 

A  mass  of  ore  that  is  nearly 
equal  in  all  dimensions  is  often 
termed  a  "chamber"  or  a 
"chambered  deposit."  Many 
chambered  deposits  are  formed 
by  the  replacement  of  lime- 
stone. Some  of  them  are  below 
shales  or  sheets  of  porphyry  or 
other  igneous  rocks ;  but  others  are  in  limestone  and  show  no  obvious 
relation  to  any  well-defined  structural  feature.  Presumably  they 
are  localized  at  intersections  of  fractures  or  where  there  has  been 
a  maximum  amount  of  fracturing.  Many  of  them  are  puzzling, 
for  the  veinlets,  filled  fractures,  or  replaced  bedding-plane  fis- 
sures which  lead  to  some  of  them  are  small  compared  with  the 
chamber  of  ore.  When  the  ore  is  removed  and  the  walls  are 
closely  scrutinized  the  openings  through  which  solutions  may 
have  entered  are  more  readily  found.  Veins  that  locally  are 
greatly  enlarged  have  been  termed  "chambered  veins"1  (see 
Fig.  99). 

Anticlinal  Deposits;  Saddle  Reefs. — In  many  districts  of 
folded  rocks  ore  deposits  are  concentrated  on  the  axes  of  folds, 
especially  at  the  crests  of  antic'.ines  (Figs.  100  to  103).  This 

1  BECKER,  G.  F. :  Geology  of  the  Quicksilver  Deposits  of  the  Pacific  Slope, 
U.  S.  Geol.  Survey  Mon.  13,  p.  411,  1881. 


FIG.     99. — Chambered     vein.       (After 
Becker,    U.  S.  Geol.  Survey.) 


202      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


m 


Wlfa 

a  i  ii>  >i 


<',*'  >'/'', ,  Y/,< 

mttM 

i  If !'!'.'/«/« 
W$$®b*f'af* 

'^iii^/Mmm. 


miKmt  \ 


200  Feet 


Scliist  Ore  zone  Gossan          Chalcocite  ore 

FIG.  100. — Vertical  section  of  Mary  Mine,  Ducktown,  Tenn. 


Fio.  101. — Vertical  section  of  Hope  Mine,  near  Philipsburg,  Mont.,  showing 
thickening  of  deposit  at  crest  of  anticline. 


FEATURE'S  OF  OPENINGS  IN  ROCKS 


203 


relation  to  the  structure  is  shown  not  only  in  schistose  rocks  that 
have  been  closely  folded,  but  also  in  rocks  that  are  not  schistose 
and  that  have  been  thrown  into  moderately  open  folds.  Five 
possible  modes  of  origin  for  such  deposits  should  be  considered. 


FIG.  102. — Deposits    formed    on    and    near     anticlines,    Tombstone,    Arizona. 
(After  Church.) 

1.  The  folding  of  ore  bodies  after  their  deposition  may  be 
attended  by  thickening  at  the  crests  of  folds  and  thinning  along 
their  limbs. 

2.  Thin  beds  of  limestone  or  other  rocks   compressed  into 
folds  and  thickened  at  crests  of  anticlines  may  have  been  replaced 
by  mineral-bearing  solutions  (see  Fig.  100). 


$»;:   :••••--•-/••>••.--•:,•• 

West  Leg  Oontrc  Country :'-':' '"Kast 

TYPICAL  SADDLE 


\ 


FALSE  SADDLE 
H21  Sandstone    |H  Quartz     El  Slate 
FIG.  103. — Sketch  showing  typical  saddle  and  "false  saddle."     (After  Richard.) 

3.  Mineral  matter  may  be  introduced  into  openings  at  axes 
of  folds  where  brittle  beds  have  been  fractured,  or  where  beds  have 
been  separated  by  movement  (see  Figs.  79,  80). 

4.  Segregation  of  mineral  matter  in  folds  may  be  accomplished 
by  solutions  during  dynamic  or  regional  metamorphism. 

5.  In  igneous  and  in  sedimentary  rocks  fissures  that  have 
approximately  parallel  strikes  and  that  cross  and  dip  in  opposite 


204      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

directions  may  be  more  heavily  mineralized  at  and  near  the  lines 
of  their  intersections  (see  in  Fig.  103). 

1.  If  rocks  are  folded  after  ores  have  formed  in  them,  the  ore 
bodies  are  likewise  folded.     Certain  textures  result,  depending 
upon  the  intensity  of  the  folding  and  the  mineral  composition  of 
the  deposits.     Many  iron-ore  deposits  in  the  Lake  Superior  re- 
gion have  been  folded  after  they  were  formed.     Examples  of 
folded  sulphide  deposits  are  also  known.     These  deposits  are 
discussed  on  pages  113  to  123. 

2.  When  a  series  of  rocks  consisting  of  sandy  or  other  heavy 
beds  inclosing  thin  limestone  layers  is  closely  folded,  the  lime- 
stone may  be  greatly  distorted.     Under  heavy  pressure  the  soft 
lime  carbonate  is  somewhat  mobile  and  is  squeezed  into  places 
where  pressures  are  lowest.     Thus  a  limestone  bed  is  likely  to 
become  thick  on  the  crests  of  anticlines  and  thin  on  the  limbs, 
where  it  may  even  be  squeezed  out  entirely.     If  subsequently 
all  the  limestone  is  replaced  by  ore,  the  ore  body  will  be  largest 
where  the  limestone  bed  was   thickest.     The  copper  sulphide 
ores  of  Ducktown,  Tenn.,  which  replace  deformed  limestone  lenses, 
are  greatly  concentrated  on  the  crests  of  anticlines  (Fig.  100). 

3.  Where  brittle  rocks  alternating  with  less  brittle  ones  are 
flexed,  fracturing  is  likely  to  be  localized  near  the  crests  of  anti- 
clinal folds  or  near  troughs  of  synclinal  folds.    Larger  fractures 
are  more  readily  formed  at  the  crest  of  an  anticline  than  at  the 
trough  of  a  syncline,  because  a  bed  there  is  nearer  the  surface 
and  under  less  pressure     Under  some  conditions,  which  can  not 
now  be  accurately  defined  but  which  doubtless  prevail  under 
moderate  load,  beds  glide  one  over  another  and  spaces  parallel 
to  the  beds  are  formed  at  the  crests  of  folds.     This  mode  of 
formation  of  openings  may  be  illustrated  by  flexing  the  sheets 
of  a  tablet  of  glazed  paper.     If  the  glued  end  is  held  firmly  and 
the  other  end  is  pushed  toward  it  without  exerting  much  down- 
ward pressure  on  the  leaves,  a  number  of  saddle-like  openings 
are  formed,  one  above  another  (Fig.  80). 

Many  bedding-plane  deposits  in  limestone  are  thicker  at  the 
crests  of  folds.  Ore  folds,  anticlinal  deposits,  saddles,  and  troughs 
may  be  developed  also  in  sandstones  and  shales,  or  in  slates,  as 
they  are  at  Bendigo,  Australia,1  where  the  saddle  reef  was  first 
recognized. 

1  DUNNE,  E.  J. :  Victoria  Min.  Dept.  Quart.  Rept.,  1888. 

RICKARD,  T.  A.:  The  Bendigo  Gold  Field.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  20,  pp.  463-545,  1800. 


FEATURES  OF  OPENINGS  IN  ROCKS  205 

4.  Some  sulphide  ores  that  follow  the  cleavage  planes' of  schists 
are  assumed  to  have  been  deposited  by  segregation  during  dy- 
namic or  regional  metamorphism,  but  probably  the  segregation  of 
metals  under  these  conditions  is  not  extensive  (see  page  118). 

5.  The  localization  of  ore  bodies  at  the  intersections  of  fissures 
is  discussed  on  page  199. 

There  are  many  examples  of  anticlinal  deposits.  The  famous 
saddle  reefs  of  Bendigo,  Victoria,  are  mentioned  above.  In  this 
region  the  prevailing  rocks  are  sandstones  and  slate.  There  are 
three  principal  productive  anticlines  which  may  be  followed  5  to 
14  miles  along  their  strike.  On  the  axes  of  these  anticlines  the 
saddles  of  auriferous  quartz  are  spaced  one  above  another  at 
intervals  in  depth  of  300  feet  or  more.  Some  of  the  ore  bodies 
have  been  stoped  for  thousands  of  feet  along  their  strike,  and 
workings  have  extended  below  4,300  feet.  The  anticlinal  axes 
are  not  horizontal  but  dip  eastward;  the  ore  bodies  along  their 
crests  plunge  at  angles  as  great  as  20°.  The  saddles  are  con- 
nected by  quartz-filled  veins  and  by  basic  dikes  that  are  later 
than  the  ore.  There  are  also  "inverted  saddles"  or  troughs  of 
ore,  but  these  are  less  valuable.  About  22  miles  south  of  Bendigo, 
in  the  Castlemaine  field, l  Silurian  rocks  are  intruded  by  granodio- 
rite.  Gold-bearing  saddle  reefs  are  developed  in  the  closely 
folded  sediments.  Veins  are  present  also  in  fissures  and  faults. 

At  Hargraves,  New  South  Wales,2  slates  and  tuffs  are  intruded 
by  granitic  rocks.  The  sediments  are  closely  folded,  and  saddle 
reefs  of  siliceous  gold  ore  are  developed  in  them  along  the  axial 
planes  of  anticlines.  Flat-lying  quartz  veins  connect  the  several 
anticlines. 

The  Rammelsberg  mine,3  in  Germany,  is  mentioned  on  page  122. 

The  deposits  of  Mysore,  India,  among  the  greatest  gold  deposits 
of  the  world,  are  characterized  by  numerous  puckerings  or  folds. 
The  country  is  an  area  of  conglomeratic  hornblende  schists  and 
quartzite,  with  which  are  associated  granite  and  aplite.  The 
principal  lode  is  the  Champion  reef,  which  is  developed  for  4 
miles  along  the  strike  and  3,740  feet  on  the  incline  in  depth. 
The  average  width  is  about  4  feet  and  the  dip  about  55°.  The 

1  BARAGWANATH,  W.:.The  Castlemaine  Gold  Field.  Victoria  Geol. 
Survey  Mem.  2,  1903. 

2PrrrMAN,  E.  F.:  Mineral  Resources  of  New  South  Wales,  pp.  33-36, 
New  South  Wales  Geol.  Survey,  1901. 

3  LINDGREN,  WALDEMAR,  and  IRVING,  J.  D. :  The  Origin  of  the  Rammels- 
berg Ore  Deposit.  Econ.  Geol,  vol.  6,  pp.  303-313,  1911. 


206      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

minerals  are  quartz,  tourmaline,  pyrite,  arsenopyrite,  pyrrhotite, 
chalcopyrite,  zinc  blende,  and  galena.1 

Anticlinal  reefs  are  developed  in  slates  and  quartzites  in  the 
gold  fields  of  Nova  Scotia.2  The  folded  metamorphosed  sedi- 
ments are  cut  by  granite.  The  minerals  present  are  quartz, 
pyrite,  chalcopyrite,  galena,  sphalerite,  and  arsenopyrite. 

The  argentiferous  lead  and  zinc  deposits  of  Broken  Hill,  New 
South  Wales,3  are  also  of  the  saddle-reef  type.  In  this  district, 
which  is  one  of  the  most  productive  in  the  world,  the  ore  bodies 
conform  to  the  planes  of  schistosity.  They  carry  abundant  garnet 
in  the  gangue,  and  it  is  possible  that  they  were  formed  by  the  re- 
placement of  calcareous  beds.  At  Ducktown,  Tenn.,4  anticlinal 
deposits  of  garnetiferous  sulphides  replace  bodies  of  deformed 
limestone. 

In  several  districts  of  the  United  States  where  rocks  are  folded 
less  closely  than  in  the  areas  above  mentioned  anticlinal  ore 
bodies  are  formed  by  replacement  in  limestone.  They  appear 
to  have  been  deposited  at  moderate  depths  by  solutions 
at  moderate  temperatures,  in  zones  of  maximum  fracturing. 
At  Hope  Hill,  near  Philipsburg,  Mont.,5  the  bedding-plane 
deposits  in  limestone  are  greatly  expanded  near  the  crests  of 
anticlines,  where  fracturing  was  probably  greatest  (see  Fig.  100). 
At  Elkhorn,  Mont.,6  sulphide  ores  are  develoepd  in  limestone 
below  a  bed  of  shale.  The  ore  bodies  are  localized  along  an  anti- 
clinal axis  that  plunges  rather  steeply  and  bifurcates  in  depth. 

1  SMEETH,  W.  F. :  Mysore,  Dept.  Mines,  Rept.  1899. 

2  FARIBAULT,  E.  R. :  The  Gold  Measures  of  Nova  Scotia  and  Deep  Mining. 
Canadian  Min.  Inst.  Jour.,  vol.  2,  pp.  119-162,  1899. 

RICKARD,  T.  A.:  The  Domes  of  Nova  Scotia.  Inst.  Min.  and  Met. 
Trans.,  vol.  21,  pp.  506-560,  1912. 

3  PITTMAN,  E.  F. :  New  South  Wales,  Geol.  Survey  Rec.,  vol.  3,  pp.  45-49, 
1892. 

JAQUET,  J.  B.:  "Geology  of  the  Broken  Hill  Lodes,"  Sydney,  1894. 
CLARK,  DONALD:   "Australian Mining  and  Metallurgy,"  p.  267,  1904. 
MOORE,  E.  S.:  Observations  on  the  Geology  of  the  Broken  Hill  Lode, 
New  South  Wales.     Earn.  Geol.,  vol.  11,  pp.  327-348,  1916. 

4  EMMONS,  W.  H.,  and  LANEY,  F.  B.:  Preliminary  Report  on  the  Mineral 
Deposits  of  Ducktown,  Tenn.     U.  S.  Geol.  Survey  Bull.  470,  p.  151,  1911. 

6  EMMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper  78, 
p.  214,  1913. 

6  WEED,  W.  H. :  Geology  and  Ore  Deposits  of  the  Elkhorn  Mining  Dis- 
trict, Jefferson  County,  Montana.  U.  S.  Geol.  Survey  Twenty-second  Ann. 
Rept.,  part  2,  p.  472,  1901. 


FEATURES  OF  OPENINGS  IN  ROCKS  207 

At  Tombstone,  Ariz.,1  argentiferous  deposits  replacing  lime- 
stone are  on  or  near  the  axes  of  anticlines.  In  the  Kelly  district, 
near  Magdalena,  N.  Mex.,2  the  ore  is  developed  in  limestone  as 
three  long  parallel  shoots,  pitching  with  the  beds  and  coinciding 
with  the  axes  of  gentle  folds.  At  the  Robinson  mine,  near  Con- 
coYd,  Maine,3  a  small  lead-zinc  deposit  in  limestone  occupies 
the  axis  of  an  anticline.  The  ore  body  of  the  Clover  Leaf  gold 
mine,  in  the  Black  Hills,  South  Dakota,4  which  is  developed  on 
a  plunging  anticlinal  fold  in  schist  and  slates,  is  greatly  thickened 
on  the  crest  of  the  fold. 

Synclinal  Deposits;  Inverted  Saddles;  Troughs. — In  some 
mining  districts  the  ores  are  concentrated  in  synclines  or  struc- 
tural troughs.  Synclinal  deposits  may  be  formed  by  several 
groups  of  processes  corresponding  to  those  mentioned  above  in 
the  discussion  of  the  origin  of  anticlinal  deposits.  They  may 
be  formed  by  the  replacement  of  limestone  beds  at  synclines,  by 
the  filling  of  openings  between  two  beds  at  synclines,  or  by  re- 
placement in  zones  of  fracturing  on  synclines.  During  dynamic 
metamorphism  tabular  deposits  may  be  folded  to  form  synclines. 
In  areas  of  closely  folded  rocks  openings  due  to  fracturing  are 
probably  less  commonly  developed  along  synclinal  axes  or  planes 
than  along  anticlinal  planes  because  in  a  folded  series  of  beds 
any  given  zone  that  is  favorable  for  the  deposition  of  ore  will 
lie  at  lower  altitudes  in  synclines  than  in  anticlines,  and  in  a 
syncline  the  favorable  zone  is  likely  to  be  covered  by  a  greater 
and  therefore  heavier  mass  of  overlying  beds.  Ores  formed  along 
anticlines  or  along  synclines  are  characteristically  developed  in 
the  deep  vein  zone  or  at  moderate  depths.  Ore  folds  are  com- 
paratively rare  in  deposits  formed  by  ascending  thermal  waters 
at  shallow  depths.  "False  saddles"  or  "false  inverted  saddles," 
both  of  which  are  formed  near  lines  of  junction  and  along  in- 
tersecting fractures,  may  be  formed  at  all  depths.  Although 
anticlinal  deposits  or  saddle  reefs  are  more  numerous  and  generally 

1  BLAKE,  W.  P. :  Geology  and  Veins  of  Tombstone,  Ariz.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  10,  p.  334,  1882. 

BLAKE,  W.  P.:  Tombstone  and  Its  Mines.     Idem,  vol.  34,  p.  668,  1904. 

2LiNDGREN,  WALDEMAR,  GRATON,  L.  C.,  and  GORDON,  C.  H.:  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Prof.  Paper  68,  p.  251,  1910. 

3  EMMONS,  W.  H. :  Some  Ore  Deposits  in  Maine  and  the  Milan  Mine,  New 
Hampshire.     U.  S.  Geol.  Survey  Bull,  432,  p.  48,  1910. 

4  IRVING,  J.  D. :  Economic  Resources  of  the  Northern  Black  Hills.     U.  S. 
Geol.  Survey  Prof.  Paper  26,  p.  212,  1904. 


208      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

more  clearly  defined  than  synclinal  deposits  or  inverted  saddles, 
examples  of  the  latter  are  not  rare.  Some  are  found  in  the  famous 
Bendigo  district  of  Victoria,  Australia,1  where  anticlinal  deposits 
are  conspicuously  developed.  The  famous  zinc  deposits  of  Frank- 
lin Furnace,  N.  J.,  form  pitching  troughs  (see  page  487).  At 
Franklin,  British  Columbia,2  the  deposits,  which  were  formed  late*r 
than  the  regional  metamorphism  of  the  associated  sedimentary 
rocks,  are  on  compressed  synclines  where  the  mashed  and  sheared 
rocks  have  afforded  channels  for  solutions.  Some  of  the 
deposits  of  Bisbee,  Ariz.,  lie  in  a  gentle  syncline.  This  fold, 
however,  appears  to  be  only  one  of  several  features  that  have 
influenced  the  localization  of  the  ore  deposits  (see  page  367). 
Other  examples  of  deposits  formed  in  open  synclines  are  known.3 
Many  ore  deposits  and  protores  enriched  by  superficial  altera- 
tion occur  in  pitching  troughs  or  synclines,  especially  where  a 
body  of  fractured  permeable  protore  lies  in  a  trough  that  is  formed 
by  impermeable  rocks.  Such  troughs  are  likely  to  be  paths  of 
descending  solutions  that  may  enrich  the  ore  by  dissolving  and 
removing  valueless  constituents  or  that  may  deposit  valuable 
ores  (see  page  301). 

FRACTURE  SYSTEMS  IN  MINING  DISTRICTS 

Although  the  fractures  and  veins  in  many  mining  districts  show 
great  differences  in  their  distribution,  there  are  certain  fracture 
patterns  which  are  well-recognized  recurring  types.  In  some  dis- 
tricts the  fractures  of  one  system  or  one  set  are  approximately 
parallel  in  strike  and  dip,  and  those  of  a  second  system  are  ap- 
proximately parallel  to  the  first  in  strike  but  dip  in  an  opposite 
direction.  Such  groups  have  been  termed  "conjugated"  frac- 
tures. A  coordinate  set  of  fractures  consists  of  a  group  of  parallel 
or  nearly  parallel  fractures  that  is  crossed  by  a  second  group  of 
nearly  parallel  fractures  striking  in  a  different  direction.  The 
fractures  of  two  conjugated  systems  or  of  a  conjugated  set  may 
be  cut  by  those  of  two  other  conjugated  systems  striking  approxi- 
mately at  right  angles  to  them.  If  the  fissures  of  the  latter 

1  RICHARD,  T.  A. :  The  Bendigo  Gold  Field.     Am.  Inst.  Min.  Eng.  Trans., 
vol.  20,  pp.  463-545,  1892. 

2  DRTSDALE,  C.  W. :  Geology  of  the  Franklin  Mining  Camp,  British  Co- 
lumbia.    Canada  Geol.  Survey  Mem.  56,  p.  166,  1915. 

3  LAWSON,  A.  C. :  Ore  Deposition  in  and  near  Intrusive  Rocks  by  Meteoric 
Waters.     Cal.  Univ.  Dept.  Geol.  Butt.,  vol.  8,  pp.  219-242,  1914. 


FEATURES  OF  OPENINGS  IN  ROCKS 


209 


likewise  dip  in  opposite  directions,  there  will  then  be  altogether 
four  systems  of  fracture,  or  two  coordinated 
conjugated  sets.     Such  systems  are  com-' 
monly  attributed  to  compressive  stresses. 

Conjugated  Systems. — Examples  of  con- 
jugated or  coordinated  systems  developed 
in  various  degrees  of  perfection  are  found 
in  many  districts.  Among  the  best-known 
examples  are  those  of  the  Nevada  City  and 
Grass  Valley  region,  California,1  where  there 
is  a  northerly  system  of  veins  that  dip 
about  35°  to  40°  east  or  west  (see  Fig. 
104),  and  an  easterly  system  that  dip  either 
north  or  south  at  high  or  low  angles.  In 
the  Ophir  district,  California,2  vein  systems 
on  the  strike  make  angles  of  intersection  of 
about  45°  (see  Fig.  105).  Other  examples 
are  in  the  Telluride  region,3  San  Juan  Moun- 
tains, Colorado,  and  the  Needle  Moun- 
tains,4 also  in  Colorado.  Foreign  examples 
include  the  tin  veins  of  Cornwall,  England, 
the  silver  veins  of  Freiberg,  Saxony,  the  +/  A  "^  "1  <« 

silver-cobalt  veins  of  Schneeberg,  Saxony, 
and  a  great  many  others. 

The  coordinated  and  conjugated  sets  of 
fractures  are  generally  supposed  to  have 
been  formed  about  the  same  time  and  by 
the  relief  of  the  same  stresses.  If  the  in- 
tersections of  veins  of  the  two  systems 

1  LINDGREN,  WALDEMAR:  The  Gold-quartz  Veins 
of  Nevada  City  and  Grass  Valley  Districts, 
California.  U.  S.  Geol.  Survey  Seventeenth  Ann. 
Rept.,  part  2,  p.  13,  1896. 

2 LINDGREN,  WALDEMAR:  The  Gold-silver  Veins 
of  Ophir,  Calif.  U.  S.  Geol.  Survey  Fourteenth 
Ann.  Rept.,  part  2,  p.  253,  1893. 

3  PURINGTON,  C.  W. :  Preliminary  Report  on  the 
Mining  Industries    of  the   Telluride  Quadrangle, 

Colorado.    U.   S.  Geol.  Survey   Eighteenth  Ann.       £* ! 

Rept.,  part  3,  p.  745,  1897. 

4  IRVING,  J.  D. :  U.  S.  Geol.  Survey  Geol.  Atlas,  Needle  Mountains  Folio 
(No.  131),  1906. 

14 


+  + ,+  /  X  •> 


210      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

should  show  that  those  of  one  system  are  everywhere  cut  by 
those  of  the  other  set,  it  would  be  inferred  that  the  two  sets  of 
fissures  were  formed  at  different  times. 

Horse-tail  Structure. — In  many  vein  systems  there  is  a  major 
fracture  from  which  numerous  minor  fractures  extend.  The 
minor  fractures  commonly  make  angles  of  30°  to  60°  with  the 
major  fracture  and  are  rudely  parallel  one  to  another.  Where 
they  are  closely  spaced  large  ore  bodies  may  be  formed  along 
them.  If  the  country  rock  between  them  is  highly  shattered 
wide  zones  of  mineralization  may  be  developed.  At  Butte, 
Mont.,1  on  the  deeper  levels  of  the  Leonard  mine,  the  axes  of 
the  principal  deposits  strike  at  a  large  angle  with  the  main  limit- 


FIG.  105. — Map  of  the  principal  vein  systems  near  Ophir  and  Auburn,  Calif. 
(Based  on  map  by  Lindgren,  U.  S.  Geol.  Survey.) 

ing  fracture.  In  some  deposits  in  the  Butte  district  zones  of 
mineralization  nearly  200  feet  wide  have  been  formed  in  the 
fractured  rock  between  two  outlining  master  fractures.  The 
cross  fractures  are  themselves  connected  by  many  still  smaller 
fractures.  The  multiplicity  of  fractures  gave  the  solutions  free 
access  to  great  bodies  of  rock,  and  mammoth  ore  deposits  were 
formed  by  replacement  of  the  shattered  zones.  In  some  of  these 
deposits  it  can  be  shown  that  the  minor  fractures  were  formed  at 
the  same  time  as  the  outlining  major  fractures.  The  minor 
fractures  do  not  cross  but  join  the  major  fractures,  and  all  carry 
similar  ore.  In  some  examples  the  area  of  cross  fracturing  is 

1  SALES,  RENO:  Ore  Deposits  of  Butte,  Montana.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  46,  pp.  13,  17  (see  particularly  Fig.  2),  1913. 

WEED,  W.  H.:  Geology  and  Ore  Deposits  of  the  Butte  District,  Mon- 
tana. U.  S.  Geol.  Survey  Prof.  Paper  74,  pp.  203-213,  1912. 


FEATURES  OF  OPENINGS  IN  ROCKS  211 

sharply  limited  in  only  one  direction  by  master  fractures,  as 
shown  in  the  Leonard  mine  (Fig.  106).  Toward  the  southeast 
the  cross  fractures  join  and  become  less  closely  spaced,  the 
highly  fractured  zone  gradually  giving  way  to  less  fractured 
rock.  It  is  believed  that  the  fracturing  is  due  to  compressive 
stresses  acting  on  a  mass  that  was  free  to  move  in  one  direction. 
If  a  block  of  wood  is  compressed  in  a  testing  machine  with  one 
end  of  the  block  free  to  move  laterally,  structure  closely  resem- 
bling the  horse-tail  structure  may  be  developed. 

Parallel  Systems. — In  many  mining  districts  all  or  nearly  all 
the  ore  veins  are  approximately  parallel.     Many  parallel  sys- 


FIG.  106. — Plan  of  a  portion  of  the  Leonard  Mine,  Butte,  Montana,  showing 
transverse  fissuring  in  Colusa-Leonard  vein.     (After  Sales.) 

terns  have  commonly  been  regarded  as  resulting  from  compressive 
stresses,  although  the  analogy  with  the  results  of  experiments  in 
deformation  is  obviously  not  so  close  as  that  with  coordinate 
systems.  Variations  in  stresses,  heterogeneity  of-  materials, 
etc.,  may  favor  the  development  of  only  one  system.  In  some 
districts  there  may  be  two  systems  of  fractures  of  which  only 
one  is  mineralized  or  filled  with  vein  materials.  The  other  system , 
where  represented  by  joint  planes  only,  is  inconspicuous.  There 
are  many  more  mining  districts  in  which  the  ore  veins  may  be 
referred  to  a  single  parallel  system  than  to  two  coordinated  or 
conjugated  sets.  Examples  of  systems  of  rudely  parallel  veins 


212      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

include  the  earlier  copper  lodes  of  Butte,  Mont.;  the  silver- 
gold  lodes  of  Philipsburg,  Mont.;  some  of  the  districts  of  Cali- 
fornia and  Idaho  (see  Fig.  107);  several  districts  in  Colorado; 
and  many  others.  This  is  probably  the  most  common  type  of 
vein  system. 

Radial  Patterns. — Systems  of  radial  igneous  dikes  are  shown 
in  many  districts,  but  radial  patterns  of  ore  veins  are  not  com- 
mon. They  are  represented  in  some  degree  of  perfection  at 
Cripple  Creek,  Colo.,1  where  the  principal  fissures  are  confined 
to  an  area  but  little  larger  than  the  volcanic  neck  in  and  around 
which  the  lodes  are  grouped  (Fig.  108).  Basic  dikes  are  arranged 
radially  about  the  volcanic  center,  and  many  of  the  veins  follow 


Hailey 


FIG.  107. — Map  of  vein  systems  in  Wood  River  district,   Idaho.     (Based  on 
map  by  Lindgren,  U.  S.  Geol.  Survey.) 

the  dikes.  The  "hubs"  of  the  Atlin  district,  British  Columbia, 
described  by  Cairnes,2  are  radial  patterns  less  perfectly  developed. 
Irregular  Patterns. — Irregular  patterns  of  ore  veins  are  not 
uncommon.  In  these  the  veins  occupy  fissures  that  can  not  be 
referred  to  systems.  Such  complex  fracture  patterns,  as  pointed 
out  by  Chambeiiin,3  may  be  regarded  as  shallow-seated  phenom- 
ena related  to  the  more  simple  movements  which  take  place 
at  greater  depths.  As  would  be  supposed,  the  patterns  of  this 

1  LINDGKEN,  WALDEMAR,  and  RANSOME,  F.  L. :   Geology  and  Gold  De- 
posits of  the  Cripple  Creek  District,  Colorado.     U.  S.  Geol.  Survey  Prof. 
Paper  54,  p.  167,  1906. 

2  CAIRNES,  D.  D. :  Portions  of  Atlin  District,  British  Columbia.     Canada 
Geol.  Survey  Mem.  37,  p.  84,  1913. 

3  CHAMBERLIN,  T.  C.:  The  Fault  Problem.     Econ.  Geol,  vol.  2,  p.  590, 
1907. 


FEATURES  OF  OPENINGS  IN  ROCKS 


213 


character  are  developed  more  commonly  in  areas  of  compara- 
tively late  deposition  where  the  deposits  were  formed  near  the 


\  / 


Granite,       The  Volcanic     The  Beacon 

Gneiss,  and  Meek        Hill  Phonolite 

Schist  Plug 


Lodes 


FIG.  108. — Plan  of  the  principal  fissures  of  the  Cripple  Creek  district,  Colorado 
on  plane  9,500  feet  above  the  sea.  (Based  on  map  by  Lindgren  and  Ransome, 
U.  S.  Geol.  Survey.) 

surface.     The  ledges  at  Goldfield,  Nev.,1  form  an  irregular  pat- 
tern (Fig.  109),  although  certain  small  portions  of  this  area  show 

1  RANSOME,  F.  L. :  Geology  and  Ore  Deposits  of  Goldfield,  Nevada.  U.  S. 
Geol.  Survey  Prof.  Paper  66,  p.  151/1909. 


214       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


tendencies  to  parallelism.     The  lodes  of  Cobalt,  Ontario,1  are 
likewise  unsystematized. 


yt 


II 
11 

i;^ 

*o&5 


1  MILLER,  W.  G.:  The  Cobalt-nickel  Arsenides  and  Silver  Deposits  of 
Temiskaming,  Ontario.  Ontario  Bur.  Mines  Rept.,  vol.  19,  part  2,  p.  6, 
1913. 


FEATURES  OF  OPENINGS  IN  ROCKS 


215 


Topographic  Expression  of  Deposits. — The  topographic  ex- 
pression of  a  deposit  depends  upon  the  relation  between  the  rates 
at  which  the  deposit  and  the  country  rock  are  eroded.  If  the 
deposit  is  more  resistant  to  erosion  than  the  country  rock,  the 
latter  will  be  removed  more  rapidly  and  the  lode  may  crop  out 
as  a  ridge  or  knob.  If  the  country  rock  is  the  more  resistant 
the  deposit  may  occur  at  the  bottom  of  a  slight  depression  where 
blocks  of  hard  vein  quartz  are  mingled  with  the  rock  debris. 
If  there  is  no  marked  difference  between  the  rates  of  erosion  of 
the  deposit  and  the  country  rock,  the  deposit  may  be  found  in 
any  topographic  position,  and  for  the  lode  deposits  of  the  western 
part  of  the  United  States  this  is  the  most  common  condition. 
There  is  in  few  places  a  conspicuous  relation  between  the  out- 
crops of  the  deposits  and  the  large  features  of  the  topography, 
although  in  many  camps  some  of  the  minor  features  of  the  relief 
are  controlled  by  the  lodes. 


a  b 

FIG.  110. — a,  Outcrop  of  lode  more  resistant  than  country  rock;  b,  outcrop  of 
lode  less  resistant  than  country  rock. 

It  is  not  unusual  to  find  differences  in  topographic  expression 
among  the  deposits  of  a  single  district,  or  even  in  a  single  lode. 
At  one  place  it  may  crop  out  as  a  ridge;  at  another,  along  a  ravine. 
The  difference  in  resistance  must  be  great  before  permanent  relief 
is  established,  for  the  rock  of  a  ridge  is  in  an  exposed  position  and 
is  therefore  the  more  readily  attacked  by  agents  of  weathering. 
In  a  region  that  has  not  been  recently  glaciated  a  resistant  ore 
may  crop  out  as  a  small  ridge.  Siliceous  ores  in  limestones  and  in 
other  soluble  rocks  generally  stand  conspicuously  above  the  sur- 
rounding country.  Magnetite  resists  erosion  longer  than  pyrite, 
and  its  deposits  may  crop  out  as  ridges.  In  the  igneous  rocks 
of  many  regions  siliceous  deposits  form  conspicuous  ridges,  but 
if  the  siliceous  deposits  are  much  fractured  they  mark  shallow 
ravines  and  saddles  where  they  cross  ridges  (see  Fig.  110). 


216      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

In  quartzite  siliceous  deposits  do  not  foi'm  conspicuous  topo- 
graphic features.  Pyrite  and  other  sulphides  alter  more  readily 
than  quartz,  and  few  highly  pyritic  ores  show  marked  topographic 
relief.  In  resistant  rocks  pyritic  ore  bodies,  especially  ore  bodies 
that  are  fractured,  may  form  depressions. 

The  great  majority  of  ore  bodies  are  rudely  tabular;  accord- 
ingly the  intersection  of  such  a  body  with  the  surface  is  in  general 
expressed  as  a  band,  which  is  usually  curved,  the  curvature  de- 
pending on  the  topography  and  the  dip  of  the  lode,  or  on  an 
actual  bend  of  the  lode  along  the  strike. 

In  a  few  districts  lodes  are  traced  almost  continuously  for 
hundreds  of  yards  by  the  topographic  expression  of  the  out- 
crops or  by  vein  matter  exposed  here  and  there  at  the  same 
general  level  as  that  of  the  surrounding  rock.  Many  large 
lodes,  however,  some  of  them  thousands  of  feet  long,  are  exposed 
originally  only  at  one  or  two  places.  The  facility  with  which 
outcrops  of  ore  bodies  are  covered  up  or  otherwise  obscured  by 
processes  of  weathering  is  surprising.  Many  valuable  deposits 
have  been  found  by  digging  in  the  surface  debris  where  fragments 
of  quartz  or  iron  oxide  suggested  to  the  prospector  the  presence 
of  a  deposit  underneath. 

The  difference  in  the  rate  of  erosion  of  the  ore  and  country 
rock  inay  under  some  conditions  give  a  hint  as  to  the  relative 
size  of  the  lode  in  depth.  As  a  rule  a  deposit  that  varies  greatly 
in  width  down  the  dip  and  is  eroded  much  less  rapidly  than  the 
country  rock  will  decrease  in  width  downward.  On  the  other 
hand,  a  lode  that  varies  in  width  down  the  dip  and  is  eroded  more 
rapidly  than  the  country  rock  will  generally  increase  in  width 
downward.  If  the  lode  is  very  resistant  and  the  country  rock 
easily  eroded,  then  the  lode  crops  out  above  the  surface,  and 
the  wider  part  of  the  lode  will  remain  exposed  for  a  longer  period 
than  the  narrower  part.  Fig.  Ill  illustrates  this  case.  The 
lode  is  more  resistant  to  erosion  than  the  country  rock  and 
crops  out  as  a  ridge.  The  solid  line  represents  an  erosion 
surface  that  shows  a  maximum  amount  of  the  hard  material  and 
may  be  called  an  "enduring"  surface;  the  dotted  line  represents 
an  erosion  surface  that  shows  a  maximum  amount  of  the  soft 
rock  and  may  be  called  a  "temporary"  surface.  If,  on  the 
other  hand,  the  deposit  is  less  resistant  than  the  country  rock, 
the  narrow  portion  is  likely  to  remain  at  the  surface  longer, 
as  shown  by  Fig.  112,  in  which  the  solid  line  represents  the 


FEATURES  OF  OPENINGS  IN  ROCKS 


217 


"enduring"  outcrop,  and  the  dotted  line  the  "temporary" 
outcrop.  Such  a  deposit  is  likely  to  increase  in  size  as  it  is  fol- 
lowed downward.  In  other  words,  a  maximum  amount  of  the 
most  resistant  material,  be  it  ore  or  country  rock,  tends  to  remain 
longest  at  the  surface  and  to  monopolize  the  outcrop.  The 
majority  of  such  deposits,  though  not  all,  will  increase  in  size 


FIG.  111. — Cross-section  of  a  lode 
which  varies  in  width  down  the  dip 
and  which  is  more  resistant  to 
erosion  than  the  country  rock. 
The  solid  lines  represent  the  "en- 
during "  surface,  the  dotted  lines  the 
"temporary"  surface. 


FIG.  112. — Cross-section  of  a  lode 
which  varies  in  width  down  the  dip 
and  which  is  less  resistant  to  erosion 
than  the  country  rock.  The  solid 
lines  represent  the  "enduring"  sur- 
face the  dotted  lines  the  "tempo- 
rary" surface. 


with  depth.  Examples  of  large  masses  of  quartzitic  ore  out- 
cropping in  limestone  that  are  underlain  by  relatively  small 
bodies  of  ore  in  limestone  are  common.  These  include  some  of 
the  most  notable  disappointments  in  mining.  A  siliceous  bed- 
ding-plane deposit  in  limestone  may  veneer  a  dip  slope,  or  a 
saddle  of  quartz  in  limestone  may  occupy  the  entire  crest  of  a 
hill  and  extend  downward  on  all  sides. 


CHAPTER  XVIII 
METASOMATIC  PROCESSES 

MECHANISM  OF  REPLACEMENT 

Metasomatism  is  a  chemical  process  by  which  a  mineral  or 
rock  is  replaced  by  another  of  different  composition  and  the  form 
of  the  earlier  body  is  preserved.  The  process  of  replacement  or 
substitution  goes  on  under  widely  varying  conditions  and  oper- 
ates in  the  formation  of  epigenetic  ores  of  every  class.  It  has 
been  active  in  connection  with  the  formation  of  some  pegmatites; 
it  is  the  dominant  process  in  the  formation  of  many  contact- 
metamorphic  ore  deposits;  it  is  operative  where  veins  are  formed 
by  solutions,  magmatic  or  meteoric,  hot  or  cold,  at  great  depths 
or  at  moderate  depths  or  near  the  surface.  Metasomatism  is 
important  in  rock  weathering,  oxidation,  and  secondary  sulphide 
deposition;  in  chalcocitization  it  is  the  dominant  process.  The 
changes,  though  not  more  clearly  shown,  are  easier  to  trace  in 
rocks  and  ores  that  have  been  altered  metasomatically  in  the 
zone  of  weathering  and  sulphide  enrichment  than  in  rocks  that 
have  been  altered  at  profound  depths  by  hot  solutions. 

An  ore  undergoing  surface  decomposition  by  weathering  is 
first  attacked  along  small  fractures  or  bedding  planes,  or  wherever 
oxidizing  solutions  can  make  their  way.  In  and  near  these  open- 
ings the  later  secondary  minerals  are  developed.  Where  the 
process  goes  on  until  the  secondary  products  predominate,  the 
original  unaltered  or  slightly  altered  material  will  remain  as 
nodules  or  irregular  bodies  surrounded  by  the  secondary  material. 
Some  minerals  show  a  very  strong  tendency  to  assume  the 
spherical  shape.  Any  irregularities  or  sharp  protuberances  are 
generally  attacked  first  because  they  present  greater  surfaces  to 
the  decomposing  solutions.  The  cores  of  unaltered  original  sub- 
stance may  become  smaller  and  smaller  until  the  new  substance 
completely  replaces  them. 

Metasomatism  takes  place  by  solution  and  reprecipitation 
in  very  small  openings.  If  large  solution  cavities  form  before 
appreciable  precipitation  begins,  minute  textures  will  not  be 

218 


METASOMATIC  PROCESSES  219 

preserved,  and  the  material  undergoing  alteration  may  collapse, 
so  that  even  its  form  will  be  destroyed ;  but  if  precipitation  of  the 
new  substance  begins  before  the  old  substance  is  completely 
dissolved  the  form  or  texture  of  the  old  substance  may  be  pre- 
served. This  frequently  happens  where  the  rock  subjected  to 
attack  contains  a  radicle  that  unites  with  one  present  in  the 
solution,  to  form  an  insoluble'  compound.  In  many  places 
the  zone  of  solution  and  reprecipitation  is  so  narrow  that  no 
space  between  the  old  and  new  substance  is  seen  even  on  close 
examination.  In  the  zone  of  oxidation  the  secondary  substances, 
such  as  carbonates  and  oxides,  usually  contain  numerous  cavities; 
their  porous  or  spongy  character,  even  in  minute  particles,  is 
generally  evident.  At  some  places,  where  a  new  mineral  has 
replaced  an  old  one  metasomatically,  a  thin  zone  occupied  by  a 
spongy  mass  is  evident  between  the  old  and  new  substances. 
Locally,  however,  dense  new  crystals  and  crystalline  bodies  may 
form  in  the  open-textured  mass,  which  later  may  become  com- 
pletely indurated  by  subsequent  deposition  of  like  material. 

Some  specimens  of  altered  rock,  on  being  broken,  separate  on 
the  plane  of  contact  between  the  old  and  new  minerals;  others 
break  across  the  contact  but  when  boiled  in  a  dye  solution  are 
stained  along  the  contact,  showing  that  there  is  a  porous  zone 
between  the  minerals  which  admits  the  dye  but  which  is  never- 
theless strong  enough  to  hold  the  old  and  new  material  together. 
Still  other  specimens  break  across  the  contact  and  show  no  stain 
after  boiling  with  dye,  even  when  examined  with  a  microscope. 
A  specimen  of  pyrite  altering  to  limonite  from  the  Southern 
Cross  mine,  Montana,1  shows  a  thick  shell  of  iron  oxide  sur- 
rounding the  pyrite  and  penetrating  it  along  fracture  planes. 
So  dense  is  the  iron  oxide  that  no  open  spaces  were  observed 
under  the  microscope;  the  specimen  was  not  stained  on  boiling 
with  dye;  yet  it  was  evidently  undergoing  oxidation — a  process 
necessitating  the  entrance  of  oxygen  and  the  escape  of  a  solution 
containing  a  sulphur  compound. 

In  metasomatic  replacement  reprecipitation  generally  succeeds 
solution  so  closely  that  they  seem  to  be  essentially  parts  of  a 
single  process.  The  absence  of  microscopically  visible  spaces  in 
many  altered  rocks  has  led  to  the  statement,  frequently  made, 

1  EMMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper  78,  p. 
185,  1913. 


220      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

that  the  new  substance  has  replaced  the  old  "molecule  by  mole- 
cule." This  expression  has  not  been  in  much  favor  in  recent 
years  because  the  theory  encounters  chemical  difficulties.  Also 
as  is  evident  from  the  following  figures,  there  is  room,  even  in  a 
submicroscopic  space,  for  myriads  of  molecules,  involving  various 
reactions. 

Meter 

Size  of  subcapillary  sheet  opening  (maximum) 0 . 0000001 

Size  of  minimum  particle  clearly  seen  by  microscope  0 . 00000025 
Size  of  molecule 0. 0000000003 

Even  the  highest-power  microscope  can  not  detect  a  capillary 
opening  of  minimum  size.  It  is  possible,  then,  that  in  metaso- 
matic  processes  material  is  transferred  through  invisible  capillary 
openings.  The  sizes  stated  above,  moreover,  are  at  ordinary 
temperatures  and  pressures.  Under  high  pressures  and  at  higher 
temperatures  water  moves  through  minute  openings  with  greater 
freedom.  Molecules  have  about  one  eight-hundredth  the  line 
dimensions  of  the  smallest  particle  ordinarily  visible  through  the 
microscope.  Evidently  the  most  complicated  chemical  reactions, 
involving  solution  and  reprecipitation  by  stages,  may  be  carried 
on  in  a  zone  so  thin  as  to  be  invisible  even  microscopically. 

When  hot  waters  circulate  along  fissures  and  deposit  ore  in 
them  they  alter  the  wall  rock  of  the  fissures.  Such  hydrother- 
mally  altered  rock  may  be  noted  along  essentially  all  veins 
deposited  by  hot  solutions.  All  rocks  are  affected — limestones, 
shales,  sandstones,  and  igneous  rocks.  The  nature  of  the  changes 
depends  upon  the  character  of  the  rocks  involved,  the  extent  of 
their  fracturing,  the  length  of  time  the  solutions  remain  active, 
and  the  temperature,  pressure,  and  composition  of  the  solutions. 
Many  examples  are  known  of  rocks  which  have  been  completely 
replaced.  Limestones,  shales,  and  igneous  rocks  may  be  entirely 
replaced  by  silica  or  by  silica  containing  various  sulphides  and 
other  minerals.  Again,  the  changes  may  be  relatively  very  slight. 
In  an  igneous  rock — for  example,  in  granite — magnetite  may  alter 
to  pyrite,  mica  may  alter  to  chlorite,  the  feldspars  may  become 
but  slightly  clouded  by  the  development  of  felty  white  mica 
(sericite),  and  quartz  may  remain  essentially  unchanged.  All 
gradations  exist  between  the  two  degrees  of  alteration,  and  the 
country  rocks  along  some  veins  show  them  all — intensely  silicified 
rock  close  to  the  fissure  passing  gradually  outward  through 
slightly  altered  rock  into  the  fresh  or  unaltered  rock.  Along 


METASOMATIC  PROCESSES  221 

some  veins  and  especially  along  some  thin  veinlets  the  zone  of 
alteration  is  only  2  or  3  inches  wide,  or  even  less;  along  others  it 
is  many  feet  wide;  and  along  wide  zones  of  shattered  rock,  filled 
with  closely  spaced  veins,  the  zone  of  alteration  may  be  measured 
in  hundreds  of  feet. 

In  hydrothermally  altered  rocks  nodules  .of  the  original  sub- 
stance surrounded  by  the  altered  material  are  less  common  than 
in  oxidized  ores.  The  thermal  solutions  have  apparently  more 
thoroughly  permeated  the  rocks.  The  changes  are  selective,  for 
the  belt  between  altered  and  unaltered  rock  comprises  (1)  a  zone 
in  which  nearly  all  the  minerals  are  hydrothermally  altered, 
(2)  a  transition  zone  in  which  only  certain  easily  altered  minerals 
are  changed,  and  (3)  a  zone  composed  of  relatively  unaltered 
rock.  This  arrangement  is  quite  different  from  that  of  pyrite 
altering  to  limonite  around  the  borders  and  in  cracks  of  pyrite 
nodules,  for  in  hydrothermal  alteration  the  new  mineral  is  de- 
veloped at  many  points  in  the  older  mineral.  The  hot  solutions 
appear  to  be  capable  of  penetrating  the  rocks  along  the  contacts 
and  cleavage  planes  of  minerals.  One  mineral  may  be  replaced 
completely,  while  surrounding  minerals  may  be  only  partly 
changed.  The  changes  are  such  that  the  border  of  the  older 
mineral  often  is  easily  recognized,  even  where  replacement  is 
complete.  The  chemical  changes  involved  are  solution  of  the 
older  mineral  and  precipitation  of  the  new  one,  one  process 
following  the  other  so  closely  that  the  texture  of  the  old  substance 
is  preserved.  When  the  changes  begin  the  fresh  rock  may  con- 
tain very  little  pore  space,  but  as  alteration  proceeds  the  pore 
space  is  increased  and  solutions  can  therefore  enter  more  and 
more  readily  the  rock  that  is  being  altered. 

A  specimen  of  fresh  granite  after  being  dipped  in  red  dye  may 
be  washed  almost  clean.  When  broken  open  it  is  seen  that  the 
dye  has  scarcely  penetrated  the  rock.  Even  after  boiling  the 
color  has  not  entered  far.  Hydrothermally  altered  rocks,  on  the 
other  hand,  are  generally  very  porous.  When  a  specimen  of  such 
a  rock  is  dipped  in  dye  it  instantly  becomes  colored  and  the  dye 
penetrates  deeply. 

In  a  rock  undergoing  hydrothermal  alteration  solution  and 
reprecipitation  take  place  in  very  minute,  mainly  submicroscopic 
spaces.  But  the  solutions  thoroughly  permeate  the  rock  and 
attack  it  at  a  great  number  of  points,  so  that  the  alteration  is 
more  nearly  uniform  than  in  the  nodular  oxidation  above  men- 


222       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tioned.  In  neither  process,  however,  are  solution  cavities  de- 
veloped that  are  large  enough  to  permit  slumping  or  the  breaking 
down  of  the  original  bodies.  In  metasomatic  or  pseudomorphic 
alteration,  even  when  the  original  material  is  completely  re- 
moved and  new  material  substituted  for  it,  the  change  is  ac- 
complished little  by  little  and  the  original  texture  remains. 
That  is,  reprecipitation  follows  solution  so  closely  that  the  rock, 
although  porous,  is  strong  enough  to  prevent  slumping.  If  the 
cavities  are  large  enough  to  weaken  the  rock  structure  suffi- 
ciently, collapse  of  the  older  rocky  material  will  result. 

Where  rocks  are  altered  by  contact-metamorphic  processes 
an  altered  belt  of  the  intruded  rock  may  surround  the  intruding 
body.  Generally  this  belt  is  not  continuous  but  is  developed 
only  where  solutions  from  the  intruding  rock  were  more  active, 
owing  to  the  character  of  the  material  intruded.  Limestones 
and  calcareous  shales  are  most  readily  altered,  but  quartzites 
and  other  rocks  are  affected  also.  The  zones  of  metamorphic 
alteration  are  not  confined  to  rocks  along  fissures,  and  generally 
there  is  no  evidence  of  extensive  fissuring.  The  solutions,  which 
were  probably  at  high  temperature  and  under  high  pressure, 
seem  to  have  entered  minute  openings  such  as  joint  cracks  and 
probably  intergranular  spaces  and  cleavage  cracks.  They  ap- 
pear to  have  soaked  into  the  rock  rather  than  to  have  circulated 
along  master  fractures,  as  when  veins  are  formed.  Close  study 
of  contact-metamorphosed  rocks  in  thin  section  rarely  shows 
zonal  arrangement  like  that  observed  where  sulphides  change  to 
sulphates  or  to  oxides — that  is,  the  original  minerals  are  not  sur- 
rounded by  rims  of  the  new  minerals,  as  is  common  in  weathering. 
A  metamorphosed  limestone  may  contain  much  of  its  original 
calcium  carbonate  as  calcite  or  original  silica  as  quartz,  inter- 
grown  intimately  and  irregularly  with  the  substances  introduced 
by  solutions,  such  as  pyrite  and  magnetite.  The  entire  mass  has 
crystallized  together — new  substances  and  old.  An  impure  lime- 
stone containing  clay  bands  may  show,  on  being  altered,  a  segre- 
gation of  new  aluminum  minerals,  such  as  andalusite  and  augite, 
along  the  former  clay  bands,  indicating  that  much  aluminum 
remained  there,  but  if  the  rock  is  thoroughly  permeated  by  the  solu- 
tions and  recrystallized  there  are  rarely  nodules  of  clay  substance 
surrounded  by  zones  of  andalusite  or  other  aluminous  silicates. 
Although  the  replaced  rock  appears  to  have  been  thoroughly 
permeated  by  the  solutions,  it  appears  not  to  have  broken  down 


METASOMATIC  PROCESSES  223 

into  a  pasty  mass,  for  there  is  rarely  a  mingling  of  the  intruded 
and  intruding  material.  The  changes  probably  go  on  by  solution 
and  reprecipitation,  as  in  the  simple  process  of  weathering. 
Contact-metamorphic  ore  is  not  so  highly  porous  as  hydrother- 
mally  altered  granite.  When  soaked  in  colored  water  it  does  not 
become  stained  so  readily  except  along  calcite  cleavage  planes. 
The  massive  garnet  and  other  heavy  silicates  commonly  formed 
during  contact  metamorphism  are  relatively  impervious  except 
where  they  are  shattered  as  by  the  blow  of  a  hammer.  The 
solutions  evidently  enter  between  grains  and  along  the  cleavage 
planes,  especially  along  those  of  calcite. 

CRITERIA  FOR  RECOGNITION  OF  REPLACEMENT  DEPOSITS 

General  Features. — A  replacement  deposit1  is  one  formed  by 
metasomatism.  Replacement  veins  are  veins  along  which  the 
wall  rocks  are  replaced  by  vein  matter  (Fig.  113).  Where  veins 


FIG.  113.— Vein  formed  by  replace-         FIG.     114.— Vein    filling    a  fissure, 

ment,  showing  inclosed  fragments  whose  showing     inclosed     fragments  whose 

structure  is  oriented  like  similar  struc-  structure    is    oriented    unlike  similar 

ture  in  wall  rock.  structure  in  wall  rock. 

1  CURTIS,  J.  S.:  Silver-lead  Deposits  of  Eureka,  Nevada.  U.  S.  Geol.  Sur- 
vey A/on.  7,  pp.  93-106,  1884. 

EMMONS,  S.  R:  The  Genesis  of  Certain  Ore  Deposits.  Am.  Inst.  Min. 
Eng.  Trans.,  'vol.  15,  pp.  125-147,  1887.  Structural  Relations  of  Ore  De- 
posits. Idem,  vol.  16,  pp.  804-839,  1888. 

LINDGREN,  WALDEMAR:  Metasomatic  Processes  in  Fissure  Veins.  Am. 
Inst.  Min.  Eng.  Trans.,  vol.  30,  pp.  578-692,  1900.  The  Nature  of  Replace- 
ment. Econ.  Geol,  vol.  7,  pp.  521-535,  1912. 

IRVING,  J.  D.:  The  Formation  of  Ore  Bodies  by  Replacement  and  the 
Criteria  by  Means  of  Which  They  May  Be  Recognized.  Canadian  Min. 
Inst.  Quart.  BuU.  17,  pp.  3-79,  1911;  also  in  BAIN,  H.  F.,  and  others: 
"Types  of  Ore  Deposits,"  pp.  220-298,  San  Francisco,  1911. 


224      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


are  formed  by  hot  waters,  the  country  rock  along  the  vein  fissures 
is  generally  altered,  but  it  is  not  always  replaced  by  ore.  Com- 
monly the  wall  rock  appears  to  have  behaved  as  a  semipermeable 
septum,  for  the  material  added  to  the  wall  rock  and  that  pre- 
cipitated in  the  vein  are  chemically  different.  Some  of  the  sub- 
stances in  the  solutions  penetrate  the  wall  rock  and  some  remain 
in  the  open  spaces.  The  metalliferous  solutions  may  not  pene- 
trate the  wall  rock  in  noteworthy  amounts  or  convert  it  to  ore ; 
the  wall  rock  may  be  changed  so  little  that  its  contacts  with  the 
fissure  filling  are  sharp. 

Pseudomorphs. — A  pseudomorph  is  a  mineral  or  group  of 
minerals  having  the  crystal  form  of  another  mineral  of  different 
composition.  Pseudomorphs  are  developed  by  metasomatism 

and  are  regarded  as  proof  of 
metasomatic  alteration.  The 
term  "pseudomorph"  has  been 
expanded  to  include  forms  other 
than  those  of  minerals.  Re- 
mains of  plants  or  animals  or 
any  bodies  of  characteristic  shape 
become  pseudomorphs  after 
alteration  of  the  material  if  the 
original  shape  is  preserved.  In 
many  replacement  deposits  the 
larger  crystals  of  the  original  rock 
are  replaced  by  many  smaller 
crystals,  and  the  new  crystals  are 
grouped  within  the  outlines  of  the  older  crystals  so  that  the  out- 
lines are  clearly  shown.  The  new  minerals  may  be  developed  in 
different  proportions  in  the  spaces  formerly  occupied  by  the  older 
minerals,  so  that  the  details  of  the  texture  of  the  older  rock  may 
be  seen  in  the  new  rock.  The  larger  features  of  an  older  rock, 
such  as  jointing  or  sheeting,  may  be  preserved  in  the  rock  that 
replaces  it.  In  replaced  sedimentary  rocks  bedding  and  jointing 
may  be  preserved.  In  some  bodies  of  vein  matter  replacement 
has  been  complete,  even  the  last  evidence  of  the  original  rock 
texture  being  destroyed.  In  many  places,  however,  there  is 
between  the  massive  vein  material  and  the  country  rock  a 
less  intensely  altered  zone  in  which  the  texture  of  the  original 
rock  is  preserved.  Pseudomorphism  (Fig.  115),  or  the  preserva- 
tion of  ancient  structure,  is  in  itself  evidence  that  the  volumes  of 


FIG.  113. — Fossil  shell  replaced  by 
native  silver.  (After  Spurr,  U.  S. 
Geol.  Survey.) 


METASOMATIC  PROCESSES  225 

old  material  are  essentially  unchanged .  Much  shrinkage  or  expan- 
sion would  alter  or  destroy  the  structure  or  render  it  indistinct. 

Banding  and  Crustification. — Some  replacement  deposits  show 
banding,  especially  where  the  replaced  rocks  were  banded  shales 
or  impure  limestones.  Banding1  is  not  unknown  also  in  normal 
limestone  that  is  replaced,  although  it  is  less  common.  Homo- 
geneous igneous  rocks  that  are  hydrothermally  altered  also  may 
show  banding.  Deposits  formed  in  open  spaces  are  much  more 
commonly  banded  than  deposits  that  have  replaced  homogene- 
ous rock.  Banding  is  developed  also  during  weathering  (Fig. 
116).  Symmetrical  crustified  banding  is  not  developed  by 
replacement. 

Cavities. — Large  cavities  are  rarely  found  in  replacement  de- 
posits, except  in  the  superficial  zone.  Veins  that  fill  fissures 

IMONITE  AND  REUtYSTALUZED  CALCITE. 
.-MALACHITE  AND   CHRYSOCOLLA 

*      -C/UC/7F  WITH  SOME  PYRITC, 

CHALCOPYRITE  AND   QUARTZ 


FIG.  116. — Section  of  partly  oxidized  ore,  Apache  No.  2  district,  New  Mexico. 
(Redrawn  from  colored  plate  by  Lindgren,  Graton  and  Gordon,  U.  S.  Geol.  Survey.) 

very  commonly  contain  many  open  spaces.  Thus  a  crustified 
vein  generally  contains  many  elongated  vugs,  and  these  tend 
to  be  oriented  in  lines  or  with  their  longer  axes  approximately 
parallel  to  the  plane  of  the  vein.  Such  an  arrangement  of  vugs 
is  rare  in  deposits  that  replace  the  country  rock. 

Crystal  Boundaries. — Minerals  that  fill  fissures  may  form 
imperfect  crystals,  the  ends  that  are  attached  to  the  walls  being 
poorly  developed.2  Crystals  of  certain  minerals  that  have  been 
deposited  by  replacement  of  the  wall  rock  may  have  sharp  bound- 
aries, all  sides  being  completely  shown  (Fig.  117).  Some  species 

1  LINDGREN,  WALDEMAR:  Processes  of  Mineralization  and  Enrichment  in 
the  Tintic  Mining  District.     Econ.  Geol,  vol.  10,  p.  231,  1915. 

LIESEGANG,  R.:  "Geologische  Diffusionen,"  p.  83,  1913. 

2  IRVING,  J.  D. :  Some  Features  of  Replacement  Ore  Bodies  and  Criteria 
by  Which  They  May  be  Recognized.    Econ.  Geol,  vol.  6,  pp.  527-561, 1911. 

15 


226      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

are  commonly  developed  in  great  perfection.  In  the  following 
list  of.  minerals1  that  are  commonly  formed  by  replacement  those 
named  first  are  more  likely  to  be  developed  with  good  crystal 
outlines:  Rutile,  tourmaline,  arsenopyrite,  pyrite,  magnetite, 
barite,  fluorite,  epidote,  pyroxene,  hornblende,  siderite,  dolo- 
mite, albite,  mica,  galena,  sphalerite,  calcite,  quartz,  orthoclase. 
Some  other  minerals,  like  chalcopyrite,  sericite  and  some  chlorites, 
very  rarely  show  crystal  outlines,,  even  under  the  microscope. 

Boundaries  of  Deposits. — Replacement  deposits  are  more 
irregular  in  outline  than  veins  filling  fissures  (Figs.  113,  114). 
Veins  may  narrow  to  thin  sheets  or  swell  to  broad  masses,  but 


FIG.  117. — Garnet  including  and  intergrown  with  cupriferous  pyrite  in  calcite; 
in  limestone  subjected  to  contact  metamorphism.  From  1,600-foot  level  of 
Ontario  mine,  Park  City,  Utah.  (After  Boutwell,  U.  S.  Geol.  Survey.) 

normally  the  changes  are  gradual.  A  replacement  deposit,  on 
the  other  hand,  may  become  very  thin  at  one  place  and  swell 
abruptly  at  another,  so  that  the  entire  deposit  may  be  a  chain  of 
large  ore  bodies  connected  by  thin  seams  of  ore,  rather  than  a 
tabular  and  nearly  uniform  mass.  Some  replacement  deposits, 
however,  are  fairly  uniform  in  width. 

Contacts. — Replacement  deposits  generally  grade  into  the 
country  rock.  In  some  deposits,  especially  those  of  copper,  gold, 
or  silver  in  igneous  rocks,  the  change  from  ore  to  country  rock 
is  so  gradual  that  the  boundary  of  the  ore  body  can  be  determined 
only  by  assays.  On  the  other  hand,  in  some  replacement  deposits 
,  WALDEMAB:  "Mineral  Deposits,"  p.  158,  1913. 


METASOMATIC  PROCESSES  227 

the  ore  may  grade  into  country  rock  through  a  zone  less  than  1 
inch  wide,  and  in  limestone  the  contact  between  mineralized  and 
unmineralized  rock  may  be  abrupt. 

Fragments. — Small  irregular  fragments  included  in  fissure 
fillings  ordinarily  have  sharp  outlines.  They  are  generally 
altered  somewhat  by  the  vein-forming  solutions,  but  as  a  rule 
their  original  shape  is  still  clearly  shown.  On  the  other  hand, 
fragments  included  in  replacement  deposits  may  be  rounded 
somewhat  by  solution,  especially  on  their  edges  that  are  most 
exposed.  This  criterion,  however, '  should  be  applied  with 
caution,  for  in  the  course  of  deposition  of  a  vein,  solutions  fre- 
quently change,  and  many  veins  during  the  process  of  deposi- 
tion are  opened  again  and  again.  At  one  period  of  deposition 
the  solutions  may  be  actively  replacing  the  wall  rock,  whereas 
at  another  period  their  action  may  be  more  feeble.  Thus,  in  a 
deposit  formed  principally  by  replacement,  sharp,  angular  frag- 
ments of  wall  rock  may  still  remain  if  they  were  broken  from  the 
walls  after  the  solutions  that  were  most  active  in  replacing  the 
wall  rock  ceased  to  be  effective,  and  before  the  solutions  that  filled 
openings  had  ceased  to  deposit  ore.  In  a  small  deposit  in  Elko 
County,  Nevada,  unaltered  angular  fragments  of  limestone  are 
inclosed  in  a  large  body  of  quartz  that  had  been  deposited  prin- 
cipally by  the  replacement  of  the  limestone.  In  some  replace- 
ment veins  larger  fragments  that  were  probably  surrounded  by 
minute  fragments  still  retain  their  shape,  but  the  minute  frag- 
ments appear  to  have  been  almost  obliterated.  Veins  in  sili- 
ceous sedimentary  rocks  and  in  igneous  rocks  more  commonly 
contain  fragments  of  the  wall  rock  with  sharp  boundaries  than 
veins  in  limestone. 

Orientation  of  Fragments. — Near  the  border  of  a  replacement 
deposit  the  rock  that  has  been  replaced  may  have  distinctive 
features  of  structure  such  as  bedding  planes  or  parallel  planes  of 
schistosity.  Fragments  included  in  the  replacement  deposit  may 
have  similar  features.  If  these  are  similarly  oriented  and  oriented 
like  the  corresponding  features  in  the  country  rock,  it  is  a  natural 
inference  that  the  material  surrounding  them  was  formed  by 
replacement  (see  Fig.  113),  for  some  of  them  would  probably  have 
been  rotated  had  they  been  broken  from  the  parent  mass  by 
movement  (see  Fig.  114). 

Variations  in  Width  Depending  on  Country  Rock. — Some 
replacement  deposits  lie  with  the  beds  (Fig.  118),  others  cut  across 


228       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

them  (Fig.  119).  Where  a  replacement  vein  in  sedimentary 
rock  cuts  across  several  beds  of  different  composition,  there  is 
generally  selective  replacement.  Siliceous  or  shaly  beds  may 
be  only  slightly  changed,  whereas  the  calcareous  members  may 


FIG.  118. — Section  across  Lake  Valley  district,  New  Mexico,  a,  Rhyolite; 
b,  andesite;  c,  Lake  Valley  limestone;  d,  e,  Percha  shale;/,  Mimbres  limestone; 
g,  ore  horizon.  (After  Endlich.) 

be  extensively  replaced.  The  deposit  may  be  wider  in  shale  than 
in  quartzite,  and  still  wider  in  limestone  than  in  shale.  In  the 
Philipsburg  quadrangle,  Montana,  the  same  system  of  fissures 
cuts  granite,  limestone,  and  shale.  The  deposits  in  granite  are 


FIQ.  119. — Section  through  Tenabo  Peak,  Nevada,  showing  irregular  deposit 
replacing  limestone  at  Garrison  mine. 

fissure  fillings;  those  in  limestone  and  shales  are  replacement  veins. 
In  that  district  some  veins  are  very  much  wider  in  limestones  than 
in  shales.  In  the  Black  Hills,  South  Dakota,  quartz  veins  cross 
thin  beds  of  dolomite  that  alternate  with  shale.  Where  the  fis- 
sures cross  the  more  soluble  dolomite  strata  the  dolomite  may  be 


ME  TA  SO  MA  TIC  PROCESSES 


229 


replaced  so  extensively  that  the  bedding-plane  deposits  become 
much  larger  than  the  deposits  filling  the  fissures  (see  Fig.  120). 
Thus  some  veins  may  grade  into  bedding-plane  or  irregular  re- 
placement deposits,  and  in  limestone  such  gradations  are  very 
common. 

Residual  Minerals. — Some  rocks  that  are  replaced  contain 
small  amounts  of  minerals  that  resist  replacement  processes. 


SCALE  r-iio 


FIG.  120. — Cross-section  of  ore  shoot  in  alternating  layers  of  shale  and  dolomite, 
Portland,  South  Dakota.  In  the  dolomite  the  ore  runs  out  to  greater  distances 
than  in  the  shale.  (After  Irving.) 

Under  some  conditions  apatite  and  zircon  are  but  little  altered 
by  solutions  that  replace  rocks  containing  them.  If  approxi- 
mately equal  quantities  of  these  resistant  substances  are  found 
in  ore  and  country  rock,  the  inference  is  warranted  that  the  deposit 
has  replaced  the  wall  rock. 


Scale  of  Feet 


CHAPTER  XIX 

MINERAL  ASSOCIATIONS  IN  VEINS 
AND  WALL-ROCK  ALTERATION 

Basis  of  Classification. — It  is  not  every- 
where possible  to  distinguish  vein  matter 
which  has  filled  open  spaces  from  that  which 
has  replaced  the  wall  rock,  and  it  is  impractica- 
ble to  discuss  one  without  the  other.  By  com- 
paring the  unaltered  country  rock  with  the 
altered  rock  along  the  veins  to  ascertain  the 
nature  of  the  changes,  it  is  possible  to  esti- 
mate the  material  added  and  that  removed  by 
the  vein-forming  solutions.  Knowledge  of  the 
results  of  the  activities  of  vein-forming  solu- 
tions affords  some  basis  for  inference  concern- 
ing their  composition  and  character.  Igneous 
rocks  in  general  are  more  nearly  uniform  in  com- 
position than  sedimentary  rocks,  and  slight 
changes  in  them  are  more  easily  followed. 
Detailed  investigations  of  these  changes  have 
been  made  in  many  districts,  and  certain  types 
of  alteration  have  been  found  to  be  common. 
The  method  followed  is  to  compare  the  chem- 
ical analysis  of  the  unaltered  rock  with  that 
of  the  altered  rock  and  to  compare  the  mineral 
analyses  obtained  by  study  of  corresponding 
thin  sections  of  the  rocks.  Valuable  data  may 
be  obtained  by  estimating  the  mineral  composi- 
tion from  the  chemical  analyses. 

As  igneous  rocks  may  be  grouped  in  various 
classes,  according  to  their  chemical  and  mineral 
composition — granites,  diorites,  gabbros,  etc. 
— so  veins  may  be  classified  according  to  their 
mineral  composition  and  the  nature  of  the 
alteration  of  their  wall  rocks.  Veins  are  not 
so  nearly  uniform  in  composition  as  many 

FIG.  121. — Plan  of  part  of  Butte  district,   Montana, 
showing  Butte  granite  hydrothermally  altered  near  veins, 
altered  areas  are  cross  lined.     (After  Sales.) 
230 


MINERAL  ASSOCIATIONS  IN  VEINS 


231 


masses  of  igneous  rocks,  and  the  classes  are  not  so  sharply 
defined;  nevertheless,  for  purposes  of  study  and  comparison  a 
grouping  according  to  mineral  composition  is  useful.  Some 
mineral  associations  in  ore  deposits  are  fairly  common,  especially 
in  deposits  formed  within  a  single  metallogenic  epoch.  Earlier 
investigators  laid  considerable  stress  upon  mineral  associations 
such  as  those  of  the  "baritic  lead  veins,"  the  "fluoritic  gold- 
quartz  veins,"  "pyritic  gold-quartz  veins."  Subsequent  de- 
velopments and  investigations  have  shown  that  these  associations 
are  not  so  nearly  standard  as  they  were  once  supposed  to  be. 
There  are  nearly  200  common  ore  and  gangue  minerals,  and  many 
types  of  ores  result  from  combinations  of  them.  Within  a 
single  district  indeed,  in  a  single  deposit,  even  the  unaltered 
ore  may  show  great  variations  in  mineral  composition.  There 
are,  nevertheless,  certain  combinations  that  are  so  common  as 
to  merit  special  consideration.  The  types  of  wall-rock  alteration 
are  more  nearly  standard  and  may  be  reduced  to  fewer  groups. 
As  the  mineral  composition  depends  on  conditions  usually  re- 
lated to  the  depth  of  formation1  the  classification  here  adopted 


DEPOSITS  FORMED  AT  ORIFICES 
OF  HOT  SPRINGS 


DEPOSITS  OF  SHALLOW  ZONE 


DEPOSITS  OF  ZONE  OF  MODERATE 
DEPTH 


DEPOSITS  OF  DEEP  ZONE 


Tufa. 

Sinter. 

Travertine. 

Baritic  fluorite  veins. 

Zeolitic  native  copper  veins. 

Chalcedonic  cinnabar  veins. 

Alunitic  kaolinic  gold  veins. 

Fluorite-tellurium-adularia  veins. 

Gold-silver-adularia  veins. 

Propylitic  veins. 

Sericitic  silver-gold  veins. 

Sericitic  copper  veins  and  disseminated 
sericitic  copper  ores. 

Sericitic  copper-silver  veins. 

Sericitic  zinc-silver  veins. 

Sideritic  lead-silver  veins. 

Sericitic  calcitic  gold  veins. 

Tourmaline-gold  veins. 

Tourmaline-copper  veins. 

Cassiterite  veins. 

Garnetiferous  lead-silver  veins. 

Garnetiferous  silver-copper  veins. 

Garnetiferous  gold  veins. 
1  LINDGREN,  WALDEMAR:  The  Relation  of  Ore  Deposition  to  Physical 
Conditions.     Econ.  Geol,  vol.  2,  pp.  105-127,  1907. 

EMMONS,  W.  H. :  A  Genetic  Classification  of  Minerals.     Econ.  Geol.,  vol. 
3,  pp.  611-627,  1908. 


232      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

may  be  regarded  as  a  refinement  of  the  classification  of  primary 
veins  presented  on  pages  49-73.  Typical  examples  are  cited 
below,  and  a  summary  of  their  general  occurrence  is  given  in  the 
foregoing  list,  in  which  the  arrangement  indicating  depth  applies 
only  to  the  four  main  groups,  not  to  the  subgroups. 

Garnetiferous  Gold  Veins.  —  Precious-metal  deposits  with  a 
garnet  gangue  are  not  common,  but  several  examples  are  known. 
At  Dahlonega,  Ga.,1  in  a  region  of  mica  schists  and  amphibolites, 
garnetiferous  gold  lodes  are  numerous  and  extensive.  Near 
the  gold  deposits,  which  occur  as  veins  and  irregular  lenses,  the 
schists  are  intruded  by  granite.  In  some  of  the  deposits  the  ore 
is  banded  and  consists  of  garnet,  quartz,  dark  mica,  and  amphi- 
bole,  the  sulphides  including  pyrite,  pyrrhotite,  chalcopyrite, 
and  galena  (see  Fig.  22).  Fluid  inclusions  containing  gas 
bubbles  and  a  solid  are  abundant.  The  deposits,  according 
to  Lindgren,  were  probably  formed  15,000  or  20,000  feet  below 
the  surface  and  at  high  temperatures  and  pressures.  Garnet 
is  a  gangue  mineral  in  gold  quartz  veins  of  Pinetuckey,  Ala.2 

Garnetiferous  Silver-copper  Veins.  —  Silver  lodes  containing 
garnet  are  rare  but  not  unknown.  Such  deposits  are  found  in 
Rossland,  British  Columbia,3  in  the  Trail  Creek  district,  a  short 
distance  north  of  the  international  boundary.  The  rocks  ex- 
posed include  Carboniferous  limestone,  quartzites,  and  shales 
with  interbedded  tuffs,  ash  beds,  and  lavas.  Above  this  series 
are  volcanic  agglomerates  and  lavas.  These  rocks  are  in- 
truded by  masses  of  monzonite,  granodiorite,  nepheline  syenite, 
etc. 

The  principal  deposits  are  fissure  fillings  and  replacement 
veins,  deposits  in  fractured  zones,  and  disseminated  deposits. 
The  most  important  lodes  have  steep  dips.  The  deposits  carry 
commercial  amounts  of  copper  and  silver.  The  gangue  minerals 
are  biotite,  quartz,  calcite,  tourmaline,  amphibole,  chlorite,  and 
garnet;  the  sulphides  include  pyrrhotite,  chalcopyrite,  pyrite, 
arsenopyrite,  marcasite,  and  other  minerals. 


WALDEMAR:  The   Gold   Deposits  of   Dahlonega,   Georgia. 
U.  S.  Geol.  Survey  Bull.  293,  p.  119,  1906. 

2  MCCASKEY,  H.  D.  :  Notes  on  Some  Gold  Deposits  of  Alabama.     U.  S. 
Geol.  Survey  Bull.  340,  p.  46,  1908. 

3  DRYSDALE,   C.   W.  :  Geology  and  Ore  Deposits  of  Rossland,   British 
Columbia.     Canada  Geol.  Survey  Mem.  77,  1915. 


MINERAL  ASSOCIATIONS  IN  VEINS         233 

Garnetiferous  Lead-silver  Veins. — At  the  St.  Eugene  mine, 
Moyie,  British  Columbia,1  one  of  the  greatest  lead  mines  of 
Canada,  the  deposits  are  lodes  of  the  deep  vein  zone.  The  ore 
bodies  are  associated  with  the  massive  purer  quartzites  of  the 
Purcell  series.  The  ore  consists  of  galena,  both  fine  and  coarse 
grained,  associated  in  places  with  zinc  blende,  pyrrhotite,  and 
magnetite.  The  gangue,  which  is  in  small  amount,  consists  of 
garnet,  actinolite,  and  a  little  quartz.  Locally  the  wall  rock  in 
the  immediate  vicinity  of  the  ore  bodies  shows  strong  metamor- 
phism  and  development  of  garnet  and  anthophyllite. 

Cassiterite  Veins. — Nearly  all  tin  veins  are  associated  with 
granitic  intrusive  rocks.  They  are  found  in  granite  or  near 
granite  contacts  in  rocks  which  the  granite  intrudes.  Almost 
everywhere  in  the  veins  some  or  all  of  the  following  minerals  are 
associated  with  cassiterite:  Topaz,  tourmaline,  muscovite, 
fluorite,  wolframite,  scheelite,  lepidolite,  zinnwaldite,  arseno- 
pyrite,  and  apatite.  These  minerals  are  deposited  in  openings 
and  replace  the  country  rock.2 

The  largest  deposits  of  lode  tin  are  those  of  the  Cornwall 
peninsula,  England  (Fig.  122),  an  area  of  Paleozoic  sedimentary 
rocks  and  lava  flows,  with  subordinate  crystalline  schists,  in- 
truded in  post-Carboniferous  time  by  large  masses  of  granite, 
some  of  which  have  outcrops  over  10  miles  in  diameter.  The 
tin  and  copper  lodes  are  in  or  near  the  granite,  and  some  of  them 
are  associated  with  dikes  of  quartz  porphyry.  Some  of  the  lodes 
are  mere  cracks  not  more  than  an  inch  wide,  but  others  are  of 
great  width  and  contain  much  crushed  or  brecciated  material. 
Fissures  and  sheeted  zones  are  filled  with  ore,  which  also  replaces 
the  walls  and  included  fragments. 

The  most  valuable  deposits  are  those  in  the  brecciated  zones, 
some  of  which  are  extensive.  The  Dolcoath  lode  is  3^  miles 
long  and  has  been  worked  through  a  vertical  range  of  over  3,000 
feet.  The  lode  ores  include  cassiterite,  stannite,  chalcopyrite, 
arsenopyrite,  wolframite,  scheelite,  and  compounds  of  cobalt, 
nickel,  and  bismuth.  The  gangue  minerals  include  quartz,  tour- 

1  SCHOFIELD,  S.  J. :  Reconnaissance  in  East  Kootenay.     Canada  Geol. 
Survey  Summ.  Rept.  for  1911,  pp.  158-164,  1912.     The  Origin  of  the  Lead- 
Silver  Deposits  of  the  East  Kootenay,  British  Columbia.     Econ.  Geol,  vol. 
7,  pp.  351-362,  1912. 

2  FERGUSON,  H.   G.,  and  BATEMAN,  A.  M.:  Geological  Features  of  Tin 
Deposits,  Econ.  Geol.  Vol.  7,  p.  209,  1912. 


234      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


MINERAL  ASSOCIATIONS  IN  VEINS          235 

maline,  topaz,  fluorspar,  and  chlorite.  The  secondary  minerals 
include  cuprite,  melaconite,  copper,  malachite,  chrysocolla, 
etc. 

Near  the  veins,  according  to  MacAlister,1  the  granite  has  under- 
gone the  following  changes :  Feldspar  is  converted  into  lepidolite 
(gilbertite),  hydro-muscovite,  tourmaline,  topaz,  fluorite,  chalco- 
pyrite,  and  cassiterite.  The  biotite  of  the  granite  is  bleached 
or  hydrated  or  converted  into  brown  tourmaline.  The  original 
quartz  of  the  granite  is  not  greatly  changed,  but  some  is  corroded. 
The  dissolved  silica,  with  that  resulting  from  the  altered  feldspar, 
is  deposited  elsewhere  as  quartz.  The  altered  granite  is  termed 
"greisen." 

Near  the  contact  the  sedimentary  rocks  are  locally  impreg- 
nated with  ore,  and  the  hydrothermal  alterations  related  to  the 
veins  may  be  superimposed  upon  contact-metamorphic  altera- 
tions. In  the  argillaceous  bands  tourmaline  is  extensively  de- 
veloped, with  andalusite,  cordierite,  and  other  minerals;  in  the 
calcareous  bands  and  in  greenstones  axinite,  pyroxene,  epidote, 
and  garnet  -are  developed  and  tourmaline  is  subordinate. 

The  solutions  contained  fluorine,  boron,  sulphur,  silica,  lithia, 
water,  and  perhaps  chlorine  and  carbon  dioxide.  The  probability 
that  they  withdrew  from  the  magma  as  it  crystallized  was  sug- 
gested by  Daubree  and  filie  de  Beaumont,  and  this  view  is  sup- 
ported by  the  recent  work  of  MacAlister2  for  the  Geological  Survey 
of  Great  Britain. 

At  Altenberg,  Saxony,3  a  small  granite  stock  of  post-Permian 
age  cuts  through  a  granite  porphyry.  The  granite  stock  is 
intersected  by  numerous  small  tin-bearing  veins,  and  the  country 
rock  is  impregnated  with  cassiterite  and  other  minerals.  Quartz, 
muscovite,  lepidolite,  tourmaline,  and  topaz  are  formed  in  large 
quantities  and  here  and  there  completely  replace  the  feldspars 
and  other  minerals  of  the  granite.  The  altered  rock  (greisen) 
is  locally  called  "zwitter." 

Tin  deposits  are  rare  in  the  United  States.  The  tin  bearing 
pegmatites  of  the  Carolinas  and  the  deposit  at  Etta  Knob,  in 
the  Black  Hills,  are  mentioned  on  pages  522-524.  Some  tin 

1  MACALISTER,   D.   A. :  Geological  Aspects  of  the  Lodes  of  Cornwall. 
Econ.  Geol,  vol.  3,  p.  374,  1908. 

2  MACALISTER,  D.  A. :  Op.  cit.,  p.  377. 

3  DALMER,  KARL:  Der  Altenberg-Graupener  Zinnerziagerstattendistrict, 
Zeitschr.  prakt.  Geologic,  pp.  313-322,  1894. 


236       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

has  been  found  near  El  Paso,  Texas, l  in  a  post-Paleozoic  granite 
that  is  cut  by  cassiterite  veins  carrying  quartz  and  wolframite 
with  greisen-like  alterations. 

Fluoritic  tin  veins,  with  tourmaline  and  topaz,  inclosed  in 
intruding  granite,  are  found  on  Seward  Peninsula,  Alaska.2 

Tourmaline  Veins. — Tourmaline  is  developed  as  a  gangue 
mineral  in  a  considerable  number  of  ore  veins.  The  cassiterite 
ores  in  which  it  is  commonly  present  are  mentioned  above.  At 
the  Cactus  mine,  in  the  San  Francisco  region,  Utah,3  copper  ores 
are  composed  of  tourmaline,  anhydrite,  and  chalcopyrite.  On 
Cable  Mountain,  near  the  head  of  Flint  Creek,  in  the  Philips- 
burg  quadrangle,  Montana,4  veinlets  of  tourmaline,  quartz, 
and  native  gold,  with  a  little  pyrite,  occupy  joint  fissures  in 
quartzite.  The  filled  portions  of  these  veinlets  are  not  more  than 
a  millimeter  thick,  but  the  surfaces  of  some  of  the  quartzite 
blocks  are  liberally  plastered  with  free  gold.  Near  the  veinlets 
tourmaline  is  developed  in  the  quartzite  through  replacement, 
but  the  visible  gold  is  limited  to  the  small  spaces. 

In  the  Meadow  Lake  district,  California,5  quartz-tourmaline- 
chalcopyrite  veins,  with  pyrite,  arsenopyrite,  pyrrhotite,  and 
zinc  blende,  cut  granitic  and  dioritic  rocks.  Tourmaline  accom- 
panies some  of  the  zeolitic  copper  ores  associated  with  trap 
in  New  Jersey.6  Many  copper  veins  of  Chile7  carry  tourmaline. 
In  these  chalcopyrite  and  pyrite  are  common  associates  of  tour- 
maline and  quartz. 

1  WEED,  W.  H. :  The  El  Paso  Tin  Deposits.     U.  S.  Geol.  Survey  Butt.  178, 
1901.     Tin  Deposits  at  El  Paso,  Tex.     U.  S.  Geol.  Survey  Bull.  213,  p.  99, 
1903. 

2  COLLIER,  A.  J. :  Tin  Deposits  of  the  York  Region,  Alaska.     U.  S.  Geol. 
Survey  Bull.  225,  pp.  154-167,  1904. 

KNOPF,  ADOLPH:  Geology  of  the  Seward  Peninsula  Tin  Deposits,  Alaska 
Geol.  Survey  Bull.  358,  1908. 

3  BUTLER,  B.  S.:  Geology  and  Ore  Deposits  of  the  San  Francisco  Region, 
Utah.    U.  S.  Geol.  Survey  Prof.  Paper  80,  p.  121,  1913. 

4  EMMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.     U.  S.  Geol.  Survey  Prof.  Paper  78,  p. 
241,  1913. 

5LiNDGREN,  WALDEMAR:  The  Auriferous  Veins  of  Meadow  Lake,  Cal. 
Am.  Jour.  Sci.,  3d  ser.,  vol.  46,  p.  201,  1893. 

6 LEWIS,  J.  V.:  Copper  deposits  of  the  New  Jersey  Triassic  Econ. 
Geology.  Vol.  2,  pp.  242-257,  1907. 

7  STELZNER,  A.  W. :  Ueber  das  Turmalinfuhrung  Kupfererzgange  von 
Chile.  Zeitschr.  prakt.  Geologic,  1897,  pp.  41-45. 


MINERAL  ASSOCIATIONS  IN  VEINS          237 

Sericitic  Calcitic  Gold  Veins. — The  principal  gold  veins  of 
California  are  in  or  near  granodiorite  bodies  that  are  intruded  in 
various  igneous,  metamorphic,  and  sedimentary  rocks.  The 
most  common  minerals  are  pyrite,  chalcopyrite,  arsenopyrite, 
galena,  and  zinc  blende,  with  quartz,  calcite,  dolomite,  and 
siderite.  More  rarely  molybdenite,  tellurides,  tetrahedrite, 
cinnabar,  and  pyrrhotite  are  developed  with  albite,  barite,  and 
fluorite.  The  sulphides  make  up  only  a  very  small  percentage 
of  the  vein  stuff.  Sericite  and  chlorite  are  common  in  the  re- 
placed wall  rock.  Adularia,  which  is  frequently  deposited  dur- 
ing mineralization  of  the  later  Tertiary  rocks,  is  probably  absent 
altogether,  and  the  manganese  minerals  rhodochrosite  and  rhodo- 
nite are  rare  if  not  unknown  in  the  gold  lodes.  In  the  granodio- 
rite near  these  deposits  the  thermal  waters  that  deposited  the 
ore  have  reduced  the  amounts  of  iron  oxides,  but  the  carbon 
dioxide  is  greatly  increased.  Much  of  the  iron  is  redeposited  as 
pyrite,  and  some  as  siderite.  Potash  is  increased,  and  soda  is 
removed.  Silica,  alumina,  and  some  soda  are  deposited  in  the 
veins  as  quartz  and  albite,  the  latter  in  minor  quantities. 

In  the  Ophir  district,  California,1  amphibolites  intruded  by 
granodiorite  are  cut  by  quartz  veins  that  fill  fissures  from  a  few 
inches  to  3  feet  wide.  The  ore  minerals  are  gold,  electrum  (a 
natural  gold-silver  alloy),  a  small  amount  of  iron,  copper,  and 
arsenical  pyrites,  with  galena,  zinc  blende,  tetrahedrite,  and 
molybdenite.  The  gangue  of  the  filled  spaces  is  mainly  quartz 
with  a  little  calcite.  The  amphibolite  schist  is  a  dark  grayish- 
green  schistose  rock  composed  of  hornblende,  feldspar,  chlorite, 
pyrite,  and  magnetite.  Its  analysis  is  given  in  the  following 
table,  together  with  that  of  the  highly  altered. amphibolite  schist. 
The  chemical  analyses  indicate  gains  of  lime,  potash,  and  carbon 
dioxide  and  losses  of  silica  and  soda.  In  places  the  wall  rocks 
carry  $2  to  $12  a  ton  in  gold  and  silver.  The  granodiorite  con- 
sists of  orthoclase,  plagioclase,  quartz,  biotite,  and  hornblende. 
The  composition  of  the  fresh  rock  and  its  altered  equivalent 
are  shown  in  the  table  below.  The  analyses  show  gains  and 
losses  similar  to  those  in  the  schist.  Silica  was  deposited,  how- 
ever, in  the  open  spaces  in  large  quantities.  Lindgren  attributes 
the  mineralization  to  hot  ascending  siliceous  and  carbonated 
solutions  containing  the  heavy  metals  with  alkaline  sulphides. 

1  LINDGREN,  WALDEMAK:  The  Gold-silver  Veins  of  Ophir,  Cal.  U.  S. 
Geol.  Survey  Fourteenth  Ann.  RepL,  part  2,  pp.  243-284,  1894. 


238      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


ANALYSES  OF  FRESH  AND  ALTERED  WALL  ROCK  AT  OPHIR,  CAL.J 
(W.  F.  Hillebrand,  Analyst) 


Amphibolite 

Granodiorite 

Fresh 
1 

Altered 
2 

Fresh 
3 

Altered 

SiO2  .  ... 

45.56 
1.11 
14.15 
1.20 
9.83 
7.86 
0.10 
Traces 
0.25 
2.30 
Traces  ? 
Traces  ? 
6.76 
1.18 
1.57 
Traces 
0.23 
4.84 
0.14 
0.03 
3.04 

37.01 
0.85 
12.99 
0.43 
3.57 
7.99 
(?) 
Traces 
0.24 
9.78 
Traces 
Traces 
5.49 
4.02 
0.13 
Traces 
0.13 
1.92 
0.06 
0.04 
15.04 

65.54 

0.39 
16.52 
1.40 
2.49 

46.13 
0.67 
15.82 
0.89 
2.27 
1.61 
(?) 
Traces 
0.09 
10.68 
Traces 
Traces 
2.13 
5.30 
0.17 
Traces 
0.12 
2.42 
0.10 
0.04 
11.24 

Ti02  
A1203  
Fe2O» 

FeO 

FeS2 

CuzS  
Ni,Zn  
MnO  
CaO  

0.06 
4.88 

SrO 

BaO 

MgO  . 

2.52 
1.95 
4.09 

K20  
Na2O  
Li20  
H2O  below  110°C.... 
H2O  above  110°C.... 
P2OS  

0.59 
0.18 

SO,  

CO,  

100.15 

99.69 

100.73             99.68 

1.  Conrad  claim;  fairly  fresh,  but  contains  calcite  and  pyrite. 

2.  Mina  Rica  vein;  typical  altered  wall  rock. 

3.  Lincoln. 

4.  Plantz  vein;  typical  altered  wall  rock. 

Nevada  City  and  Grass  Valley2  are  among  the  most  produc- 
tive gold  mining  districts  of  California.  The  area  includes 
Carboniferous  metamorphosed  sedimentary  rocks  (Calaveras) 
compressed  into  isoclines,  and  associated  igneous  rocks  which 
are  less  deformed.  Above  these  are  the  Mariposa  (Jura-Trias) 
slates,  with  associated  diabases  and  serpentines.  These  rocks 
also  are  folded  and  metamorphosed  but  were  not  nearly  so 

1  LINDGREN,  WALDEMAR:  The  Gold-silver  Veins  of  Ophir,  Cal.  U.  S.  Geol. 
Survey  Fourteenth  Ann.  Rept.,  part  2,  p.  249,  1894. 

2  LINDGREN,  WALDEMAR:  The  Gold-quartz  Veins  of  Nevada  City  and 
Grass  Valley  Districts,  California.     U.  S.  Geol.   Survey  Seventeenth  Ann. 
Rept.,  part  2,  p.  110,  1896. 


MINERAL  ASSOCIATIONS  IN  VEINS 


239 


intensely  compressed  as  the  rocks  of  the  Calaveras  formation. 
They  were  intruded,  presumably  in  early  Cretaceous  time,  by 
great  bodies  of  granodiorite.  Strong  fissure  veins  many  of  them 
making  conjugated  systems,  were  formed  after  the  solidification 
of  granodiorite.  The  ore  veins,  which  are  genetically  related  to 
the  granodiorite,  were  formed  before  the  Neocene  sedimentary 
formations  and  associated  lavas  were  deposited.  The  richer  ores 
carry  from  $10  to  $20  a  ton  in  gold,  and  are  of  higher  grade  than 
the  California  deposits  in  general.  The  ore  minerals  are  quartz, 
calcite,  chalcedony,  magnesite,  sericite,  mariposite  (chrome  mica), 
gold,  tellurides,  pyrite,  pyrrhotite  (subordinate),  chalcopyrite, 
galena,  zinc,  blende,  scheelite,  arsenopyrite,  tetrahedrite,  stepha- 
nite,  and  cinnabar.  The  metasomatic  alteration  of  the  granodio- 
rite is  similar  to  that  at  Ophir  described  above.  The  analyses 
of  fresh  and  altered  rock  are  given  in  the  subjoined  table.  The 

ANALYSES  OF  FRESH  AND  ALTERED  ROCK,  NEVADA  CITY-GRASS  VALLEY 
REGION,  CALIFORNIA 


Granodiorite 

Diabase 

Fresh 

1 

Altered 
2 

Fresh 
3 

Altered 
4 

SiO2 

66.65 
0.38 
16.15 
1.52 
2.36 
0.02 

56.25 
0.25 
17.65 
0.76 
2.64 
2.87 

51.01 
0.98 
11.89 
1.57 
6.08 
1.73s 
Trace 
Trace 
10.36 

8.87 
0.15 
4.17 
0.24 
2.09 
0.17 

45.74 

0.36 
5.29 
0.13 
2.06 
0.49 

0.26 
23.85 

0.94 
1.29 
0.11 
0.22 
1.07 
0.07 
18.91 

TiOa 

A12O3  

Fe203  
FeO  
FeS2  
Cu2S 

MnO...  
CaO  

0.10 
4.53 
0.07 
1.74 
2.65 
3.40 
0.18 
0.72 
0.10 

None 
4.46 
0.03 
1.69 
6.01 
0.30 
0.30 
2.36 
0.21 
4.82 

BaO 

MgO  
K2O  
Na2O  
H2O+  below  110°C.... 
H2O-  above  110°C  
P206  
CO2 

100.57 

.100.60 

99.31 

100.79 

i  Near  Nevada  City.    LINDOREN,  WALDEMAR:  Op.  tit.,  p.  38. 
i  Bellefontaine  tunnel.     Idem,  p.  149. 
*  Above  Maryland  mine.     Idem,  p.  66. 
North  Star  mine.     Idem,  p.  149. 


240      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


alteration  of  diabase  presents  some  noteworthy  features:  the 
diabase  of  the  Maryland  area,  Grass  Valley,1  is  a  dark-green 
rock  composed  of  lath-shaped  plagioclase,  augite  and  hornblende, 
with  considerable  original  pyrite  and  pyrrhotite.  The  altered 
diabase  is  a  grayish-green  rock  with  a  fine-grained  groundmass 
and  small  greenish  crystals.  The  analyses  of  this  rock  indicate 
losses  of  considerable  iron,  aluminum,  sodium,  and  magnesium 
and  of  some  silica;  potash,  lime,  and  carbon  dioxide  have  been 
added  in  notable  amounts. 

MINERAL  COMPOSITION  OF  ALTERED  ROCK  AT  GRASS  VALLEY,  CALIFORNIA 


Granodiorite 


Diabase 


Quartz  

25.00 

35.00 

Sericite  

61.46 

21.20 

Calcite  

7.23 

42.15 

Magnesite  

2.70 

0.71 

Siderite  

0.58 

Rhodonite  

Rutile  

0.25 

0.36 

Pyrite  

2.87 

0.50 

Apatite  

0.46 

0.15 

Sideritic  Lead  Veins. — The  Wood  River  district,  near  Hailey, 
Idaho,2  has  produced  much  silver,  lead,  and  gold.  It  is  an  area 
of  Carboniferous  sandstones,  shales,  and  limestones  intruded  by 
granite  (quartz  monzonite),  near  the  contact  of  which  heavy 
silicate  minerals  are  developed  in  the  sedimentary  rocks .  Neocene 
lavas  rest  unconformably  on  the  granite  and  older  rocks.  The 
principal  deposits  are  replacement  veins  in  calcareous  shale  and 
subordinately  in  quartz  monzonite  and  quartz  diorite.  The 
gangue  is  siderite,  and  the  chief  ore  mineral  is  galena.  Other 
minerals  are  pyrite,  zinc  blende,  arsenopyrite,  and  tetrahedrite, 
with  a  little  quartz.  The  ore  carries  50  per  cent,  of  lead  and  from 
50  to  100  ounces  of  silver  and  $2  to  $5  in  gold  to  the  ton.  The 
deposits,  though  smaller,  are  of  higher  grade  than  those  of  the 
Coeur  d'Alene  district.  Analyses  of  the  fresh  quartz  monzonite 
and  its  hydrothermally  altered  equivalent  are  given  below.  The 

1  Idem,  p.  66. 

'LiNDGREN,  WALDEMAR:  The  Gold  and  Silver  Veins  of  Silver  City,  De 
Lamar,  and  Other  Mining  Districts  in  Idaho.  U.  S.  Geol.  Survey  Twentieth 
Ann.  Rept.,  part  3,  p.  211,  1900. 


MINERAL  ASSOCIATIONS  IN  VEINS 


241 


loss  of  potash  is  noteworthy.  The  table  also  includes  analyses  of 
the  fresh  and  altered  quartz  diorite,  in  which  the  changes  caused 
by  the  solutions  are  seen  to  be  of  the  same  general  character. 

ANALYSES  OF  FRESH  AND  ALTERED  WALL  ROCKS,  HAILEY,  IDAHOI 
(W.  F.  Hillebrand,  Analyst) 


Quartz    monzonite 
Idaho-Democrat 
mine 

Quartz  diorite 
Crresus  mine 

Fresh 

Altered 

Fresh 

Altered 

SiO2  
TiO2  
A12O3 

68.42 

0.50 
15.01 
0.97 
1.93 
None 
0.06 
2.60 
0.03 
0.12 
1.21 
4.25 
3.22 
Trace 
0.54 
0.73 
0.13 
0.20 
0.02 

71.93 
0.40 
12.21 
0.64 
2.99 
None 
0.18 
2.59 
None 
Trace 
0.58 
3.29 
0.23 
Trace 
0.37 
2.06 
0.10 
1.95 
0.18 
0.13 
None 
0.09 
Trace 
None 
None 

57.78 
1.01 

16.28 
1.02 
4.92 
0.02 
0.15 
6.65 
0.07 
0.12 
4.60 
2.22 
3.25 
Trace 
0.34 
0.92 
0.30 
0.15 
0.02 

58.01 
1.08 
15.72 
0.64 
3.87 
None 
0.17 
2.15 
None 
Trace? 
2.07 
4.79 
0.10 
Trace 
0.31 
2.71 
0.31 
2.86 
1.25 
1.52 
0.12 

0.86 
0.05 
1.65 

Fe2O3  
FeO  

CoO  +  NiO 

MnO 

CaO 

SrO  
BaO  
MgO  
K2O  
Na2O  
Li2O 

H2O  below  105°C  
H2O  above  105°C  
P2O6  

CO2  
S  
Fe  
Co  +  Ni  

Zn 

Pb 

Cu  

AS  ,  

99.95              99.92 

99.88            100.24 

The  Coeur  d'Alene  district,  Idaho,2  is  an  area  of  pre-Cambrian 
quartzites  and  siliceous  slates  intruded  by  monzonite  and 
monzonite  porphyry.  The  most  productive  deposits  are  lead- 

1LiNDGREN,  WALDEMAR:  Op.  cit.,  pp.  219-221. 

2RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  The  Geology  and  Ore  Deposits 
of  the  Coeur  d'Alene  District,  Idaho.  U.  S.  Geol.  Survey  Prof.  Paper  62, 
p.  134,  1908. 


242      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

silver  lodes  in  the  siliceous  sedimentary  rocks.  The  ore  minerals 
include  galena,  sphalerite,  pyrite,  chalcopyrite,  and  some 
argentiferous  tetrahedrite  and  stibnite.  Siderite  is  the  most 
abundant  gangue  mineral;  quartz  and  some  barite  are  also  present. 
The  ore  has  been  formed  largely  by  replacement  of  the  "siliceous 
country  rock  along  fissures.  The  veins  have  great  vertical  ex- 
tent, and  in  the  lower  levels  pyrrhotite  and  magnetite  appear. 
Tourmaline  and  siderite  are  deposited  in  the  country  rock  beyond 
the  limits  of  ore  deposition  (see  p.  493). 

Sericitic  Copper-silver  and  Sericitic  Zinc-silver  Veins. — The 
Butte  district,  Montana  (see  page  357),  is  an  area  of  quartz 
monzonite  ("granite")  intruded  by  aplitic  granite  and  rhyolite 
porphyry  and  partly  covered  by  rhyolite.  Huge  east-west 
replacement  veins  traverse  all  these  rocks  except  the  rhyolite, 
and  these  veins  are  crossed  by  northwest  replacement  veins  that 
were  formed  in  and  along  fault  fissures.  Of  the  east-west  lodes 
three  systems  are  noteworthy.  These  are  the  Anaconda  and 
Bell-Speculator  systems,  which  are  copper-silver  veins,  and  north 
of  them  the  Rainbow  lode,  which  yields  silver  and  zinc.  Hydro- 
thermal  alteration1  is  extensive;  nearly  everywhere  in  the  ore- 
bearing  district  the  granite  is  slightly  chloritized,  and  along  the 
borders  of  the  three  great  east-west  vein  systems  and  also 
along  the  northwest  fault  fissures  the  country  rock  is  highly 
sericitized.  Where  veins  are  thickly  spaced  the  sericitization 
locally  extends  over  areas  hundreds  of  feet  wide,  and  much  of 
the  ore  consists  of  granite  impregnated  by  copper  or  zinc  minerals. 
The  ascending  hot  solutions  first  attacked  augite,  hornblende, 
biotite,  and  magnetite  and  formed  chlorite,  epidote,  secondary 
silica,  and  pyrite.  Orthoclase  and  plagioclase  feldspars  were 
converted  to  sericite  and  silica;  chalcopyrite,  enargite,  and  other 
copper  minerals  were  formed  in  the  granite  along  the  copper  lodes, 
and  zinc  blende  with  some  galena  along  the  zinc-silver  lodes. 
Locally  fluorite,  rhodonite,  and  rhodochrosite  are  present  in 
both  copper  and  zinc  lodes,  but  the  great  bulk  of  the  altered  rock 

IWEED,  W.  H.,  EMMONS,  S.  F.,  and  TOWER,  G.  W.:  TL  S.  Geol.  Survey 
Geol.  Atlas,  Butte  Folio  (No.  38),  1897. 

WEED,  W.  H.:  Geology  and  Ore  Deposits  of  the  Butte  District,  Montana. 
U.  S.  Geol.  Survey  Prof.  Paper  74,  p.  87,  1912. 

SALES,  RENO:  Ore  Deposits  at  Butte,  Montana.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  46,  pp.  3-109,  1913. 

KIRK,  C.  T.:  Conditions  of  Mineralization  in  the  Copper  Veins  at  Butte, 
Mont.  Econ.  Geol.,  vol.  7,  p.  40,  1912. 


MINERAL  ASSOCIATIONS  IN  VEINS 


243 


along  both  copper  and  zinc  lodes  is  simply  sericite  with  residual 
and  secondary  quartz  and  sulphides. 

It  was  long  believed  that  the  earlier  easterly  veins  were  es- 
sentially free  from  enargite  and  that  this  mineral  was  developed 
principally  in  the  northwesterly  fault  fissures,  but  it  is  now  known 
that  veins  of  both  systems  carry  enargite.  Sales,  however,  has 
shown  that  the  metallization  is  related  broadly  to  zones  (see 
Fig.  123).  There  is  a  central  copper  zone  occupying  the  great 
area  of  altered  granite  in  the  vicinity  of  the  Mountain  View  mine, 
in  which  the  ores  are  characteristically  free  from  sphalerite  and 


FIG.  123. — Plan  of  Butte  district,  Montana,  at  altitude  4,600  feet,  illustrating 
general  distribution  of  ore  types  with  reference  to  the  central  copper  zone. 
(After  Sales.) 

manganese  minerals.  This  zone  is  represented  by  the  shaded 
area  in  Fig.  123.  An  indeterminate  zone  of  irregular  width 
nearly  surrounds  the  central  copper  zone;  in  it  the  ores  are  pre- 
dominantly copper  but  are  rarely  free  from  sphalerite,  and  near 
the  outer  boundaries  (A— A  and  A' -A',  Fig.  123),  rhodonite  and 
rhodochrosite  are  of  common  occurrence.  This  intermediate 
zone  is  bordered  by  a  peripheral  zone  of  undetermined  width  in 
which  copper  has  not  been  found  in  large  quantities.  The  vein 
filling  is  chiefly  quartz,  rhodonite,  sphalerite,  pyrite,  and  rho- 
dochrosite. In  this  peripheral  zone  are  included  the  manga- 
nese-silver veins  of  the  Alice,  Moulton,  Black  Rock,  Elm  Orlu, 


244      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

and  Magna  Charta  mines,  on  the  north,  and  the  Emma,  Ophir, 
Travonia,  and  others,  on  the  south. 

Sericitic  Copper  Veins  and  Disseminated  Copper  Ores.  —  The 
Clifton-Morenci  district,  Arizona,1  includes  pre-Cambrian  granite 
and  schists,  overlain  by  Paleozoic  and  Mesozoic  sedimentary  rocks 
(page  376),  which  have  been  intruded  by  stocks  and  dikes  of 
granitic,  monzonitic,  and  dioritic  porphyries.  The  contact 
metamorphism  of  the  sedimentary  rocks  of  this  area  and  their 
accompanying  ores  are  described  on  page  40.  Great  dis- 
seminated deposits  of  chalcocite  ore  are  developed  in  the 
porphyry. 

The  following  analyses  and  abridged  notes  from  Lindgren's 
descriptions  show  the  changes  that  have  taken  place  in  the 
porphyry  as  a  result  of  hydrothermal  processes.  The  normal 
fresh  quartz  monzonite  porphyry  is  a  light-gray  rock  composed 
of  orthoclase,  plagioclase,  quartz,  and  green  biotite  in  a  micro- 
crystalline  groundmass  with  much  quartz  and  orthoclase.  Some 
secondary  minerals,  such  as  sericite,  epidote,  chlorite,  serpentine, 
and  pyrite,  are  present  in  small  amounts  but  do  not  greatly  alter 
the  composition  of  the  rock,  which  is  represented  by  analysis  1  in 
the  accompanying  table.  Analysis  2  represents  a  specimen  taken 
adjoining  a  2-inch  pyrite  vein  —  a  soft  white  chalky  rock  contain- 
ing scattered  pyrite  and  showing  on  a  few  seams  a  little  chalco- 
cite. The  locality  where  this  specimen  was  obtained  is  600  feet 
below  the  surface  and  somewhat  below  the  main  chalcocite  zone, 
although  there  is  a  little  oxidation  and  chalcocitization  of  the 
vein.  .  The  rock  is  a  felted  mass  of  sericite  with  some  granular 
quartz  and  is  cut  by  veinlets  of  kaolin.  Pyrite  is  present  in 
grains  and  crystals.'  Zinc  blende,  chalcopyrite,  and  molybdenite 
occur  in  small  amounts  as  irregular  grains  and  aggregates;  also 
some  rutile  and  zircon. 

Sample  3,  taken  about  300  feet  below  the  surface,  is  a  hard 
white  porphyry  with  small  pyrite  crystals.  The  original  struc- 
ture is  almost  lost,  but  under  the  microscope  the  outlines  of 
feldspar  crystals  are  visible.  The  rock  consists  chiefly  of  a  very 
fine  sericite  felt  with  granular  quartz;  the  quartz  also  occurs  as 
veinlets  with  pyrite. 

Sample  4,  the  disseminated  chalcocite  ore,  is  a  soft  white  chalky 
rock,  cut  by  many  small  seams  of  pyrite  and  chalcopyrite.  In 


WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.     U.  S.  Geol.  Survey  Prof.  Paper  43,  p.  168,  1905. 


MINERAL  ASSOCIATIONS  IN  VEINS 


245 


thin  section  the  porphyry  structure  is  retained,  but  the  feldspars 
are  entirely  converted  into  sericite  felt;  the  groundmass  consists 
of  granular  quartz,  filled  with  sericite.  Pyrite  occurs  in  crystals 
and  anhedrons,  largely  in  the  altered  feldspars  but  also  with 
small  masses  of  granular  quartz. 

ANALYSES  OF  FRESH  AND  ALTERED  PORPHYRY  FROM  MINES  AT  MORENCI, 


(W.  F.  Hillebrand,  analyst) 


1 

2 

3 

4 

5 

SiO2  
A12O3  
Fe2O3  
FeO 

68.04 
17.20 
0.34 
0  67 

46.67 
20.92 
0.37 
0  36 

69.55 
16.43 
0.46 
0  11 

64.88 
16.41 

|    0.65 

72.78 
15.35 
f    0.55 
1     0  10 

MgO  :  

CaO 

1.05 
2  21 

0.85 
0  15 

0.62 
0  15 

1.12 
0  11 

0.89 
0  14 

Na2O 

5  33 

0  16 

0  17 

0  12 

0  36 

K2O 

2  65 

4  33 

5  05 

4.96 

5.00 

H2O  -  
H20  +  
TiO2  
ZrO2  
CO2  

0.60 
1.23 
0.41 
0.01 
None 

0.94 
5.01 
0.43 

Trace? 
None 

1.00 
2.69 
0.41 
Trace 
None 

0.83 
2.74 
0.38 
Trace 
None 

1.21 
3.22 
0.45 
Trace 
None 

P205  

SO  3 

0.12 

0.15 
0  18 

0.05 
0  10 

0.12 
0  10 

0.05 
0  08 

MnO  

0.06 

None 

None 

Trace? 

None 

BaO 

0  10 

0  04 

0  05 

0  07 

0.02 

SrO  
Li2O  
V203  
FeS2  
Cu2S  
Zn  

0.03 
Trace? 
Trace 
0.24 
0.02 

None 
Trace 

19.18 
0.24 

None 
Trace? 

3.09 
0.07 

Trace 
Trace 

4.96 
2.42 
None 

None 
Trace 

0.06 

ZnS  
Mo  
MoS2  

0.03 
None 

0.32 
0.20 

None 

None 

100.34 

100.50 

100.00 

99.87 

100.26 

1 .  Fresh  monzonite  porphyry.     Ryerson  mine,  first  level. 

2.  Altered  monzonite  porphyry.     Ryerson  mine,  first  level;  drift  on  Humboldt  vein  at 
end  of  small  crosscut  in  foot  wall  40  feet  east  of  Humboldt  claim  line. 

3.  Altered  (silicified)  porphyry.     Ryerson  mine,  intermediate  level,  550  feet  west  of  West 
Yankie  shaft. 

4.  Altered  porphyry  within  chalcocite  zone.     Ryerson  mine,  lower  adit  level,  Humboldt 
vein,  from  stopes  70  feet  wide,  80  feet  above  level. 

5.  Altered  porphyry  from  the  surface. 


246      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Sample  5  is  from  -the  surface  of  the  northeast  spur  of  Copper 
Mountain,  where  the  rock  formed  brownish-gray  outcrops;  the 
porphyry  structure  is  still  visible.  A  thin  section  shows  it  to  be 
an  entirely  sericitized  rock  with  pseudomorphs  of  feldspars  and 
biotite.  The  groundmass  consists  mainly  of  fine-grained  quartz, 
with  sericite  foils. 

The  mineral  components  of  these  rocks,  calculated  by  L.  C. 


MINERAL  COMPOSITION   OF   FRESH  AND  ALTERED   PORPHYRY  FROM 
MINES  AT  MORENCI,  ARIZ. 


Quartz  
Orthoclase  (mol.).... 
Albite  (mol.)  . 
Anorthite  (mol.)  
Apatite 

21.35 
13.12 
45.26 
9.52 

0  28 

19.13 
0  36 

49.33 
0.12 

43.03 
0.30 

50.81 
0.12 

Zircon  .  . 

0  02 

Magnetite  
Ilmenite  
Rutile  

0.41 
0.81 

0.43 

0.41 

0.38 

0.45 

Sericite  

3.75 

38.50 

43.44 

44.94 

44.29 

Kaolin 

17  90 

Chlorite 

"3  04 

Serpentine  
Epidote  
Alunite  

60.98 
"0.59 

'2.81 
0.46(?) 

d1.71 
0.43 
0.26(?) 

•2.74 
0.26(?) 

/2.71 
"0.36 
0.21(?) 

Water  (below  100°C.) 
FeS2 

0.60 
0  24 

0.94 
19  18 

1.00 
3  09 

0.83 
4  96 

1.21  . 

0.06 

Cu2S 

0  02 

0  24 

0  07 

2  42 

ZnS  
MoS2  

0.03 

0.32 
0.20 

. 

100.02 

100.47 

99.86 

99.86 

100.22 

"Amesite.   MgO:FeO  =  6:l. 

6  0.39  per  cent.  MgO,  0.19  per  cent.  H2O  residue,  with  0.40  per  cent.  SiOz — too  high  in 
HjO  for  serpentine. 

c  0.84  per  cent.  MgO,  0.29  per  cent.  FeO,  0.53  per  cent.  HzO  residue,  with  1.15  per  cent. 
SiOj — too  high  in  H2O  for  serpentine. 

d  0.62  per  cent.  MgO,  0.39  per  cent.  HzO  residue,  with  0.70  per  cent.  SiOs  corresponds 
fairly  well  to  deweylite. 

•  1.11  per  cent.  MgO,  0.49  per  cent.  HzQ  residue,  with  1.12  SiOz — too  high  in  H2O  for 
serpentine. 

f  0.89  per  cent.  MgO,  0.95  per  cent.  HsO  residue,  with  0.87  per  cent.  SiO2 — too  high  in 
HiO  for  serpentine. 

'  Al»Oi:FejG»-4:l. 


MINERAL  ASSOCIATIONS  IN  VEINS         247 

Graton  from  analyses  and  thin  sections,  are  shown  in  the  table 
on  page  246. 

This  table  shows  the  altered  rocks  to  consist  chiefly  of  sericite, 
pyrite,  quartz,  and  serpentine.  Kaolin  occurs  only  in  sample  2, 
the  only  one  taken  immediately  adjacent  to  an  important  seam 
or  vein.  The  silica  has  not  been  materially  changed,  except  in 
No.  2,  where  it  is  lowered  to  correspond  to  the  high  percentage  of 
kaolin  and  pyrite.  Alumina  is  generally  constant  but  has  been 
increased  in  No.  2;  this  increase  is,  however,  probably  due  to 
conversion  of  sericite  to  kaolin,  with  attendant  setting  free  of 
some  chalcedonic  or  opaline  silica.  MgO,  TiO2,  Zr02,  and  P2O5 
remain  nearly  constant.  Practically  all  CaO  and  Na2O  have 
been  carried  away  and  FeS2  and  K2O  have  been  added.  The 
alteration  indicates  waters  deficient  in  carbonates  but  rich  in 
potash,  iron,  and  silica. 

The  Bingham  district,  Utah,1  is  an  area  of  Carboniferous 
(Bingham)  quartzite  which  includes  numerous  beds  and  lentils 
of  limestone.  These  rocks  are  intruded  by  monzonite  and  mon- 
zonitic  porphyries  which  take  the  form  of  stocks,  sills,  and  dikes. 
The  deposits  in  monzonite  and  monzonitic  porphyry  include 
veins  and  disseminated  copper  ores.  The  alterations  at  the 
Last  Chance  lode2  result  in  decreases  in  calcium,  sodium,  and 
magnesium,  and  increases  of  potassium,  iron,  and  sulphur. 
There  is  little  variation  in  silica,  alumina,  and  titanic  oxide. 
The  monzonite  that  contains  the  disseminated  chalcocite  ores  is 
hydrothermally  altered  over  wide  areas;  the  alterations,  so  far 
as  may  be  judged  by  microscopic  studies,  are  similar  to  those 
at  the  Last  Chance  mine.  In  the  fresh  monzonite  the  chief  con- 
stituents are  orthoclase,  plagioclase,  augite,  biotite,  and  a  little 
hornblende  and  quartz.  Pyrite  and  chalcopyrite  occur  spar- 
ingly on  parting  planes.  Sericite,  potash  feldspar,  and  biotite 
are  developed  by  hydrothermal  alteration.  The  sericitized  rock 
is  crossed  by  numerous  unsystematized  fractures  so  closely 
spaced  that  at  many  places  it  is  almost  impossible  to  cleave  a 
hand  specimen  of  the  usual  size  without  breaking  it  along  a 
fracture.  The  copper  occurs  in  these  thin  crossings  and  is 
disseminated  through  the  bleached  wall  rock. 

1  BCHJTWELL,   J.   M.,   KEITH,   ARTHUR,  and  EMMONS,  S.   F.:  Economic 
Geology  of  the   Bingham   Mining   District,   Utah.     U.   S.   Geol.   Survey 
Prof.  Paper  38,  1905. 

2  Idem,  p.  178. 


248      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Sericitic  Silver-gold  Veins. — The  sericitic  silver-gold  veins  of 
Granite  County,  Montana,1  include  several  parallel  lodes  that 
are  traceable  for  a  mile  or  more  along  the  strike.  They  cut 
quartz  monzonite  of  early  Tertiary  age.  The  granite  walls  of 
the  fissure  fillings  have  not  been  replaced  by  ore  to  any  great 
extent,  and  nowhere,  so  far  as  known,  can  the  granite  of  the 
walls  or  horses  be  worked  at  a  profit.  The  metasomatic  changes 
which  the  ore-depositing  solutions  have  produced  in  the  granite 
are  nevertheless  noteworthy  and  clear  in  their  expression.  These 
solutions  have  attacked  nearly  all  minerals  in  the  granite  and 
have  deposited  large  quantities  of  calcite  and  sericite  with  con- 
siderable quartz.  The  alteration  extends  to  varying  distances, 
but  the  greatest  alteration  is  confined  to  a  zone  less  than  20  feet 
on  each  side  of  the  vein. 

Gold-silver-adularia  Veins. — The  gold-silver  veins  of  De 
Lamar  and  Silver  City,  Idaho,  are  post-Miocene  deposits  that  were 
formed  very  near  the  present  surface.  These  veins  are  in  rhyo- 
lite,  basalt,  and  granite.  The  surface  at  the  time  of  deposition, 
as  shown  by  Lindgren,2  was  not  more  than  700  to  2,000  feet 
above  the  deposits  mined.  The  gangue  consists  of  quartz  and 
adularia,  with  some  chalcedony.  Much  of  the  quartz  is  pseudo- 
morphous  after  calcite  or  barite.  Small  quantities  of  chlorite, 
epidote,  barite,  siderite,  and  calcite  are  present  also.  The  ore 
minerals  are  gold,  silver,  argentite,  proustite,  pyrargyrite,  poly- 
basite,  miargyrite  (AgSbS2),  chalcopyrite,  zinc  blende,  galena, 
pyrite,  and  a  little  marcasite.  Some  of  the  rich  ore  was  formed 
very  near  the  old  surface  and  is  probably  primary.  The  analyses 
of  the  altered  rhyolite  show  that  potash  and  aluminum  have 
been  removed  as  well  as  soda.  The  removal  of  both  the  alkalies, 
the  presence  of  marcasite  and  chalcedony,  and  the  extensive 
kaolinization  of  the  walls  are  noteworthy. 

Chloritic  Alteration  in  Granitic  Rock. — In  many  metallif erous 
districts  three  principal  types  of  alteration  are  recognized — 
chloritic  alteration  in  a  zone  extending  far  out  from  the  deposits, 
extensive  sericitization  within  this  zone,  and  silication  near  the 
master  fractures.  Where  the  fractures  are  closely  spaced  the 

1  EMMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper  78, 
1912. 

J  LINDGREN,  WALDEMAR:  The  Gold  and  Silver  Veins  of  Silver  City, 
De  Lamar,  and  Other  Districts  in  Idaho.  U.  S.  Geol.  Survey  Twentieth 
Ann.  Rept.,  part  3,  p.  165,  1899. 


MINERAL  ASSOCIATIONS  IN  VEINS          249 

area  of  sericitic  alteration  may  be  hundreds  of  feet  wide  and  the 
chloritic  alteration  may  extend  over  thousands  of  feet.  At 
Butte,  Mont.,  the  chloritic  alteration  consists  chiefly  of  the 
development  of  chlorite  and  pyrite  in  the  iron-bearing  minerals, 
and  the  clouding  of  feldspar  by  incipient  sericitization.  Some 
epidote  and  secondary  quartz  also  are  developed.  This  phase 
appears  in  practically  all  the  granite  within  the  mineral-bearing 
area  at  Butte.  Alteration  of  this  type  is  attended  by  relatively 
little  addition  or  subtraction  of  material.  It  is  believed  to  have 
been  brought  about  by  gases  or  by  waters  that  deposited  ore  and 
sericitized  the  wall  rock  near  the  ore-bearing  fissures.  Although 
less  extensive,  the  chemical  changes  brought  about  by  chloritic 
alteration  are  chemically  similar  to  those  accomplished  by 
sericitization. 

Chloritic  Alteration  in  Lavas  (Propylitic  Alteration). — In  many 
districts,  particularly  in  certain  western  camps,  where  ore  deposits 
were  formed  in  lavas  during  Miocene  and  Pliocene  time,  the 
country  rock  is  altered  over  wide  areas.  Near  the  surface,  rocks 
fracture  more  readily  than  at  greater  depths,  because  they  are  not 
held  down  by  so  great  a  mass  of  overlying  rocks.  Hot  waters 
and  gases  ascending  along  major  fractures  spread  out,  entering 
cracks  and  joints  and  mineralizing  practically  the  entire  country 
rock  between  the  veins.  In  such  a  district  the  rocks  in  an  area 
of  more  than  a  square  mile  may  be  altered,  and  if  the  district 
lies  in  a  mountain  range  the  mineralized  area  may  be 
noted  by  an  observer  miles  away.  Experienced  prospectors 
appreciate  the  significance  of  the  alterations,  which  give  what 
some  designate  a  "kindly  look"  to  the  rock.  Andesite  and 
basalt  are  generally  changed  to  a  dull  green;  rhyolite  to  a  paler 
green  and  locally  to  a  chalky  white. 

A  common  type  of  such  alteration  is  the  propylitic  phase, 
in  which  the  following  changes  are  noteworthy:  Minerals  con- 
taining iron,  such  as  hornblende,  pyroxene,  and  biotite,  are 
altered  to  chlorite,  epidote,  and  pyrite.  Glassy  feldspars  lose 
their  glassy  habit.  A  glassy  groundmass  is  recrystallized  largely 
to  feldspar  and  quartz,  with  some  chlorite,  epidote,  and  calcite. 
Some  sericite  is  nearly  always  developed.  Chemically  there  is  a 
loss  of  sodium  and  generally  of  alkaline  earths;  sulphur  and  water 
are  added.  These  changes 'are  very  common  in  andesites,  dacites, 
and  basalts.  In  rhyolite  less  chlorite  and  more  sericite  may  be 
formed,  and  quartz  may  be  added  in  considerable  quantities  by 


250      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  silication  of  iron-bearing  minerals  and  feldspars.  In  many 
districts  the  alteration  increases  near  the  veins,  where  less  chlorite 
is  developed  and  more  sulphides  and  quartz.  As  in  granite, 
three  types  of  alteration  are  distinguished  in  some  districts — 
chloritic  alteration  extending  over  wide  areas,  sericitic  alteration 
nearer  the  veins,  and  silication  of  the  wall  rock  adjoining  the 
main  fractures. 

Sericitic  and  Propylitic  Alteration  Compared. — Sericitic 
alteration  and  propylitic  alteration  are  the  most  common 
types  of  hydrothermal  metamorphism  of  igneous  rocks  near 
veins  that  are  formed  at  moderate  and  at  shallow  depths. 
They  are  generally  attended  by  similar  chemical  changes,  al- 
though the  changes  involved  in  sericitization  are  greater.  This 
is  shown  by  the  calculations  of  Reber,1  who  employed  the 
circular  diagram  first  used  by  Ransome.2  On  this  diagram, 
reproduced  as  Fig.  124,  are  plotted  10  pairs  of  analyses  of  fresh 
rocks  and  of  corresponding  hydrothermally  altered  rocks.  The 
16  radii  represent  radicles  or  molecules  of  the  analyses  and  are  so 
named.  The  points  where  the  radii  are  met  by  the  heavy  circle 
represent  the  compositions  of  the  10  unaltered  rocks.  In  each 
corresponding  altered  rock  the  percentage  losses  are  plotted 
on  the  16  radii  toward  the  center  of  the  circle,  and  gains  are 
plotted  outward-from  the  heavy  line.  The  points  showing  losses 
or  gains  so  plotted  are  connected  by  lines  solid  or  broken  in 
"different  ways  so  that  changes  brought  about  by  alteration  may 
easily  be  compared  by  following  these  lines  around  the  circle. 
It  is  evident,  on  inspecting  the  diagram,  that  potash,  and  com- 
bined water,  are  practically  always  increased  whether  the 
alteration  is  by  sericitization  or  propylitization,  that  soda  and 
magnesia  are  always  reduced,  and  that  there  is  generally  a  loss 
of  lime.  Iron  oxides  decrease,  considerable  iron  being  changed 
to  sulphide.  Carbon  dioxide  is  commonly  added. 

The  propylitic  and  sericitic  alterations  at  Tonopah,  Nevada, 
have  been  closely  studied.  The  Tonopah  district,3  is  a  fault 
mosaic  of  Tertiary  eruptive  rocks,  probably  of  Miocene  and 
Pliocene  age.  The  first  rocks  erupted  were  andesites,  and  these 

1  REBER,   L.    E.,    JR.:  The   Mineralization   at    Clifton-Morenci.     Econ. 
Geol,  vol.  11,  p.  566,  1916. 

2  RANSOME,  F.  L.:  The  Geology  and  Ore- Deposits  of  Goldfield,  Nevada. 
U.  S.  Geol.  Survey  Prof.  Paper  66,  p.  181,  1909. 

3SPURR,  J.  E.:  Geology  of  the  Tonopah  Mining  District,  Nevada, 
U.  S.  Geol.  Survey  Prof.  Paper  42,  p.  213,  1905. 


MINERAL  ASSOCIATIONS  IN  VEINS 


251 


were  followed  by  rhyolites  and  dacites  as  flows,  intrusives,  and 
tuffs.  The  first  andesite,  termed  by  Spurr  the  early  andesite, 
was  followed  by  the  later  andesite,  which  is  more  basic.  The 
principal  veins  were  formed  after  the  early  andesite  was  erupted 
and  before  the  later  andesite  and  consequently  do  not  extend 


A1203 


Fe208 


ZnS 


FeS» 


MgO 


TiOz 


Na20 


Fio.  124. — Diagram  showing  results  of  sericitization  and  propylitization.  1, 
San  Francisco  (Utah)  moiizonite,  sericitic(?) ;  2,  San  Francisco  monzonite,  sericitic; 
3,  Hailey,  (Idaho)  diorite,  sericitic  and  propylitic;  4,  Morenci  granite  porphyry 
or  monzonite,  sericitic  and  propylitic;  5,  Leadville  white  porphyry,  sericitic  and 
propylitic;  7,  Rimini  monzonite,  sericite  tourmaline;  8,  Willow  Creek  (Idaho) 
granodiorite,  sericitic  and  propylitic;  9,  British  Columbia  diorite,  propylitic; 
10,  Hauraki,  (New  Zealand,  andesite,  propylitic.  (After  Reber.) 

into  the  overlying  rocks.  The  veins  were  formed  by  hot  ascend- 
ing waters,  chiefly  through  replacement  along  sheeted  zones. 
The  primary  ores  have  a  gangue  of  quartz,  adularia,  sericite,  and 
carbonates  and  contain  argeritite,  polybasite,  stephanite,  silver 
selenide,  gold,  chalcopyrite,  pyrite,  and  some  galena  and  zinc 
blende. 


252      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  wall  rock  is  greatly  altered  by  the  vein-depositing  solu- 
tions. In  some  places  the  early  andesite  is*  altered  chiefly  to 
quartz,  sericite,  and  adularia.  In  others  it  is  altered  chiefly 
by  the  deposition  of  calcite  and  chlorite.  These  phases  of 
alteration  grade  into  each  other  and  were  caused  by  the  same 
waters.  The  maximum  effect  of  these  waters  was  the  formation 
of  the  mineral  veins  along  the  major  channels.  Near  the  veins 
they  accomplished  the  quartz-sericite-adularia  alteration;  farther 
away,  the  calcite-chlorite  or  propylitic  alteration. 

The  nature  of  the  hydrothermal  alterations  is  shown  by  the 
table  on  page  254,  which  includes,  in  order,  analyses  representing 
the  slightly  altered  chloritic  or  propylitic  phases,  the  more 
intensely  altered  phases,  and  the  highly  siliceous  metal-bearing 
phase.  The  following  notes  are  abridged  from  Spurr's 
descriptions. 

Specimen  1  is  a  dense  dark-green  rock  obtained  670  feet 
below  the  surface.  It  contains  small  phenocrysts  in  a  fine 
microlitic  groundmass.  Among  the  phenocrysts  the  feldspars, 
probably  andesine-oligoclase,  are  prominent.  They  are  largely 
altered  to  calcite  with  a  little  quartz.  Abundant  pseudomorphs 
after  hornblende  consist  of  dark  blue-green  chlorite  and  iron  oxide. 
Pseudomorphs  after  biotite  consist  of  fine  muscovite,  with  a  little 
calcite  and  hematite. 

Specimen  2  is  green  but  much  lighter  than  No.  1.  It  shows 
small  phenocrysts  in  a  fine  microlitic  groundmass,  with  much 
felty  devitrified  glass.  The  feldspar  phenocrysts  (andesine), 
are  only  slightly  decomposed.  The  ferromagnesian  minerals 
are  entirely  altered. 

Specimen  3,  from  a  depth  of  218  feet,  is  a  purple  rock  with 
rather  abundant  phenocrysts  of  white  andesine  in  a  glassy  ground- 
mass.  The  feldspars  are  only  partly  altered  to  sericite;  biotite 
has  altered  to  muscovite,  with  a  small  amount  of  siderite  and 
hematite,  which  forms  a  zone  around  the  edge.  Sericite  and 
talc  form  pseudomorphs  after  hornblende,  with  inclusions  and 
heavy  rims  of  magnetite.  Quartz  forms  pseudomorphs  after 
pyroxene  or  biotite,  with  a  little  calcite  and  hematite  around  the 
borders.  Other  pseudomorphs  consist  of  quartz  and  sericite. 

Specimen  4  has  a  pale  pinkish-purple  groundmass  with  white 
phenocrysts.  This  shows  what  was  originally  a  microlitic  glassy 
groundmass,  containing  abundant  secondary  quartz  and  sericite, 
with  limonite,  hematite,  and  siderite.  Pseudomorphs  after 


MINERAL  ASSOCIATIONS  IN  VEINS          253 

biotite  phenocrysts  consist  of  muscovite,  with  a  little  siderite. 
Other  phenocrysts,  possibly  of  hornblende,  are  represented  by 
pseudomorphs  of  quartz,  sericite,  and  a  little  siderite. 

Specimen  5,  from  the  180-foot  level,  near  the  Mizpah  vein, 
is  light  salmon-pink  and  shows  phenocrysts  of  feldspar,  whiter 
than  the  rest  of  the  rock.  No  original  mineral  remains.  The 
groundmass,  of  which  the  structure  may  be  distinguished,  is 
altered  to  quartz  and  sericite,  with  a  little  iron  oxide.  The 
pseudomorphs  after  phenocrysts  are  well  defined.  Those  after 
feldspar  form  aggregates  of  felty  muscovite,  with  a  little  quartz. 
Those  after  biotite  consist  of  muscovite,  with  a  little  siderite. 
Pseudomorphs  after  hornblende  or  pyroxene,  or  both,  are  barely 
distinguishable  from  the  groundmass.  They  consist  of  sericite 
and  quartz,  with  some  siderite,  which  marks  the  outlines  of  the 
original  phenocrysts.  In  this  rock  the  secondary  quartz  varies 
in  grain,  some  areas  becoming  more  coarsely  crystalline. 

Specimen  6  is  a  hard  white  rock  with  nests  of  fine  granular 
secondary  adularia  and  quartz.  The  feldspar  is  oligoclase,  mostly 
altered  to  adularia.  This  alteration  can  be  seen  in  all  stages, 
up  to  the  complete  pseudomorph.  A  little  sericite  accompanies 
the  adularia.  Traces  of  original  ferromagnesian  phenocrysts  can 
be  determined. 

Specimen  7,  from  the  hanging  wall  of  the  Mizpah  vein,  300- 
foot  level,  is  a  light-gray,  nearly  white  rock,  with  dull  luster. 
It  is  so  much  altered  as  to  be  hardly  recognizable  and  consists 
of  an  aggregate  of  quartz  and  oxidized  pyrite.  The  quartz 
is  segregated  into  little  areas  and  veinlets.  Phenocrysts  of  feld- 
spar are  indicated  by  pseudomorphous  areas  characterized  by 
different  groupings  of  the  quartz  and  sericite;  those  of  ferromag- 
nesian minerals  are  marked  by  similar  differences  of  grouping 
and  by  greater  abundance  of  the  iron  minerals.  The  decomposi- 
tion products,  however,  are  all  similar.  In  many  areas  the 
vestiges  of  the  phenocrysts  have  been  effaced. 

Specimen  8  is  ore  material  of  the  Mizpah  vein,  300-foot  level, 
and  shows  dense  quartz  intermixed  irregularly  with  kaolinic 
material.  The  microscope  shows  fine  to  moderately  coarse 
granular  quartz,  with  much  sericite.  Mixed  with  the  quartz 
in  the  finer-grained  areas  is  adularia  in  crystals  of  characteristic 
rhombic  section. 

Spurr  concludes  that  the  mineralizing  waters  were  charged  with 
an  excess  of  silica  and  probably  of  potash,  together  with  silver, 


254      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


ANALYSES  OF  DIFFERENT  PHASES  OF  ALTERED  EARLIER  ANDESITE, 
TONOPAH,  NEV. 


1 

2 

3 

4 

5 

6 

7 

8 

SiO2  

55.60 

58.47 

60.45 

71.14 

72.98 

73.50 

76.25 

91.40 

A1203  

16.70 

16.85 

17.78 

15.24 

14.66 

14.13 

12.84 

4.31 

Fe2O3  

2.23 

2.04 

5.86 

1.77 

1.01 

1.51 

0.54 

0.77 

FeO  

3.51 

3.12 

0.25 

0.26 

0.16 

0.26 

0.33 

0.11 

MgO  

2.60 

3.84 

1.55 

0.16 

0.33 

0.21 

0.56 

0.18 

CaO  

4.27 

1.35 

1.04 

0.09 

0.18 

0.12 

0.16 

None 

Na2O  

4.08 

4.30 

3.58 

0.24 

None 

0.24 

0.12 

0.06 

KzO  

3.17 

3.14 

2.11 

6.31 

6.03 

5.11 

3.20 

1.68 

H20  -  

0.88 

1.10 

2.86 

0.85 

0.97 

1.07 

2.14 

0.46 

HZO   +  

3.06 

3.59 

2.93 

2.87 

2.95 

2.81 

3.17 

0.98 

TiO2  

0.72 

0.77 

0.81 

0.48 

0.44 

0.47 

0.37 

0.07 

ZrO2 

Undet. 

0  02 

CO2  

2.76 

0.52 

None 

None 

None 

None 

None 

None 

P205  

0.28 

0.35 

0.28 

0.05 

0.16 

0.09 

0.12 

0.04 

SO3  

None 

None 

None 

0.05 

0.17 

None 

Cl 

F 

0  12 

Trace 

s 

None 

0  02 

0  03 

None 

FeS2  

0.49 

0.06 

NiO. 

(a) 

MnO  

Undet. 

0  26 

(a) 

(a) 

(a) 

(a) 

(a) 

0  06 

BaO  

0.12 

0.11 

0.07 

0.17 

(a) 

0.19 

0  02 

99.98 

100.42 

99.63 

99.70 

99.87 

99.91 

99.80 

100.16 

(a)   Not  looked  for. 

gold,  antimony,  arsenic,  etc.;  that  they  also  contained  carbonic 
acid  and  sulphur,  with  some  chlorine  and  fluorine;  but  that  they 
were  noticeably  deficient  in  iron. 

Fluoritic  Tellurium-adularia  Gold  Veins. — The  Cripple  Creek 
mining  district,  Colorado,1  is  an  area  of  gneiss,  granite,  and 
schists  cut  by  a  volcanic  neck  composed  mainly  of  breccia  which 
has  been  intruded  by  syenite,  latite-phonolite,  phonolite,  trachy- 
dolerite,  and  basic  dikes.  The  ore  deposits  are  veins  or  lodes  and 
irregular  replacement  deposits.  The  ores  are  later  than  all  the 
rocks,  and  there  is  a  rudely  radial  arrangement  of  veins  and 
dikes  with  respect  to  the  center  of  the  volcanic  neck.  Many  of 

1  LINDGREN,  WALDEMAR,  and  RANSOME,  F.  L. :  Geology  and  Gold  Deposits 
of  the  Cripple  Creek  district,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper  54, 
1906. 


MINERAL  ASSOCIATIONS  IN  VEINS         255 

the  veins  follow  phonolite  or  basic  dikes.  The  fissures  are  gener- 
ally narrow,  and  much  of  the  ore  has  formed  through  replacement 
of  wall  rock.  The  fissures  show  little  pre-mineral  displacement, 
nor  are  the  veins  extensively  faulted.  According  to  Lindgren 
and  Ransome,  the  fissures  were  formed  at  about  the  same  time 
as  the  intrusion  of  the  basic  dikes  and  represent  a  late  phase  of 
volcanic  activity. 

Calaverite  is  the  chief  primary  constituent,  and  native  gold 
is  rarely  present  in  the  unoxidized  ores.  Pyrite  is  widely  dis- 
tributed, and  galena,  sphalerite,  tetrahedrite,  stibnite,  and  molyb- 
denite are  sparingly  present.  As  gangue  minerals  quartz,  fluorite, 
and  dolomite  predominate;  celestite  is  common  and  is  in  many 
places  changed  to  quartz.  Sylvanite,  krennerite,  chalcedony, 
opal,  calcite,  rhodochrosite,  barite,  wavellite,  adularia,  sericite, 
and  roscoelite  are  present. 

The  hydrothermal  alteration  is  not  intense,  but  in  the  breccia 
it  is  spread  over  wide  areas ;  f erromagnesian  minerals  are  changed 
to  carbonates,  pyrite,  and  fluorite;  feldspar  and  feldspathoids 
are  changed  to  sericite  and  adularia.  Among  the  metasomatic 
minerals  pyrite  and  tellurides  are  common,  zinc  blende  rare. 
Sericite  forms  in  orthoclase  and  is  abundant  in  sodic  minerals 
such  as  nepheline,  sodalite,  and  analcite.  Roscoelite,  the  green 
vanadium  mica,  is  developed  at  some  places.  Adularia  appears 
in  considerable  amount  in  the  vein  filling  and  in  the  wall  rock. 
In  cavities  it  is  more  abundant  than  quartz,  which  is  relatively 
rare  except  where  silicates  have  been  altered.  Dolomite,  calcite, 
and  some  siderite  have  formed.  Purple  fluorite  is  widely 
distributed. 

Alunitic  Kaolinic  Gold  Veins.— The  Goldfield  district,  Nev.,1 
is  a  low  domical  uplift  of  Tertiary  lavas  and  lake  beds  that  rest 
upon  older  granitic  and  metamorphic  rocks,  small  areas  of  which 
are  exposed  here  and  there  by  erosion  of  the  Tertiary  beds.  The 
Tertiary  rocks  as  mapped  by  Ransome  include  rhyolite,  latite, 
andesite,  dacite,  dolerite  basalt,  conglomerate,  and  tuff.  The 
igneous  rocks  are  mainly  flows,  but  some  are  intrusive.  The 
rocks  are  greatly  shattered  and  highly  altered,  although  the 
geologic  structure  is  relatively  simple.  Some  of  the  ores  are  in 
andesite,  but  the  most  productive  deposits  are  in  an  intrusive 
dacite  which  probably  cuts  through  andesite.  In  the  sulphide 

1  RANSOME,  F.  L.:  Geology  and  Ore  Deposits  of  Goldfield,  Nev.  U.  S. 
Geol.  Survey  Prof.  Paper  66,  1909. 


256      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

ores  native  gold  and  pyrite  are  accompanied  by  minerals  con- 
taining copper,  silver,  antimony,  arsenic,  bismuth,  tellurium,  and 
other  elements.  In  some  ores  the  gold  occurs  in  small  particles 
so  closely  crowded  together  in  flinty  quartz  as  to  form  yellow 
bands  or  blotches.  Crustification  is  characteristic  of  the  richest 
ores,  fragments  of  silicified,  alunitized,  and  pyritized  rock  being 
covered  by  shells  of  gold  and  sulphides. 

The  deposits  were  formed  near  the  surface,  and  the  fissuring 
or  fracturing  is  very  irregular.  Their  age  is  probably  Pliocene, 
and  it  is  believed  that  the  surface  at  the  time  of  deposition  was 
not  more  than  1,000  feet  above  the  present  surface.  The  minerals 
of  the  ore  include  alunite,  barite,  gypsum,  kaolinite,  quartz,  native 
gold,  pyrite,  marcasite,  tellurides,  goldfieldite,  enargite,  fama- 
tinite,  and  bismuthinite.  As  is  generally  the  case  where  ascend- 
ing hot  waters  enter  a  complexly  fractured  zone  near  the  sur- 
face, they  have  migrated  far  from  the  principal  channels,  and 
large  areas  are  greatly  altered.  Over  an  area  of  several  square 
miles  fresh  rocks  are  almost  entirely  lacking. 

As  shown  by  Ransome,  there  are  three  types  of  alteration. 
The  less  intense  type  is  propylitic  and  consists  of  the  develop- 
ment of  quartz,  calcite,  chlorite,  epidote,  and  pyrite  at  the 
expense  of  the  rock-making  minerals  and  the  groundmass  of 
dacite,  andesite,  and  rhyolite. 

In  a  second  type  of  alteration  the  rock  is  changed  to  a  com- 
paratively soft  light-colored  mass  of  quartz,  kaolinite,  alunite, 
and  pyrite.  The  quartz  is  flinty  or  cryptocrystalline.  Much 
of  the  gold  is  associated  with  such  alteration. 

A  third  type  of  alteration  is  essentially  similar,  but  the  end 
products  are  more  siliceous,  and  the  materials  form  hundreds 
of  craggy  points  and  ledges  that  are  characteristic  of  the  topog- 
raphy of  the  district.  Although  most  of  these  quartz  ledges 
contain  no  ore  bodies,  nearly  all  the  ore  near  the  surface  has  been 
found  in  or  alongside  such  ledges. 

The  alunitized  area,  shown  in  Fig.  125,  is  surrounded  nearly 
everywhere  by  propylitic  rocks.  The  known  ore  bodies  are  con- 
fined to  the  alunitic  rocks  but  are  not  so  widely  distributed. 

The  chemical  changes  are.  shown  by  the  analyses  on  page  258. 
The  following  notes  are  condensed  from  Ransome's  descriptions.1 

1  RANSOME,  F.  L.:  Op.  tit.,  pp.  185-199.  The  Association  of  Alunite 
with  Gold  in  the  Goldfield  District,  Nevada.  Econ.  Geol,  vol.  2,  pp.  666- 
692,  1907. 


MINERAL  ASSOCIATIONS  IN  VEINS 


257 


The  fresh  dacite  (analysis  1)  is  of  comparatively  uniform  com- 
position. It  is  dark  gray  and  contains  moderately  abundant  lab- 
radorite  phenocrysts  and  less  numerous  phenocrysts  of  augite  and 
biotite  with  a  few  grains  of  quartz,  in  a  dark  aphanitic  groundmass. 


Generally  alunitized  areas 


FIG.  125. — Diagram  showing  the  portions  of  the  Goldfield  district,  Nevada, 
in  which  the  rocks  have  undergone  extensive  alunitic  alteration.  (After  Ransome, 
U.  S.  Geol.  Survey.) 

In  the  altered  dacite  (analysis  2)  the  rock  is  recrystallized,  with 
the  exception  of  quartz  phenocrysts,  to  an  aggregate  of  quartz, 
kaolinite,  alunite,  and  pyrite.  As  they  stand,  the  two  analyses 
show  close  agreement  in  silica,  alumina,  titanic  oxide,  and  phos- 
phorus pentoxide.  When  all  the  iron  in  both  analyses  is  calcu- 

17 


258      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


OOCCOO>OOOl>-*CO->l<O'-* 

M   II  +  I   I   II ++ 


d<No6d°t^«5o 


od(Ndddd^H( 


•  to  « <£ 

iSII 


lculated  to  summation 
grams,  in  100  cubic  ce 
grams,  in  100  cubic  ce 
grams,  in  100  cubic  ce 
grams,  in  100  cubic  ce 
in  grams  of  each  constitu 
of  each  constituent  in  p 
of  each  constituent  in  p 


nal 
na 
,  r 


MINERAL  ASSOCIATIONS  IN  VEINS          259 

lated  as  ferrous  oxide,  that  of  the  altered  rock  shows  a  loss  of 
only  0.7  per  cent.,  so  that  in  this  respect  also  the  two  analyses  are 
very  close  together.  The  analysis  of  the  altered  rock  shows  a  loss 
of  practically  all  the  lime  and  magnesia,  most  of  the  soda,  and 
one-half  of  the  potash.  It  has  gained  much  combined  water, 
nearly  6  per  cent,  of  sulphuric  anhydride,  and  nearly  4  per  cent, 
of  sulphur.  Such  a  direct  comparison  of  analyses,  while  indicat- 
ing in  this  case  the  general  character  of  the  change  that  has  taken 
place  in  the  dacite,  is  really  a  comparison  of  equal  masses  of  the 
two  rocks,  or,  strictly  speaking,  of  the  two  powders  as  prepared 
and  weighed  for  analysis. 

The  volumes  of  the  rocks  considered  in  large  masses  have  not 
greatly  changed,  but  as  the  fresh  dacite  has  a  porosity  of  0.9 
per  cent,  and  the  altered  rock  a  porosity  of  9.9  per  cent.,  correc- 
tions are  made  for  porosity. 

In  columns  Ic  and  2c  are  given  the  number  of  grams  of  each 
constituent  in  100  cubic  centimeters  of  each  rock,  the  .figures 
being  obtained  by  multiplying  the  percentage  figures  in  columns 
la  and  2a  by  the  respective  specific  gravities  of  the  fresh  and 
altered  dacite.  The  gains  and  losses  of  each  constituent  are 
given  in  grams  in  column  3  and  in  percentages  of  total  initial 
mass  in  column  4.  Column  5  gives  the  percentage  of  loss  of 
each  constituent  and  displays  clearly  the  nature  of  the  change. 

The  iron  has  been  converted  to  pyrite.  The  iron  oxides  in 
column  Ic  correspond  to  11.08  grams  of  metallic  iron;  the  pyrite 
in  column  2c  corresponds  to  8.4  grams  of  iron  and  9.6  grams  of 
sulphur.  The  iron  originally  in  the  dacite  was  more  than 
enough  to  form  all  the  pyrite  in  the  altered  rock ;  the  loss  of  iron 
is  over  27  per  cent. 

The  norm  or  theoretical  composition  of  the  fresh  dacite  and  the 
mineral  composition  of  the  altered  rock  are  given  on  page  260. 

The  agent  of  hydrothermal  metamorphism,  according  to 
Ransome,  was  a  strongly  acid  solution  carrying  hydrogen  sul- 
phide, sulphuric  acid,  and  possibly  sulphur  dioxide,  which  was 
capable  of  decomposing  the  silicates  of  the  dacite  and  carry- 
ing part  of  their  constituents  away,  reacting  with  iron  of  magne- 
tite and  silicates  to  form  pyrite,  with  potash  and  aluminum  to  • 
form  alunite,  and  with  aluminum  silicates  to  form  kaolinite. 

Ransome  shows  that  the  ores  were  probably  formed  by  ascend- 
ing hot  solutions  that  became  oxidized  near  the  surface.  The 
hot  ascending  waters  carried  sulphides,  as  shown  by  the  exten- 


260      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

NORM  OF  ORIGINAL  DACITE 

Quartz 17.34 

Orthoclase 15.01 

Albite -. 25.68 

Anorthite 21 . 96 

Zircon 0.37 

Diopside 5.21 

Enstatite. 4. 63 

Magnetite 4 . 87 

Ilmenite 1.52 

Apatite 0.34 

96.93 
Water...  2.95 


MINERAL  COMPOSITION  OF  -ALTERED  DACITE 

Quartz  (SiO2) 49.38 

Kaolinite  (Al2O32SiO2.2H2O) 23 . 99 

Alunite  (K2O.3A1263.4SO3.6H2O) 15.73 

Pyrite  (FeS2) 7 . 20 

Water  (H2O) 2.53 

Other  constituents. .  .  1 . 17 


100.00 

sive  change  to  pyrite  of  the  iron  that  was  originally  present  in 
the  dacite  and  other  rocks  as  a  constituent  of  mi  gnetite  and 
silicates.  This  change  was  not  confined  to  the  TOCKS  now  alu- 
nitized  but  extended  outward  to  what  has  been  called  the  pro- 
pylitic  aureole  of  alteration.  The  changes  in  the  dacite,  notwith- 
standing the  development  of  the  potassium  and  sodium  bearing 
mineral  alunite,  do  not  show  any  addition  of  alkalies  to  the  origi- 
nal rock,  but  such  evidence  does  not  prove  that  no  alkali 
sulphides  were  present  in  the  primary  solutions. 

Zeolitic  Native  Copper  Veins. — The  copper  deposits  of  Kewee- 
naw  Point,  Michigan1  extend  northeastward  for  about  70  miles. 

1  PTTMPELLY,  RAPHAEL:  The  Metasomatic  Development  of  the  Copper- 
bearing  Rocks  of  Lake  Superior.  Am.  Acad.  Arts  and  Sci.  Proc.,  vol.  13,  pp. 
253-309,  1878. 

IRVING,  R.  D. :  The  Copper-bearing  Rocks  of  Lake  Superior.  U.  S.  Geol. 
Survey  Mon.  5,  1883. 

WADSWORTH,  M.  E. :  Origin  and  Mode  of  Occurrence  of  the  Lake  Supe- 
rior Copper  Deposits.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  27,  pp.  669-696, 
1898 


MINERAL  ASSOCIATIONS  IN  VEINS          261 

They  are  in  pre-Cambrian  diabasic  lava  and  associated  conglom- 
erates which  dip  about  45°  or  less  to  the  northwest  (see  page 
395).  The  diabase  is  extensively  fractured,  and  small  veinlets 
of  copper  ore  are  commonly  developed  in  cracks  near  the  lodes. 
The  larger  part  of  the  ore,  however,  replaces  the  diabase  and 
conglomerate.  The  diabase  lavas  and  conglomerates  are  ex- 
tensively altered  by  hot  waters.  Zeolites,  chlorite  (delessite), 
epidote,  laumontite,  prehnite,  datolite,  adularia,  calcite,  quartz, 
and  other  minerals  are  developed  metasomatically  and  also  line 
vesicles  in  the  lavas.  Some  chalcocite  and  bornite  are  found, 
but  nearly  all  the  copper  is  in  the  native  state.  A  little  silver  is 
present.  Alteration  is  more  intense  in  the  vesicular  or  other 
permeable  beds.  The  massive  centers  of  the  great  lava  flows  are 
fresh,  but  locally  these  also  are  crossed  by  small  veins  of  native 
copper. 

Although  the  deposits  have  been  developed  for  about  a  mile 
below  the  surface,  no  consistent  change  in  the  character  of  the 
mineralization  with  increasing  depth  is  noted.  It  is  believed  that 
the  ores  and  associated  minerals  were  formed  before  the  beds 
were  tilted,  probably  soon  after  the  lavas  were  poured  out  (see 
page  399). 

Chalcedonic  and  Calcitic  Cinnabar  Veins. — Nearly  all  quick- 
silver deposits,  the  world  over,  are  found  in  areas  of  comparatively 
late  igneous  activity,  and  it  is  believed  that  all  the  valuable 
deposits  have  been  formed  relatively  near  the  surface.  In  the 
United  States  the  larger  deposits  are  on  the  Pacific  coast  and  in 
Texas  (see  page  499).  On  the  Pacific  coast  the  quicksilver  ores 
are  found  in  many  different  rocks,  but  the  ores  themselves  have 
characteristic  associations.  The  gangue  minerals  are  quartz, 
chalcedony,  calcite,  barite,  and  gypsum.  In  some  deposits  bitu- 
men is  noted.  The  associated  sulphides  are  cinnabar,  pyrite, 
chalcopyrite,  marcasite,  stibnite,  and  realgar. 

Many  of  the  deposits  are  huge  irregular  masses  formed  in 
brecciated  rock  along  fissures.  Becker1  termed  these  "cham- 

RICKARD,  T.  A.:   "The  Copper  Mines  of  Lake  Superior,"  p.  164,  1905. 

LANE,  A.  C.:  The  Keweenawan  Series  of  Michigan.  Mich.  Geol.  and  Biol. 
Survey,  ser.  4,  vols.  1  and  2,  1911. 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K:  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  52,  pp.  423-426,  573-596, 1911. 

1  BECKER,  G.  F. :  Geology  of  the  Quicksilver  Deposits  of  the  Pacific  Slope. 
U.  S.  Geol.  Survey  Mon.  13,  pp.  410,  472,  1888. 


262      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

bered  veins."  The  solutions  depositing  the  ores  have  apparently 
made  out  from  the  major  fractures,  but  the  details  of  rock  altera- 
tion by  hydrothermal  processes  are  not  available. 

At  Steamboat  Springs,  Nev.,1  on  the  east  front  of  the  Sierra 
Nevada,  6  miles  from  the  Comstock  lode,  are  mines  that  were 
formerly  worked  for  quicksilver.  Mercury  sulphide  and  other 
minerals  are  being  deposited  there  at  the  present  day.  The 
country  rock  is  granite,  which  is  cut  by  dikes  of  granite  porphyry 
and  covered  locally  by  metamorphosed  sedimentary  rocks, 
succeeded  by  andesitic  lavas.  Hot  waters  issue  from  the 
granite,  and  a  flow  of  basalt  near  the  mine  is  altered  by  the 
waters  which  deposited  the  ore.  The  present  vents  of  the 
springs  are  along  a  series  of  fissures  extending  for  about  1  mile. 
Some  of  the  openings  retain  their  sheet-like  character ;  others  are 
pipe-like.  The  granite  is  highly  altered  and  at  some  places 
silicified.  Steam,  hydrogen  sulphide,  carbon  dioxide,  and  sul- 
phur dioxide  escape  from  the  springs  and  fissures.  The  spring 
deposits  contain  sulphides  of  antimony  and  arsenic,  ferric  hy- 
droxide, lead  sulphide,  copper  sulphide,  mercuric  sulphide,  gold, 
and  silver,  together  with  traces  of  zinc,  manganese,  cobalt,  and 
nickel.  The  gangue  minerals  are  quartz,  chalcedony,  opal, 
carbonates,  sulphur,  and  sulphates.  The  temperature  of  the 
water  ranges  from  75°C.  to  84.5°C.  A  sample  collected  by  Becker 

COMPOSITION  OF  WATER  ISSUING  AT  STEAMBOAT  SPRINGS,  NEV. 

Parts  per  million 

FeCO3 0.29 

MgCO, 0.99 

CaCO3 15.77 

Ca3P2O8 1.37 

KC1. ! 97 . 35 

Li2SO4 56.50 

NaCl 1,414.75 

NaHS ' 3.58 

Na2SO4 Ill  .47 

NaHCO3 290 . 23 

Na2CO3 43.14 

Na2B4O7 313.68 

Na,Si4O9. .'. 390.90 

Na2SbS3 1.00 

Na2AsS3 8.66 

A12O3 0.25 

HgS.nNa2S ' '.  . .  Trace 

1  BECKER,  G.  F.:  Op.  cit.,  p.  331. 


MINERAL  ASSOCIATIONS  IN  VEINS          263 

for  analysis  deposited  on  cooling  arsenic  and  antimony  sul- 
phides and  silica  on  the  sides  of  the  vessel.  An  analysis  of  the 
water  and  of  material  deposited  from  it  shows  that  its  probable 
composition  prior  to  oxidation  was  that  indicated  on  page  262. 

The  springs  thus  offer  an  example  of  the  formation  of  fissure 
veins  by  hot  ascending  waters  at  the  present  day.  To  ascertain 
the  source  of  the  metals,  Becker  had  assays  made  of  large  samples 
of  the  granite  through  which  the  waters  pass.  It  was  found  to 
contain  small  quantities  of  several  metals  but  no  quicksilver. 

Lindgren1  visited  Steamboat  Springs  in  1901  and  found  that 
a  shaft  had  been  sunk  30  feet  through  the  deposit  of  sinter. 
At  a  depth  of  25  feet  pebbles  of  andesite  and  granite  were  en- 
countered, with  abundant  hot  water.  Attached  to  the  pebbles 
were  crystals  of  pyrite,  stibnite,  and  opaline  silica. 

At  Sulphur  Bank,  Cal.2  in  a  region  of  late  volcanic  activity, 
where  hot  springs  now  issue,  mercury  sulphide,  pyrite,  and 
chalcedony  are  being  deposited.  The  spring  waters  carry  hydro- 
gen sulphide,  sulphur  dioxide,  and  dissolved  metals.  The 
gases  escaping  at  the  surface  deposit  sulphur,  which  was  for- 
merly exploited. 

Baritic  Fluorite  Veins.  —  Some  baritic  fluorite  veins  are  formed 
very  near  the  surface.  Those  at  Wagon  Wheel  Gap,  Colo.,  and 
at  Ojo  Caliente,  N.  Mex.,  described  under  the  next  heading,  are 
formed  along  fissures  through  which  hot  waters  now  rise.  These 
waters  issue  at  the  surface  and  deposit  much  travertine.  With- 
out much  doubt  these  veins,  like  the  quicksilver  deposits  of 
Steamboat  Springs,  Nev.,  and  Sulphur  Bank,  Cal.,  are  now  in 
process  of  formation.  Both  barite  and  fluorite  are  formed  also 
at  moderate  depths  and  are  noted  in  many  examples  of  ores 
described  above. 

Deposits  Formed  at  the  Orifices  of  Hot  Springs.  —  Deposits 
formed  at  the  orifices  of  hot  springs,  although  of  little  value  as 
sources  of  the  metals,  have  great  interest  in  connection  with  the 
genesis  of  metalliferous  deposits,  for  some  of  them  contain  small 
amounts  of  the  more  valuable  metals.  At  some  of  these  springs 
the  waters  issue  from  fissures  in  which  low-grade  veins  have 


WALDEMAR:  The    Occurrence    of    Stibnite    at    Steamboat 
Springs,  Nevada.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  36,  pp.  27-31,  1905. 

?  LE  CONTE,  JOSEPH,  and  RISING,  W.  B.  :  The  Phenomena  of  Metalliferous 
Vein  Formation  now  in  Progress  at  Sulphur  Bank,  Cal.  Am.  Jour.  Sci., 
3d  ser.,  vol.  24,  pp.  23-33,  1882. 


264      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

been  deposited,  without  much  doubt,  by  solutions  from  the 
sources  of  the  waters  now  issuing  along  the  fissures.  Deposits 
formed  at  the  orifices  of  hot  springs  are  in  a  strict  genetic  sense 
sedimentary  deposits,  but  they  form  a  connecting  link  between 
sediments  and  epigenetic  vein  deposits  and  for  that  reason  they 
are  treated  here.  Hot  springs  are  widely  distributed.  A 
few  are  found  in  regions  of  ancient  volcanic  activity  and  are 
doubtless  fed  by  ground  water  heated  while  traveling  a  deep 
circuitous  route  through  fractures  or  other  openings  in  rocks. 
By  far  the  larger  number  are  found  in  regions  of  late  volcanic 
activity.  These  include  the  springs  having  the  higher  tempera- 
tures, although  some  springs  in  areas  of  late  volcanic  activity 
are  no  hotter  than  springs  in  regions  where  there  are  no  evidences 
of  late  volcanic  action.  In  many  areas  containing  deposits 
formed  at  moderate  and  shallow  depths  hot  springs  are  common. 
They  are  found  near  Creede,  Idaho  Springs,  Dunton,  and  Ouray, 
Colo.;  Boulder,  Mont.;  the  Comstock  lode,  Goldfield,  and  Cornu- 
copia, Nev. ;  and  at  many  other  places.  The  waters  of  many  of 
these  springs1  carry  considerable  concentrations  of  chlorides  and 
carbonates,  with  appreciable  quantities  of  sulphates  and,  in  some, 
borates.  Sodium  and  potassium  are  the  principal  positive 
radicles,  though  lime  and  iron  are  common  also.  Silica  and 
metallic  sulphides  are  present  in  some  hot-spring  waters,  and 
fluorides  are  not  uncommon. 

As  waters  issue  at  points  of  least  pressure,  hot  springs  are 
generally  found  along  the  apexes  of  fissures  at  the  lowest  places, 
or  along  stream  beds  that  they  cross.  Consequently  the  material 
deposited  by  hot  springs  is  generally  mixed  with  loose  rock  or 
other  material  that  accumulates  on  flood  plains  or  along  streams. 
The  materials  deposited  at  the  orifices  of  hot  springs  in  regions 
of  late  volcanic  activity  are  generally  of  simple  composition. 
Limonite,  silica,  and  lime  carbonate  are  most  common.  Silica 
occurs  as  chalcedony  and  also  as  crystalline  quartz,  but  generally 
in  crystals  that  are  smaller  than  those  of  ore  veins.  As  hot 
springs  generally  deposit  abundant  hydrous  iron  oxide  and 
silica,  many  of  these  deposits  resemble  gossans  that  are  formed 
by  the  weathering  of  sulphide  ores,  and  it  is  not  surprising  that 
outcrops  of  sulphide  veins  have  not  infrequently  been  mistaken 
for  hot-spring  deposits.  Those  having  abundant  lime  carbonate, 

1  EMMONS',  W.  H.,  and  HARRINGTON,  G.  L.:  A  Comparison  of  Waters  of 
Mines  and  Hot  Springs.  Earn.  Geol,  vol.  8,  p.  654,  1913. 


MINERAL  ASSOCIATIONS  IN  VEINS 


265 


however,  differ  greatly,  for  lime  carbonates  is  seldom  stable  in 
sulphide  ores  under  conditions  of  weathering  near  .the  surface. 
Gold,  silver,  lead,  zinc,  antimony,  arsenic,  mercury,  and  other 
metals  are  found  in  sinters  and  tufas,  but  rarely  in  workable 
quantities.  Examples  of  hot-spring  deposits  that  carry  metals 
are  numerous;  many  of  them  are  cited  by  Clarke.1 

The  hot  springs  of  Wagon  Wheel  Gap  are  about  7  miles  south- 
east of  Creede,  Colo.,  a  camp  which  has  produced  much  gold, 
silver,  lead,  and  zinc.  The  rocks  are  all  effusives  of  Miocene 
age.2  The  springs  are  on  the  flood  plain  of  Goose  Creek,  above 
which  mountains  rise  abruptly  1,000  feet  or  more.  From  all 
the  springs  bubbles  of  gas  issue  continually,  and  extensive  de- 
posits of  travertine  have  been  formed  on  the  flood  plain,  where 
they  have  mingled  with  the  wash.  Analyses  of  the  waters  of  the 
springs  are  given  below. 

Fig.  126  is  a  sketch  of  the  region  near  the  springs  and  the  slope 


ANALYSES  OP  WATERS  OF  THE  HOT  SPRINGS  OF  WAGON  WHEEL  GAP,  COLO. 

[Recalculated  by  G.  L.  Harrington  to  parts  per  million  from  analyses  made 

at  Colorado  College  in  1904] 


1 

2 

3 

SiO2  

Not  det. 

84 

73 

Fe  

3 

2 

1 

Al  

2 

1 

Trace 

Ca  

46 

66 

17 

Mg  

53 

15 

15 

Na 

347 

393 

360 

K     . 

52 

133 

156 

Li  

8 

11 

9 

COS  

582 

353 

431 

SO4  

161 

457 

135 

Cl  

66 

195 

210 

Total 

1,320 

1,710 

1,407 

"  Carbonic  acid  and  water  in  bicarbonates.  " 
Temperature  

1,062 
135°F. 

788 
150°F. 

1,055 
140°F. 

1  CLARKE,  F.  W. :  The  Data  of  Geochemistry,  3d  ed.  U.  S.  Geol.  Survey 
Bull.  616,  pp.  179-215,  1916. 

2EMMONs,  W.  H.,  and  LARSEN,  E.  S.:  The  Hot  Springs  and  the  Mineral 
Deposits  of  Wagon  Wheel  Gap,  Colorado.  Econ.  Geol.,  vol.  8,  pp.  235-246, 
1913. 


266      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

of  the  high  ridge  which  lies  east  of  them.  This  ridge  is  strewn 
with  many  tons  of  large  barite  crystals,  and  with  float  of  fluorite, 
jasper,  calcite,  and  opal,  sprinkled  liberally  with  pyrite  crystals. 
This  material,  now  oxidized  to  limonite,  carries  0.007  ounce  of 
gold  and  0.283  ounce  of  silver  to  the  ton. 

About  100  feet  east  of  spring  1  a  lode  occupies  a  zone  of  sheeted 
rhyolite  3  feet  or  more  wide.  This  zone  includes  veins  of  barite 
and  fluorite,  and  pyrite  is  found  along  the  veins  in  the  country 
rock,  which  is  highly  altered.  At  600  feet  or  more  above  the 
level  of  Goose  Creek,  in  a  higher  tunnel  about  200  feet  long,  the 
lode  is  well  exposed.  Fluorite  and  barite  are  developed  in  open 
spaces  along  the  fissure,  and  pyrite  is  abundantly  present  in 


FIG.  126.  —  Sketch  showing  position  of  hot  springs  and  associated  mineral 
deposits  of  Wagon  Wheel  Gap,  Colorado. 

the  walls.  Analyses  of  this  material  by  Edmund  Newton, 
of  the  Minnesota  School  of  Mines,  showed  0.03  ounce  of  gold 
and  4.55  ounces  of  silver  to  the  ton. 

A  partial  analysis  of  the  travertine  made  by  George  Steiger 
is  as  follows: 

Per  cent  Per  cent. 

Pb  .................       None  F  ...................       0.220 

ZnO  ................       0.007  Cu  .................       None 

BaO  ................       0.045 

These  veins  are  exploited  for  fluorite,  which  is  used  by  smelters 
at  Pueblo  as  a  flux. 
At  Ojo  Caliente,  N.  Mex.,1  about  50  miles  north  of  Santa  Fe, 


WALDEMAR:  The  Hot  Springs  of  Ojo  Caliente  and  Their 
Deposits.     Econ.  Geol,  vol.  5,  pp.  22-27,  1910. 


MINERAL  ASSOCIATIONS  IN  VEINS          267 

hot  springs  issue  from  andesitic  tuffs  and  sands  which  lie  in  the 
valley  of  a  small  creek  and  above  which  is  reddish  pre-Cambrian 
gneiss.  About  200  feet  above  the  bed  of  the  gulch  a  shaft  was 
sunk  on  a  vein  2  or  3  feet  wide,  filled  with  rock  fragments  loosely 
cemented  by  fluorite  crusts.  The  vein  matter  is  oxidized,  con- 
taining limonite  and  oxide  of  manganese;  the  latter  is  reported 
to  contain  silver.  About  500  feet  southwest  of  the  shaft  and 
directly  in  the  line  of  the  vein,  which  can  be  traced  in  this  direc- 
tion for  200  feet,  is  a  small  hill  about  75  feet  vertically  above  the 
shaft.  The  top  of  this  hill  is  covered  to  the  extent  of  about  half 
an  acre  by  a  tufaceous  hot-spring  deposit.  A  pit  3  feet  deep  is 
sunk  in  a  cellular  mass  composed  of  calcite,  limonite,  and  0.4 
per  cent,  of  fluorite,  with  traces  of  gold  and  silver. 

At  Boulder  Hot  Springs,  Mont.,1  deposits  bearing  metals  are 
now  forming.  The  country  rock  is  granite  (quartz  monzonite), 
a  part  of  the  great  granite  batholith  that  extends  southward 
beyond  Butte.  At  many  places  the  eroded  granite  surface  shows 
numerous  jasper  reefs,  formed,  according  to  Weed,  along  the 
apexes  of  fissures  through  which  hot  solutions  rose.  Some  of 
the  hot  springs  issue  along  such  reefs  at  others  the  water  rises 
from  rocky  debris.  The  hot  waters  carry  soda,  lime,  and  mag- 
nesia, as  chlorides,  sulphates,  and  carbonates,  with  a  little  iron, 
sulphur,  and  hydrogen  sulphide.  Some  of  the  fissures  are  sheeted 
zones  about  6  feet  wide.  The  granite  is  highly  altered  near  the 
fissures,  sericite  and  kaolinite  replacing  feldspar  and  quartz. 
Chalcedony  and  jasperoid  are  developed  in  open  spaces,  to- 
gether with  calcite,  quartz,  and  stilbite;  adularia  was  identified 
in  a  veinlet  near  the  springs.  The  vein  filling  carries  0.05  ounce 
of  gold  and  0.4  ounce  of  silver  to  the  ton;  the  altered  granite 
carries  a  trace  of  gold  and  0.4  ounce  of  silver.  Some  of  the 
altered  granite  is  stained  with  copper,  and  some  of  the  red 
jasperoid  shows  patches  of  crystalline  hematite.  Pyrite  is 
present.  According  to  Weed,  the  metals  and  gangue  minerals 
were  deposited  by  solutions  such  as  are  now  issuing  from  the 
fissures. 

At  Norris  Geyser  Basin,  in  Yellowstone  National  Park,2  where 

1  WEED,  W.  H.:  Mineral-vein  Formation  at  Boulder  Hot  Springs,  Mont., 
U.  S.  Geol.  Survey  Twenty-first  Ann.  Rept.,  part  2,  p.  233,  1900. 

2  WEED,  W.  H.,  and  PIRSSON,  L.  V. :  Occurrence  of  Sulphur,  Orpiment, 
and  Realgar  in  the  Yellowstone  National  Park.     Am.  Jour.  Sci.,  3d  ser.,  vol. 
42,  pp.  401-405,  1901. 


268      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

hot  springs  issue  through  Tertiary  rhyolites,  layers  of  waxy  red 
travertine  and  realgar,  %  inch,  thick,  covered  by  and  alternating 
with  amorphous  orpiment,  are  found  in  siliceous  sinter.  Cleav- 
age planes  indicating  incipient  crystallization  were  identified 
under  the  microscope. 

Near  Anaconda,  Mont.,1  there  is  a  hot-spring  deposit  of 
economic  value.  The  area  is  one  of  tilted  Mesozoic  limestone 
overlain  byrhyolite  tuffs  upon  which  a  travertine  cap  is  deposited. 
Some  of  the  deposit  is  eroded,  but  in  places  old  cones  remain. 
The  waters  now  issuing  have  a  temperature  of  about  100°F. 
In  channels  and  trenches  they  deposit  limonite  which  locally 
replaces  the  travertine.  The  limonite,  according  to  Weed, 
carries  50  cents  to  $1.50  in  gold  to  the  ton  and  has  been  used  for 
flux  at  the  Washoe  plant,  the  gold  being  recovered  by  smelting 
the  travertine  with  copper  ores. 

1  WEED,  W.  H. :  Economic  Value  of  Hot  Springs  and  Hot-spring  Deposits. 
U.  S.  Geol.  Survey  Bull.  260,  p.  601,  1905. 


;.-.:•      CHAPTER  XX 

METALLOGENIC   PROVINCES   AND   METALLOGENIC 
EPOCHS 

Metallogenic  Provinces. — Petrologists  have  long  used  the 
term  " petrographic  province"  for  districts  or  regions  that  con- 
tain bodies  of  igneous  rocks  which,  though  differing  somewhat 
in  composition  and  character,  nevertheless  exhibit  certain  similar 
features  that  indicate  similar  genetic  relations.  Thus  the  batho- 
lithic  intrusive  rocks  of  western  Montana  are  quartz  monzonites; 
those  of  the  Pacific  coast  are  granodiorites;  the  rocks  of  the 
Cripple  Creek  region  in  Colorado  are  phonolites,  syenites,  etc., 
characterized  by  high  percentages  of  alkalies.  Similarly  the 
ore  deposits  in  some  regions  have  certain  characteristics  that 
suggest  nearly  related  genesis.  Thus  the  zinc  and  lead  deposits 
of  southwestern  Wisconsin,  of  eastern  Tennessee,  and  of  south- 
western Missouri  were  all  formed  in  Paleozoic  limestone  and  are 
all  remote  from  igneous  rocks.  Mineralogically  these  deposits 
are  closely  similar,  and,  though  differing  in  structural  relations, 
they  may  be  placed  in  one  group  in  the  interior  zinc-lead  province. 
In  eastern  Ontario  near  Cobalt  there  are  many  small  districts 
that  contain  silver  ores  resembling  those  of  Cobalt,  which 
mineralogically  are  rare  and  distinctive  types.  The  copper, 
nickel,  and  silver  deposits  of  the  Lake  Superior  region  in  general 
may  be  characterized  as  ores  deficient  in  sulphur  or  containing 
less  sulphur  than  the  common  ores  of  these  metals.  The  copper 
ores  of  Keweenaw  Point,  Michigan,  carry  the  native  metal;  the 
nickel-copper  ores  of  the  Sudbury  region,  in  Ontario,  contain 
much  pyrrhotite,  a  sulphide  with  less  sulphur  than  the  com- 
moner iron  sulphide  pyrite.  The  silver  ores  of  Cobalt  and  many 
districts  near  by  are  made  up  largely  of  arsenides  and  native 
metal.  These  deposits,  though  differing  greatly  as  to  struc- 
tural relations  and  origin,  are  all  in  or  near  basic  igneous  rocks. 
The  gold  belt  of  California  may  be  considered  a  metallogenic 
province,  for  its  deposits  are  closely  similar  in  composition  and 
character.  Because  these  deposits  have  been  extensively  eroded, 
many  of  them  have  furnished  the  material  for  enormous  placers. 

269 


270      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

In  the  Southwest,  in  Utah,  Nevada,  Arizona,  and  New  Mexico, 
deposits  of  disseminated  copper  ores  have  been  formed  in  con- 
nection with  intrusions  of  granitic  and  monzonitic  porphyries. 
These  deposits  have  since  been  deeply  eroded,  and  conditions 
have  been  favorable  for  much  secondary  concentration.  With 
respect  to  both  primary  and  secondary  features  they  constitute 
a  well-defined  group. 

The  geologic  processes  that  operate  in  any  region,  such  as 
deformation  and  denudation,  affect  the  deposits  it  contains,  and 
these  deposits  throughout  the  area  in  which  the  processes  were 
operative  may  be  expected  to  show  certain  similar  features. 
Thus  a  great  many  deposits  may  be  grouped  in  a  relatively  small 
number  of  provinces,  and  such  a  grouping  is  found  to  be  useful 
for  study  and  comparison.  On  the  other  hand,  a  relatively  small 
area  may  contain  deposits  of  totally  different  character  formed 
at  different  times  or  even  in  the  same  geologic  epoch.  The 
regional  grouping  is  useful,  however,  if  difference  of  age  and 
origin  of  deposits  are  properly  recognized. 

Metallogenic  Epochs.  —  A  metallogenic  epoch  is  a  division  of 
geologic  time.  Ore  deposits  have  been  forming  since  the  earliest 
periods  recorded  in  the  rocks,  and  they  are  forming  today,  but 
they  have  not  formed  at  an  equal  rate  throughout  the  geologic 
ages.  Just  as  they  have  been  concentrated  regionally  in  provinces 
and  subprovinces,  so  also  they  have  formed  more  abundantly 
during  one  period  or  another.  Iron  deposits  were  formed  most 
abundantly  in  North  America  in  pre-Cambrian  time,  especially 
in  the  later  Huronian  epoch,  but  they  formed  abundantly  also 
in  the  Clinton  epoch  of  Silurian  time  and  less  abundantly  in  the 
Devonian,  Carboniferous,  and  later  periods.  Gold  deposits1 
were  formed  in  the  pre-Cambrian,  early  Cretaceous,  early  Ter- 
tiary, and  middle-late  Tertiary  epochs.  Silver  deposits  also 
were  abundantly  formed  in  pre-Cambrian  and  especially  in  Ter- 
tiary time.  Many  copper  deposits  were  formed  in  the  United 
States  in  pre-Cambrian,  Cretaceous,  and  early  Tertiary  time, 
but  only  sparingly  in  the  middle  and  late  Tertiary.  The  age 
relations  of  deposits  of  the  principal  metals  are  mentioned  in 
the  following  chapters,  where  deposits  of  the  metals  are  treated 
separately. 


WALDEMAR:  The  Geological  Features  of  the  Gold  Production 
of  North  America.  Am.  Inst.  Min.  Eng.,  Trans,  vol.  33,  pp.  790-845, 
1903;  Metallogenetic  Epochs.  Econ.  Geol,  vol.  4,  pp.  409-420,  1909. 


CHAPTER  XXI 

COMPOSITION  AND  SOURCES  OF  ASCENDING 
THERMAL  METALLIFEROUS  WATERS 

Chemical  Composition. — Investigation  of  the  composition  of 
the  metalliferous  deposits  and  of  the  nature  of  hydrothermal 
alteration  along  fissures  should  throw  some  light  on  the  com- 
position of  the  metalliferous  solutions  that  have  deposited 
ores.  The  hot  waters  circulating  in  the  fissures  have  deposited, 
besides  the  heavy  metals,  potash,  boron,  fluorine,  carbonates, 
sulphates,  and  other  compounds.  Many  regions  where  igneous 
rocks  have  lately  been  intruded  contain  hot  springs.  Nearly 
all  these  springs  carry  alkali  chlorides,  and  many  carry  carbonates 
and  sulphates.  A  few  carry  also  boron  and  fluorine;  at  some  of 
them  hydrogen  sulphide  and  carbon  dioxide  gas  escape.  Some 
of  these  hot  springs  issue  from  fissures  where  ores  are  probably 
being  deposited  today,  and  it  is  a  warranted  assumption  that  the 
waters  of  the  hot  springs  are  closely  similar  to  the  residues  of 
solutions  such  as  have  deposited  metalliferous  veins.  The 
average  composition  of  58  hot  springs  in  regions  of  volcanic 
rocks  is  stated  below,  where  they  are  compared  with  mine  waters 
and  also  with  waters  of  hot  springs  in  areas  where  there  is  no 
evidence  of  recent  volcanism.1 

Ascending  hot  waters  vary  considerably  in  composition,  and 
the  wall-rock  alterations  they  accomplish  likewise  show  great 
variations.  Yet,  on  reviewing  all  these  alterations  and  the 
analyses  of  the  waters  of  hot  springs  associated  with  igneous 
activity,  the  conclusion  is  justified  that  ascending  thermal  waters 
in  general  are  complex  solutions,  containing  varying  amounts 
of  sodium,  potassium,  alkaline  earths,  and  heavy  metals  dis- 
solved as  chlorides,  carbonates  and  sulphides,  and  subordinately 

1  EMMONS,  W.  H.,  and  HARRINGTON,  G.  L. :  A  Comparison  of  Waters  of 
Mines  and  Hot  Springs.  Econ.  Geol,  vol.  8,  pp.  653-659,  1913. 

271 


272      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


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THERMAL  METALLIFEROUS  WATERS 


273 


A.  Average  of  58  analyses  of  hot  springs  in  areas  of  comparatively  late 
volcanic  activity. 

B.  Average  of  17  analyses  of  hot  springs  in  areas  of  comparatively  remote 
volcanic  actiyity. 

C.  Average  of  42  analyses  of  mine  waters. 

1.  Parts  per  million. 

2.  Percentage  of  dissolved  material. 

3.  Results  obtained  by  dividing  weight  of  ion  by  its  valence  and  parts  per 
million  by  the  quotient. 

4.  Reacting  values  of  columns,  3,  A,  B,  C,  in  appropriate  chemical  groups. 

5.  Reacting  values  reduced  to  percentages  of  halogens,  etc.,  and  of  metals. 


ANALYSES  OF  HOT  SPRINGS  AND  MINE  WATERS. — (Continued) 


Reacting  values 

Percentages 

Hot 
springs 
(A) 

Hot 

springs 

Mine 
waters 

Hot 
springs 

Hot 
springs 

Mine 
waters 

Alkalies 

48.15 
6.89 
3  12 

0.416 
5.065 

3.53 
100.48 
60.82 
158.19 

2.77 

7.06 
200.96 
108  36 

64.0 

7.6 
12.4 

2.2 
63.5 
34.3 

100.0 

Alkali  earths  and  other  metals 
Hydrogen  

Strong  acids  
Weak  acids  

35.24 
21.98 

70.48 

25.82 
13.78 

1.095 
4.135 

0.832 
1.358 

Primary  salinity 

Secondary  salinity  
Tertiary  salinity  
Primary  alkalinity  
Secondary  alkalinity  

Alkalinity  

8.772 

23.5 
12.5 

80.0 

110.08 

10.962 

316.38 

100.0 

100.0 

39.60 

8.770 

108.36 

Acidity  

as  boron  and  fluorine  compounds  and  probably  sulphates.  These 
substances  are  present  in  different  proportions  in  different  solu- 
tions, and  they  accomplish  different  results,  even  under  nearly 
similar  conditions.  Probably  no  two  deposits  formed  by  such 
waters  have  exactly  the  same  composition,  and  a  single  deposit 
may  differ  greatly  in  different  parts.  Each  type,  however,  grades 
into  other  types.  Like  igneous  rocks,  they  are  not  set  off  by 
sharp  divisions,  each  differentiated  from  all  others.  That  solu- 
tions of  compositions  suggested  by  the  analyses  given  above  are 

18 


274      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

capable  of  depositing  ores  of  the  various  metals  is  evident  from  a 
review  of  the  chemical  relations  of  the  metals.  A  chloride  solu- 
tion will  dissolve  iron,  copper,  gold,  lead,  zinc,  and  mercury,  as 
well  as  antimony  and  arsenic.  Silver  is  soluble  in  a  concentrated 
alkaline  chloride  solution.  Thus  a  chloride  solution  would  itself 
be  capable  of  transporting  all  these  metals. 

Carbonate  solutions  with  excess  carbon  dioxide  will  dissolve 
iron,  copper,  lead,  silver,  and  zinc.  Gold  is  not  dissolved  in 
carbonate  solutions  unless  alkaline  sulphides  or  chlorides  are 
present. 

A  solution  of  alkali  sulphide  will  dissolve  iron,  gold,  lead,  zinc, 
and  mercury.  Antimony  and  arsenic  also  are  very  soluble  in 
alkali  sulphide  solution,  and  in  a  concentrated  solution  copper 
is  dissolved.  The  presence  of  antimony  and  arsenic  in  an  alkali 
sulphide  solution  increases  the  solubility  of  copper. 

Ascending  hot  waters  probably  carry  abundant  gases.  Gases 
bubble  up  at  many  hot  springs,  and  in  some  regions  of  recent 
volcanism  mine  workings  are  filled  with  gas.  Carbon  dioxide  is 
commonly  present,  and  hydrogen  sulphide  escapes  from  some 
springs.  These  gases  tend  to  keep  certain  metals  in  solution. 
When  they  escape  such  metals  are  thrown  down.  The  escape 
of  gases,1  together  with  cooling,  decrease  of  pressure,  partial 
oxidation,  dilution,  etc.,  favors  the  precipitation  of  metals  in 
the  upper  regions  of  the  earth's  crust.  Without  doubt  the  metals 
are  deposited  most  abundantly  within  a  zone  not  more  than  2 
or  3  miles  deep,  and  generally  less — in  the  upper  part  rather  than 
the  lower  part  of  the  "zone  of  fracture"  (see  page  100).  In 
some  deposits  carrying  bonanzas  of  gold  and  silver  the  maximum 
deposition  appears  to  have  taken  place  probably  not  more  than 
2,000  feet  below  the  surface  at  the  time  of  deposition. 

Sources  of  Metalliferous  Thermal  Waters.— Epigenetic  ores 
have  been  grouped  in  six  classes.  They  are  deposited  by  aqueous 
solutions,  hot  and  cold.  Pegmatites  and  contact-metamorphic 
deposits  are  formed  by  solutions,  either  liquid  or  gaseous,  that 

ITOLMAN,  C.  F.,  JR.,  and  CLARK,  J.  D.:  The  Oxidation,  Solution,  and 
Precipitation  of  Copper  in  Electrolytic  Solutions  and  the  Dispersion  and 
Precipitation  of  Copper  Sulphides  from  Colloidal  Suspension,  with  a  Geo- 
logical Discussion.  Econ.  Geol.,  vol.  9,  pp.  559-592,  1914. 

SIEBENTHAL,  C.  E. :  Origin  of  the  Lead  and  Zinc  Deposits  of  the  Joplin 
Region,  Missouri,  Kansas,  and  Oklahoma.  U.  S.  Geol.  Survey  Bull.  606, 
pp.  33-81,  1915. 


THERMAL  METALLIFEROUS  WATERS          275 

have  emanated  from  cooling  magmas.  Ore  bodies  formed  by  cold 
solutions  are  deposited  mainly  by  waters  of  meteoric  origin — 
that  is,  by  rain  water  that  has  sunk  into  the  ground.  The  three 
classes  of  vein  deposits — those  formed  in  the  deep  zone,  at 
moderate  depths,  and  at  shallow  depths — are  supposed  to  have 
been  deposited  by  hot  solutions,  because  such  deposits  are  essen- 
tially confined  to  areas  where  igneous  rocks  are  present,  and 
along  them  the  wall  rocks  show  characteristic  alterations  unlike 
the  changes  that  are  known  to  result  from  weathering.1  Such 
changes  are  assumed  to  have  been  accomplished  by  hydrothermal 
processes  (see  pages  230-263). 

Many  deposits  of  this  character  have  been  formed  below  im- 
pervious beds,  as  if  the  solutions  that  deposited  them  had 
been  halted  in  their  upward  journey  by  those  beds.  Others 
extend  downward  for  thousands  of  feet.  They  do  not  show  the 
zonal  arrangement  that  characterizes  many  ore  bodies  formed  or 
altered  by  descending  cold  solutions.  For  these  and  other 
reasons  the  solutions  that  deposited  them  are  supposed  to  have 
been  ascending. 

The  sources  of  such  hot  ascending  solutions  have  been  the 
subject  of  much  controversy.  Three  hypotheses  may  be  stated: 
(1)  The  deposits  of  the  several  vein  zones  may  have  been  de- 
posited by  meteoric  waters;  (2)  they  may  have  been  deposited 
by  magmatic  waters  or  by  mixtures  of  meteoric  and  magmatic 
waters;  or  (3)  some  may  have  been  deposited  by  meteoric  and 
others  by  magmatic  waters. 

The  third  hypothesis  has  not  met  with  much  favor,  probably 
because  the  most  valuable  lode  deposits — those  of  the  precious 
metals,  copper,  mercury,  etc. — exhibit  certain  features  so  nearly 
similar  that  any  hypothesis  that  is  assumed  to  explain  the  origin 
of  one  must  be  considered  as  a  possible  explanation  of  the  others. 

The  first  hypothesis  assumes  that  the  bulk  of  the  ores  of  the 
three  vein  zones  have  been  formed  by  ascending  meteoric  waters 
and  that  magmatic  waters  have  little  or  no  part  in  their  genesis. 
This  hypothesis,  which  was  accepted  by  Posepny,2  was  urged 
by  him  as  opposing  the  hypothesis  of  "lateral  secretion"  ad- 

1  STEIDTMANN,   EDWARD:  A  Graphic  Comparison  of  the  Alteration  of 
Rocks  by  Weathering  with  Their  Alteration  by  Hot  Solutions.     Econ.  Geol., 
vol.  3,  pp.  381-399,  1908.     STEPHENSON,  E.  A. :   Studies  in  Hydrothermal 
Metamorphism.     Jour.  Geol,  vol.  14,  pp.  180-199,  1916. 

2  POSEPNY,  FRANZ  :  The  Genesis  of  Ore  Deposits.     Am.  Inst.  Min.  Eng. 
Trans.,  vol.  23,  pp.  197-369,  1893. 


276      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

vanced  by  Sandberger,1  who  maintained  that  the  metals  were 
dissolved  from  rocks  near  by — essentially  from  those  that  formed 
the  vein  walls — and  concentrated  in  the  lodes  by  movement  of 
ground  water  to  the  master-  fissures  in  which  the  ore  was  precipi- 
tated. The  conception  of  the  deeper  meteoric  circulation  was 
developed  more  comprehensively  by  Van  Hise.2  T.  A.  Rickard3 
once  compared  the  circulation  of  water  to  that  which  takes  place 
in  a  house  that  is  heated  by  hot  water.  He  argued  that  the 
solutions  sank  because  they  were  cold  and  after  becoming  heated 
rose  because  they  were  pushed  up  by  colder  and  therefore  heavier 
columns  of  water.  LeConte4  also  elucidated  this  theory.  Becker5 
had  adopted  it  to  explain  the  genesis  of  the  Comstock  lode  and 
of  the  Pacific  coast  quicksilver  deposits.  Becker  has  discussed 
the  chemistry  of  the  hypothesis  at  some  length.  He  urged  that 
descending  waters,  probably  carrying  sulphates,  come  into  regions 
of  higher  temperature,  where  they  meet  reducing  substances. 
It  has  been  stated  frequently  that  carbon  will  reduce  sulphates 
to  sulphides.  Although  this  has  not  been  accomplished  experi- 
mentally at  temperatures  such  as  prevail  near  the  surface  of  the 
earth,  reduction  by  hydrocarbon  compounds  is  a  possible  reac- 
tion at  temperatures  well  above  100°.  Meteoric  waters,  warmed 
in  depth,  might  actively  dissolve  metals  and  carry  them  as  sul- 
phides, chlorides,  and  carbonates.  In  depths  where  rocks  are 
alkaline  the  solutions  would  become  alkaline.  It  is  not  unlikely 
that  the  alkalies  would  become  concentrated  under  the:  3  con- 
ditions and  that  they  would  dissolve  certain  metals  present  in 
rocks  which  they  traversed.  Siebenthal  has  shown  that  solu- 
tions charged  with  carbon  dioxide  under  pressure  will  dissolve 
metallic  sulphides  and  generate  hydrogen  sulphide.  When 
the  pressure  is  released,  the  carbon  dioxide  and  hydrogen  sul- 

1  SANDBERGER,  FRIDOLIN  VON:  tlber  die  Bildung  von  Erzgangen  mittelst 
Auslaugung  des  Nebengesteins.     Berg  u.  Huttenm.  Zeitung.,  vol.  39,  pp.  329- 
331,  337-339,  390-392,  402-405,  1880. 

2  VAN  HISE,  C.  R. :  Some  Principles  Controlling  the  Deposition  of  Ores. 
Am.  Inst.  Min.  Bag.  Trans.,  vol.  30,  pp.  27-177,  1900. 

s  RICKARD,  T.  A.:  The  Genesis  of  Ore  Deposits;  discussion  of  Posepny's 
paper.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  24,  p.  950,  1894. 

*LE  CONTE,  JOSEPH:  The  Genesis  of  Ore  Deposits;  discussion  of  Po- 
sepny's paper.  Am.  Inst.  Min.  Eng.,  Tra-s.,  v  1.  24,  pp.  996,  1006,  1894. 

8  BECKER,  G.  F.:  Geology  of  the  Comstock  Lode  and  the  Washoe  Dis- 
trict. U.  S.  Geol.  Survey  Mon.  3,  1882;  Geology  of  the  Quicksilver  Deposits 
of  the  Pacific  Slope.  U.  S.  Geol.  Survey  Mon.  13,  pp.  449-450,  1881. 


THERMAL  METALLIFEROUS  WATERS          277 

phide  escape  and  the  metallic  sulphides  are  deposited  (see  page 
479).  It  is  well  established  also  that  many  igneous1  and  sedi- 
mentary2 rocks  contain  small  amounts  of  lead,  zinc,  and  some 
other  metals.  So  far  as  the  chemistry  of  the  process  is  concerned, 
there  is  nothing  inherently  improbable  in  the  hypothesis  that 
the  ascending  metalliferous  thermal  waters  are  of  meteoric 
origin. 

This  hypothesis  assumes  that  heat  from  the  intruding  magma 
warms  the  descending  column  of  cold  water  and  causes  it  to 
rise  because  it  is  hotter  and  therefore  lighter.  The  heat  that  is 
generated  by  a  great  mass  of  molten  magma  is  doubtless  sufficient 
to  stimulate  such  circulation  through  a  long  period  if  openings 
are  available  and  favorably  spaced.  Thus,  there  is  likewise 
nothing  inherently  impossible  with  respect  to  the  physics  of  the 
processes  on  which  this  hypothesis  relies. 

In  the  geologic  relations  of  the  deposits  this  hypothesis  finds 
its  most  serious  difficulties.  These  are  discussed  below. 

1.  Underground  observations  in  deep  mines  show  that  the 
deep-water  circulation  is  much  less  than  it  was  once  supposed 
to  be.  Many  deep  mines  are  dry.3  There  is  a  tightening  of 
the  ground  a  few  hundred  feet  "below  the  surface,  and  the  deeper 
circulation,  except  in  porous  sediments  and  where  fracturing  is 
pronounced,  is  very  sluggish  (see  page  133). 

It  may  be  urged  that  during  periods  of  intrusion  the  rocks 
surrounding  the  intrusive  masses  are  shattered,  and  that  meteoric 
waters  circulate  more  freely  at  such  periods  because  openings 
are  more  numerous.  These  openings  are  formed  at  an  opportune 
time — when  circulation  is  made  active  by  the  stimulus  of  heat 
derived  from  igneous  rocks  near  by.  The  fact  that  the  deposits 
at  considerable  depths  are  dry  when  mines  are  opened,  it  has 

1  EMMONS,    S.    F. :   Geology  and   Mining  Industry  of  Leadville,   Colo. 
U.  S.  Geol.  Survey  Mon.  12,  pp.  574-584,  1886. 

ROBERTSON,  J.  D.,  in  WINSLOW,  ARTHUR:  Lead  and  Zinc  Deposits.  Mo. 
Geol.  Survey,  vol.  7,  pp.  479-481,  1894. 

2  CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.     U.  S.  Geol.  Survey 
Bull  616,  pp.  627-631,  1916. 

3  KEMP,  J.  F. :  The  R61e  of 'the  Igneous  Rocks  in  the  Formation  of  Veins, 
in  POSEPNY,  FRANZ:  "The  Genesis  of  Ore  Deposits,"  pp.  681-809,  1902. 

FINCH,  J.  W.:  The  Circulation  of  Underground  Aqueous  Solutions  and 
the  Deposition  of  Lode  Ores.  'Colo.  Sci.  Soc.  Proc.,  vol.  7,  pp.  193-202,  1904. 

RICKARD,  T.  A. :  Waters,  Meteoric  and  Magmatic.  Min.  and  Sci.  Press, 
ol.  95,  pp.  872-875,  1908. 


278      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

been  argued,  is  inconclusive  because  the  openings  in  the  rocks 
may  have  been  filled  by  the  mineralizing  solutions  themselves, 
and  the  cementation  of  openings  may  have  rendered  the  ground 
surrounding  the  deposits  tight  or  impervious.  However,  in 
many  regions  little  evidence  of  cementation  of  openings  on  a 
large  scale  in  the  country  rock  near  lode  deposits  can  be  found. 

An  example  of  sluggish  circulation  is  disclosed  by  developments 
at  Cripple  Creek,  Colo.  The  central  feature  of  the  district  is  a 
mass  of  volcanic  breccia  filling  the  throat  of  an  ancient  volcano 
and  inclosed  in  granite  and  schists.1  The  breccia  is  highly  porous 
and  soaked  with  water,  whereas  the  surrounding  rocks  are  prac- 
tically impervious.  Lindgren  and  Ransome  have  compared 
the  volcanic  complex  to  a  "sponge  in  a  cup."  The  conditions 
for  circulation  outside  of  the  volcanic  throat  are  unfavorable. 
If  the  Cripple  Creek  deposits  have  been  formed  through  the 
agency  of  meteoric  waters,  these  must  have  been  derived  from 
the  surrounding  granite;  they  could  not  have  been  derived  from 
the  very  small  body  of  the  volcanic  plug.  If  such  waters  were 
effective  when  the  ores  were  formed,  they  must  have  descended 
to  great  depths2  near  the  granite  mass  and  have  been  driven  up 
by  volcanic  heat  in  and  near  it.  But  considering  the  impermeable 
character  of  the  granite  area,  this  hypothesis,  according  to 
Lindgren  and  Ransome,  is  untenable. 

2.  If  ascending  hot  metal-bearing  waters  are  assumed  to  be 
meteoric  waters  that  have  gathered  their  Ibad  of  metals  from  the 
rocks  they  traversed,  and  if  they  deposit  those  metals  in  and  near 
igneous  rocks  because  they  are  stimulated  to  ascend  by  heat 
from  such  rocks,  it  might  be  supposed  that  all  intrusive  bodies 
would  similarly  stimulate  the  deep  circulation.  But  a  great 
many  intrusive  rocks  are  practically  barren  of  ores,  as  are  also 
the  rocks  that  surround  them.  This  feature  of  deposition  is 
illustrated  in  many  mining  districts.  At  Tonopah,  Nev.,3  there 


1  CROSS,  WHITMAN,  and  PENROSE,  R.  A.  F.,  JR.  :  Geology  and  Mining 
Industries    of  the  Cripple  Creek  District,  Colorado.     U.  S.  Geol.  Survey 
Sixteenth  Ann.  Rept.,  part  2,  pp.  1-209,  1895. 

LINDGREN,  W  \LDEMAR,  and  RANSOME,  F.  L. :  Geology  and  Gold  Deposits 
of  the  Cripple  Creek  District,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper  54, 
1906. 

2  LINDGREN,  WALDEMAR,  and  RANSOME,  F.  L. :  Op.  cit.,  p.  227. 
3SptJRR,  J.  E.:  Geology  of  the  Tonopah  Mining  District.     U.  S,  Geol. 

Survey  Prof.  Paper  42,  1905. 


THERMAL  METALLIFEROUS  WATERS          279 

are  many  igneous  intrusives,  but  the  valuable  ore  bodies  were 
formed  only  in  connection  with  one  rock  unit,  the  early  andesite 
(later  called  Mizpah  trachyte).1  At  Goldfield,  Nev.,2  there  are 
likewise  many  intrusive  and  extrusive  igneous  bodies,  but  all  the 
valuable  ore  bodies  are  in  or  near  dacite  or  andesite.  At  Butte, 
Mont.,3  the  copper  ores  are  found  in  the  region  that  has  been 
intruded  by  quartz  porphyry.  In  several  other  districts  which 
lie  within  the  great  granite  mass  that  contains  the  Butte  deposits 
but  which  yield  no  copper,  the  major  events  of  geologic  history 
have  been  broadly  similar  to  those  at  Butte,  except  that  no  quartz 
porphyry  has  been  intruded.  It  is  inferred,  therefore,  that  there 
is  a  genetic  connection  between  the  quartz  porphyry  dikes  at 
Butte  and  the  copper  ores  and  that  they  may  both  be  derived 
from  the  same  parent  igneous  mass. 

In  these  and  in  many  other  districts  ore  deposits  are  restricted 
to  places  in  and  around  certain  intrusives  to  which  they  are 
genetically  related,  and  they  were  deposited  in  relatively  brief 
geologic  epochs,  presumably  during  the  cooling  of  certain  magmas 
that  supplied  material  for  the  rocks  with  which  they  are  asso- 
ciated. It  is  not  inferred  that  the  deposits  were  formed  by  solu- 
tions that  emanated  from  the  rocks  immediately  associated 
with  them.  Many  of  the  deposits  are  veins  traversing  such  rocks; 
they  are  commonly  referred  to  a  deeper  magmatic, source — a 
parent  mass  perhaps  not  yet  exposed  by  erosion,  a  mass  that  sup- 
plied not  only  the  material  for  dikes  and  associated  igneous  rocks 
but  also  the  solutions  that  deposited  the  ores  which  cut  these 
rocks  and  are  grouped  around  them. 

The  igneous  history  of  the  Georgetown  region,  Colorado,  is 
very  complicated.  There  are  many  dikes  of  different  character, 
and  the  ores  appear  to  be  genetically  related  to  them,  certain 
types  of  ores  occurring  in  regions  where  dikes  of  certain  character 
predominate.4  Many  additional  examples  similarly  suggest  a 
local  relation  of  certain  types  of  ores  to  certain  types  of  magmas. 

1  SPURR,  J.  E. :  Geology  and  Ore  Deposits  of  Tonopah,  Nevada.     Econ. 
Geol,  vol.  10,  p.  719,  1915. 

2  RANSOME,  F.  L. :  Geology  and  Ore  Deposits  of  Goldfield,  Nevada.     U.  S. 
Geol.  Survey  Prof.  Paper  66,  1909. 

3 SALES,  RENO:  Ore  Deposits  at  Butte,  Montana.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  46  pp.  80,  81,  1914. 

4  SPURR  J  E.,  GARRET,  G.  H.,  and  BALL,  S.  H.:  Economic  Geology  of 
the  Georgetown  Quadrangle,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper 
63,  1908. 


280      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

A  very  common  sequence  of  events1  in  mining  districts  is  in- 
trusion, fissuring  and  faulting,  ore  deposition. 

3.  In  some  districts  several  types  of  ores  show  a  rude  zonal 
arrangement.     At  Butte  there  is  a  central  area  of  copper  ores 
containing  chalcocite,  enargite,  pyrite,  and  quartz.     This  area 
is  surrounded  by  a  belt  of  copper  ores  with  which  sphalerite  and 
manganese  minerals  are  practically  everywhere  associated,  and 
this  belt  in  turn  is  surrounded  by  a  belt  of  manganiferous  zinc 
ores,  some  of  which  contain  much  silver.2    At  Tintic,  Utah,3 
also,  there  are  belts  of  mineralization  related  to  a  central  mass 
of  intrusive  monzonite  (see  page  463) ,  and  in  some  other  districts 
a  similar  arrangement  of  types  of  ores  with  respect  to  certain 
intrusives  is  indicated.     The  theory  that  hot  meteoric  waters 
have  deposited  ores  that  are  so  arranged  is  not  satisfactory.     It 
is   difficult   to  harmonize  such  great  differences  as  must  have 
existed  in  the  waters  that  deposited  the  different  ores  if  the  waters 
are  assumed  to  have  derived  their  metal  content  within  areas 
so  small  as  the  mineralized  areas  above  cited,  especially  as  the 
meteoric  solutions  must  have  traversed  rocks  of  essentially  the 
same  character  during  the  period  when  it  is  assumed  that  they 
were  gathering  the  metals  from  the  country  rocks.     On  the  other 
hand,  this  zonal  arrangement  might  readily  be  produced  by 
progressive  precipitation  from  a  magmatic  center. 

4.  In  many  districts  veins  having  essentially  the  same  com- 
position are  found  in  rocks  of  various  kinds.     At  Philipsburg, 
Mont.,  silver  deposits  of  similar  character  are  found  in  veins  in 
granite  and  crossing  shales  and  limestones.     Except  in  the  lime- 
stones, where  carbonates  are  more  abundant,  the  veins  carry 
similar  ores.4    If  the  vein  stuff  represents  material  that  was 
dissolved  out  of  the  surrounding  rocks  and  deposited  in  fissures, 
there  should  be  differences  in  the  vein  stuff  corresponding  to 
differences  in  the  composition  of  the  surrounding  rocks.     Thus 
it  appears  that  in  some  districts  the  solutions  which  are  assumed 


1  SPUBR,  J.  E. :  The  Relation  of  Ore  Deposition  to  Faulting.     Econ.  Geol, 
vol.  11,  pp.  601-622,  1916. 

2  SALES,  RENO:  Op.  cti.,  p.  58. 

3LiNDGREN,  WALDEMAR:  Processes  of  Mineralization  and  Enrichment 
in  the  Tintic  Mining  District.  Econ.  Geol,  vol.  10,  pp.  225-240,  1915. 

4  EMMONS,  W.  H.,  and  CALKINS,  F.  C.:  Geology  and  Ore  Deposits  of  the 
Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper  78, 
p.  187,  1913. 


THERMAL  METALLIFEROUS  WATERS          281 

to  have  traversed  rocks  of  the  same  character  have  deposited 
different  ores,  and  in  other  districts  the  solutions  which  are 
assumed  to  have  traversed  rocks  of  different  character  have 
deposited  similar  ores.  Evidently  the  composition  of  the  ore- 
depositing  solutions  in  many  districts  was  substantially  inde- 
pendent of  the  rocks  they  traversed. 

5.  The  ore  deposits  in  many  districts  are  grouped  around 
intrusive  masses  of  certain  composition  and  character,  whereas 
other  intrusive  masses  near  by  are  barren  of  ores.  In  some 
districts  the  ores  occur  around  certain  parts  of  igneous  masses. 
Over  large  areas  the  relations  are  so  common  and  so  consistent 
that  it  is  not  logical  to  account  for  them  by  mere  coincidence. 

In  Utah1  ore  deposits  are  widely  distributed.  The  larger 
intrusive  bodies  are  laccoliths  and  stocks.  Of  the  stocks  some 
have  been  deeply  eroded  or  truncated  near  their  bases;  others 
have  had  merely  their  apexes  or  tops  removed.  The  apically 
truncated  stocks  are  monzonitic  or  dioritic  in  composition;  the 
basally  truncated  stocks  are  more  siliceous,  ranging  from  grano- 
diorite  to  granite.  According  to  Butler  the  deposits  associated 
with  the  laccoliths  and  basally  truncated  stocks  are  commercially 
unimportant,  but  those  associated  with  apically  truncated  stocks 
have  great  value.  Butler  believes  that  the  absence  of  large 
deposits  associated  with  laccoliths  is  to  be  attributed  to  the  fact 
that  after  intrusion  the  magmas  of  laccoliths  were  sealed  off 
from  the  deep-seated  sources.  Great  ore  deposits  are  rarely 
associated  with  small  laccoliths,  because  the  amounts  of  metals 
in  the  magmas  that  form  laccoliths  are  insufficient  and  differen- 
tiation in  them  is  incomplete.  In  Utah  differentiation  in  stocks, 
according  to  Butler,  was  greater.  The  mobile  constituents  of 
the  magmas — particularly  water  carrying  metals  in  solution — • 
rose  toward  the  surface,  while  the  heavier  minerals  that  were 
first  formed  sank  to  greater  depths,  rendering  the  magmas  more 
siliceous.  When  the  mobile  constituents  arrived  at  places  where 
the  magmas  had  solidified  and  fractured,  they  were  guided  out- 
ward by  the  fissures  and  on  reaching  favorable  places  the  metals 
they  carried  were  deposited.  Butler  regards  the  more  deeply 
truncated  stocks  as  remnants  of  larger  stocks  from  which  the  more 
highly  mineralized  portions  have  been  eroded. 

In  the  gold-bearing  belt  of  California,  as  pointed  out  by  Lind- 

1  BUTLER,  B.  S. :  Relation  of  Ore  Deposits  to  Different  Types  of  Intrusive 
Bodies  in  Utah.  Econ.  Geol.,  vol.  10,  pp.  101-122,  1915. 


282      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

gren,1  the  gold  deposits  are  nearly  all  located  in  metamorphic, 
igneous,  and  sedimentary  rocks  around  the  edges  of  the  great 
granodiorite  batholith  which  forms  the  high  Sierra  and  in  smaller 
outlying  and  doubtless  related  intrusive  bodies  of  granodiorite 
(see  page  430).' 

6.  Pegmatite  veins  are  products  of  the  crystallization  of  the 
more  mobile  constituents  of  molten  magmas  —  constituents  that 
are  precipitated  at  lower  temperatures  than  the  bulk  of  the  mag- 
matic  materials.     In  some  regions  pegmatites  composed  of  feld- 
spar and  quartz  pass  by  decrease  of  feldspar  into  quartz  veins. 
In  the  Yukon  district,  Alaska,2  quartz-feldspar  (alaskite)  dikes 
pass  gradually  into  quartz  veins  by  decrease  of  feldspar.     All 
changes  may  be  noted  in  a  single  dike  or  vein.     The  quartz  veins 
contain  pyrite  and  calcite,  and  similar  veins  near  by  carry  gold. 
At  Silver  Peak,  Nev.,3  alaskite  and  gold  quartz  veins  form  two 
ends  of  a  differentiation  series  between  which  rock  types  showing 
all  gradations  are  represented.     This  subject4  is  discussed  briefly 
on  page  26. 

7.  For  many  reasons,  already  stated,  it  is  believed  that  contact- 
metamorphic  deposits  are  formed  by  magmatic  solutions.5    In 
some  regions  lode  deposits  grade  mineralogically  into  contact- 
metamorphic   deposits  and  become  more  numerous  and  more 
typically  developed  away  from  the  contacts.     Such  relations 
are  shown  in  the  Coeur  d'Alene  district,  Idaho.     There,  accord- 
ing to  Ransome,6  the  ores  of  the  Granite  and  Sixteen  to  One 
mines,  which  are  in  the  contact  zone  encircling  one  of  the  largest 
monzonitic  masses,  contain  garnet,  biotite,  diopside,  and  quartz, 
with  magnetite,  pyrrhotite,  galena,  sphalerite,  and  pyrite,  but 
no  siderite.     Near  the  contacts  are  found  veins  containing  pyr- 


,  WALDEMAR:  Characteristics  of  the  California  Gold-quartz 
Veins.  Geol.  Soc.  America  Bull.  vol.  6,  pp.  221-240,  1896. 

2  SPURR,  J.  E.  :  Geology  of  the  Yukon  Gold  District,  Alaska.  U.  S.  Geol. 
Survey  Eighteenth  Ann.'Rept.,  part  3,  p.  312,  1898. 

3SPURR,  J.  E.:  Ore  Deposits  of  the  Silver  Peak  Quadrangle,  Nevada. 
U.  S.  Geol.  Survey  Prof.  Paper  55,  1906. 

4  See  also  SPURR,  J.  E.  :  A  Consideration  of  Igneous  Rocks  and  Their 
Segregation  and  Differentiation  as  Related  to  the  Occurrences  of  Ores. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  33,  pp.  288-340,  1902. 

6LiNDGREN,  WALDEMAR:  The  Character  and  Genesis  of  Certain  Contact 
Deposits.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  31,  pp.  226-244,  1901. 

6  RANSOME,  F.  L.,  and  CALKINS,  F.  C.  :  The  Geology  and  Ore  Deposits 
of  the  Coeur  d'Alene  District,  Idaho.  U.  S.  Geol.  Survey  Prof.  Paper  62, 
p.  136,  1908. 


THERMAL  METALLIFEROUS  WATERS          283 

rhotite,  and  magnetite.  Away  from  the  contacts  the  deposits 
are  the  typical  sideritic  galena  ores  of  the'  Coeur  d'Alene 
district,  and  these  are  connected  by  many  gradations  with  the 
garnetiferous  ores  near  the  contacts. 

8.  Fluid  inclusions  are  found  in  many  lode  ores,  especially  in 
those  formed  in  the  deeper  zone  and  at  moderate  depths.     Some 
contain  gas  bubbles,  salt  cubes,  and  a  dark  solid,  probably  a 
metallic  substance.     Many  of  the  fluid  inclusions  in  pegmatites 
and  in  some  igneous  rocks  contain  gas  bubbles.     The  distribution 
and  character  of  the  fluid  inclusions  in  rocks  and  in  some  ores 
suggests  a  genetic  relation. 

9.  As  is  shown  on  pages  284-287  there  are  strong  reasons  to 
believe  that  magmas  contain  an  abundant  supply  of  water — 
ample  at  least  to  account  for  ore  deposition  by  thermal  waters. 

Summary. — Certain  conclusions  seem  to  be  warranted  by  the 
facts  and  conditions  reviewed  above.  Igneous  rocks  or  magmas 
are  the  sources  of  practically  all  ore  deposits,  whether  they  are 
syngenetic  or  epigenetic,  for  all  geologic  bodies,  whether  sedi- 
mentary, igneous,  or  metamorphic,  are  formed  of  material  de- 
rived from  igneous  magmas.  Some  ore  deposits  and  protores 
have  been  formed  directly  .by  consolidation  of  molten  magmas. 
Others  have  been  formed  by  cold  meteoric  solutions  that  have 
leached  the  metals  from  sedimentary  or  from  other  rocks.  Such 
deposits  have  certain  marked  characteristics  (see  pages  74  to 
83).  Other  deposits  equally  characteristic  (pages  49  to  73) 
have  been  formed  by  hot  ascending  solutions.  This  conclusion 
is  inevitable,  for  the  deposits  are  almost  invariably  associated 
closely  with  igneous  bodies  that  are  exposed.  The  structural 
relations  of  many  of  the  deposits  indicate  that  they  have  been 
formed  by  ascending  solutions.  There  is  much  evidence  that 
such  solutions  have  been  derived  from  magmatic  sources,  for 
in  some  districts  the  ores  have  been  deposited  only  at  places 
where  and  during  periods  when  volcanic  processes  were  active. 
Moreover,  in  many  districts  the  deposits  are  substantially  inde- 
pendent of  the  rocks  they  traverse  but  are  grouped  with  respect 
to  the  position  of  a  certain  igneous  mass  or  with  respect  to  certain 
rock  types.  If  the  localization  of  the  deposits  were  due  simply 
to  the  heat  supplied  by  the  igneous  bodies  and  to  the  stimulation 
of  the  meteoric  circulation  by  such  heat,  the  deposits  would 
probably  have  formed  in  connection  with  all  intrusives  in  a 
region,  because  the  meteoric  and  structural  conditions  must  have 


284      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

been  substantially  similar  during  more  than  one  period  of  intru- 
sion. During  different  intrusions  meteoric  waters  undoubtedly 
mingle  with  magmatic  waters,  especially  in  the  ore  zones  at 
moderate  and  shallow  depths.  It  is  shown  below  that  magmas 
contain  fluids  sufficient  to  supply  the  solutions  that  alter  rocks 
hydro-thermally  and  deposit  ores  and  to  supply  contributions  to 
hot  springs.  It  is  a  rational  conclusion  that  ascending  thermal 
waters  are  in  large  measure  of  magmatic  origin.  It  does  not 
follow  that  no  metalliferous  thermal  waters  are  essentially  or 
entirely  of  meteoric  origin. 

The  Water  Content  of  Molten  Magmas. — In  considering  the 
sources  of  ascending  thermal  waters,  the  question  arises  whether 
molten  magmas  contain  sufficient  water  to  supply  the  solutions 
that  accomplish  hydrothermal  alteration,  deposit  epigenetic 
ores,  and  mingling  with  surface  waters  issue  as  hot  springs. 
Direct  observations  are  difficult,  yet  there  are  many  data  bearing 
on  the  problem. 

1.  Practically   all  igneous   rocks   contain  hydrous   minerals. 
Micas  are  almost  invariably  present  in  deep-seated  rocks,  and 
in  pegmatites  micas  and  many  other  hydrous  minerals  are  common. 
These  minerals  are  primary.     They  were  formed  when  the  molten 
magma  cooled;  the  water  they  contain  was  a  part  of  the  magma. 
Clarke  estimates  the  amount  of  combined  water  in  the  average 
rock  as  1.42  per  cent.,  the  average  of  959  determinations.1 

2.  Many  rocks  contain  fluid  inclusions.     These  are  present  in 
quartz,  feldspars,  and  other  minerals  but  are  most  common  in 
quartz.     In  some  quartz  the  inclosed  fluids  are  under  pressure 
and  the  inclusions  are  so  abundant  that  the  quartz  breaks  with 
explosive  violence  when  shattered  with  a  hammer.     Not  much 
is  known  of  the  composition  of  the  fluid,  but  probably  water, 
carbon  dioxide,   and  other  gases  are  generally  present.     The 
fact  that  they  are  now  under  heavy  pressure  suggests  that  they 
were  formed  when  the  magma  solidified. 

3.  Volcanic  glasses  represent  magmas  that  were  chilled  so 
suddenly  that  there  was  no  opportunity  for  their  crystallization. 
Some  glasses  contain  high  percentages  of  water,  considerably 
more  than  the  average  crystalline  rocks.     One  analysis  shows  5.04 
per   cent.2    Spherulites  and  lithophysse3  are  believed  to  have 

1  CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.     U.  S.  Geol.  Survey 
Butt.  616,  p.  27,  1916. 

2  CLARKE,  F.  W.:  Op.  tit.,  p.  435. 

3lDDiNGS,  J.  P.:  Geology  of  the  Yellowstone  National  Park.  U.  S. 
Geol.  Survey  Mon.  32,- part  2,  p.  418,  1899. 


THERMAL  METALLIFEROUS  WATERS          285 

been  localized  by  the  presence  of  excess  of  steam  or  other  gases 
that  facilitated  incipient  crystallization.  That  great  quantities 
of  gases  have  escaped  when  lavas  were  extravasated  is  shown  in 
nearly  every  volcanic  field.  Where  pressure  on  the  lavas  is 
relieved,  especially  at  the  tops  of  flows,  vesicular  and  pumiceous 
phases  are  developed  by  escape  of  the  gases  in  the  rapidly  cooling 
and  viscous  magmas.  Some  formations  are  made  up  of  pumice 
hundreds  of  feet  thick.  In  the  Yellowstone  National  Park  and 
the  adjacent  part  of  Montana,  in  the  San  Juan  region,  Colorado, 
and  in  other  volcanic  regions  there  are  beds  of  volcanic  tuff  many 
hundreds  and  even  thousands  of  feet  thick,  extending  over  areas 
of  hundreds  of  square  miles,  made  up  very  largely  of  minute  wind- 
blown air-laid  volcanic  dust  which  has  been  broken  up  as  it  was 
blown  out  of  volcanic  vents.  This  material  is  for  the  most  part 
comminuted  lava  and  pumice  mixed  with  fragments  of  more 
compact  rock.  These  formations  illustrate  the  explosive  violence 
of  expanding  gas  on  a  huge  scale. 

4.  Not  only  do  surface  lavas  exhale  gases,  but  great  volumes 
of  gases  rise  in  areas  of  recent  volcanic  activity  long  after  the 
surface  flows  have  cooled.  There  is  reason  to  suppose  that  such 
gases  are,  in  part  at  least,  if  not  chiefly,  of  magmatic  origin. 
At  Cripple  Creek,  Colo.,1  great  quantities  of  gases  enter  the 
mine  workings.  When  the  barometer  is  low  they  are  so  abundant 
as  to  cause  discomfort  to  the  miners  and  even  suffocation.  They 
consist  essentially  of  nitrogen,  carbon  dioxide,  and  water,  as 
shown  in  the  analysis  below. 

VOLUMETRIC  ANALYSIS  OF  GAS  FROM  THE  ELKTON  MINE,  CRIPPLE  CREEK, 
COLO. 

Water  vapor  ..............  1.4  Hydrogen  ................  0.0 

Hydrocarbon  vapors  .......  0.0  Methane,  etc  .............  0.0 

Carbon  dioxide  ............  14  .  7  Nitrogen  .................  76  .  8 

Heavy  hydrocarbons  .......  0.0  Argon  ...................  1.5 

Oxygen  ...................  5.6 

Carbon  monoxide  ..........  0.0  100.0 

This  gas  may  be  considered  a  mixture  of  about  25  per  cent,  air, 
59  per  cent,  nitrogen,  and  14.7  per  cent,  carbon  dioxide,  with 
1.4  per  cent,  water  vapor.  •  . 


,  WALDEMAR,  and  RANSOME,  F.  L.:.  Geology  and  Gold  De- 
posits of  Cripple  Creek,  Colo.  U.  S.  Geol.  Survey  Prof.  Paper  54,  p. 
257,  1906. 


286      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

On  account  of  their  composition  and  quantity  it  is  believed 
that  these  gases  were  derived  from  deep-seated  cooling  igneous 
rocks.  Brun1  has  stated  that  volcanic  rocks  recently  formed 
contain  much  chlorine,  but  very  little  water  or  hydrogen,  and 
that  volcanic  eruptions  are  essentially  anhydrous.  He  reports 
observations  made  by  him  at  many  active  volcanoes,  among  them 
Kilauea.  His  conclusions  have  lost  force,  however,  since  Day 
and  Shepherd2  collected  water  from  the  molten  lava  of  Kilauea. 

Nearly  all  rocks,  when  heated  to  redness,  give  off  large  quanti- 
ties of  gas.3  This  gas  is  a  mixture  consisting  mainly  of  hydrogen, 
carbon  dioxide,  carbon  jnonoxide,  hydrogen  sulphide,  and  marsh 
gas.  The  volumes  of  gas  evolved  from  many  rocks  are  several 
times  the  volumes  of  the  rocks  themselves.  Most  of  these  gases 
are  believed  to  be  formed  by  heating  hydrous  and  other  minerals.4 
Whether  the  gases  are  occluded  in  the  rocks  or  are  constituents 
of  hydrous  minerals,  they  indicate  the  abundant  sources  of  fluids 
that  are  capable  of  dissolving  and  depositing  metals.  Calcula- 
tions by  Gautier  show  that  a  cubic  kilometer  of  granite  would 
yield  26,400,000  metric  tons  of  water  and  an  amount  of  hydrogen 
that  on  burning  would  yield  4,266,000  tons. 

The  volume  of  the  earth  is  about  260,000,000,000  cubic  miles, 
and  its  average  specific  gravity  is  about  5.57.5  The  volume  of 
the  ocean  is  302,000,000  cubic  miles.6  Thus  the  volume  of  the 
ocean  is  approximately  0.12  per  cent,  of  the  volume  of  the  solid 
earth.  The  weight  of  water,  volume  for  volume,  is  about  18 
per  cent,  that  of  the  weight  of  the  average  of  solid  earth  matter. 
The  weight  of  the  ocean  is  thus  about  0.02  per  cent,  that  of  the 
earth. 

According  to  Clarke,7  the  average  igneous  rock  contains  1.42 
per  cent,  of  combined  water  (H2O+)  and  0.47  per  cent,  not  com- 

1  BRUN,  ALBERT:  "Recherches  surl'exhalaison  volcanique,"  Geneva,  1911. 
1  DAY,  A.  L.,  and  SHEPHERD,  E.  S.:  Water  and  Volcanic  Activity.     Geol. 
Soc.  America  Bull.  vol.  24,  pp.  573-606,  1913. 

3  GAUTIER,  ARMAND:  La  genese  des  eaux  thermales  et  ses  rapports  avec 
le  volcanisme.     Annales  des  mines,  10th  ser.,  vol.  9,  pp.  316-390,  1906. 

4  CHAMBERLIN,  R.  T. :  The  Gases  in  Rocks.     Carnegie  Inst.,  Washington 
Pub.  106,  1908. 

'CHAMBERLIN,  T.  C.,  and  SALISBURY, •  R.  D.:  "Geology,"  vol.  1,  p.  9, 
1905. 

"CLARKE,  F.  W.:  Op.  cit.,  p.  22. 

7  CLARKE,  F.  W. :  Op.  cit.,  p.  27.  The  estimate  was  obtained  by  averaging 
the  water  content  of  645  samples. 


THERMAL  METALLIFEROUS  WATERS          287 

bined  (H20  — ).  If  the  water  contained  in  the  entire  rocky 
part  of  the  earth  is  estimated  as  1.42  per  cent,  of  its  mass,  it  is 
evident  that  a  loss  of  only  1.4  per  cent,  of  its  water  would  be 
sufficient  to  yield  all  the  water  of  the  ocean.  Thus  there  appears 
to  be  an  adequate  source  in  the  earth  itself  to  supply  thermal 
waters,  whether  or  not  the  ocean  represents,  as  is  thought  by  some, 
the  long  accumulations  of  magmatic  or  juvenile  waters  that  have 
issued  at  its  surface  through  the  geologic  ages. 

The  age  of  the  ocean1  is  estimated  as  between  50  and  70  million 
years.2  If  the  ocean  has  been  derived  from  the  central  rocky 
sphere  through  additions  of  water  from  hot  springs  and  volcanic 
vents,  such  additions  would  be  equivalent  to  about  5  cubic 
miles  of  juvenile  or  magmatic  waters  a  year. 

Origin  of  Magmas. — Whatever  may  be  accepted  as  a  working 
hypothesis  to  account  for  the  thermal  waters  that  have  deposited 
ore  veins,  the  problems  relating  to  the  origin  of  magmas  that 
solidify  to  form  igneous  rocks  are  of  vital  importance.  The  fact 
that  many  magmas  contain  small  amounts  of  the  metals  is  estab- 
lished. Many  analyses  of  igneous  rocks  reveal  small  amounts  of 
lead,  zinc,  gold,  etc.  The  ultimate  sources  of  all  sedimentary 
rocks  are  igneous  rocks  or  solidified  magmas.  Thus  the  hot 
solutions  that  have  deposited  ores  must  have  obtained  the  metals 
originally  in  igneous  bodies,  either  directly  by  magmatic  dif- 
ferentiation or  less  directly  by  the  process  of  leaching  magmatic 
products.  The  existence  of  the  ores  is  itself  evidence  that  they 
have  come  from  igneous  bodies,  for  igneous  bodies  are  essentially 
the  sources  of  all  other  rocks  that  make  up  the  earth. 

Formerly  the  nebular  hypothesis  of  Kant  and  Laplace  was 
widely  credited.  This  hypothesis  assumes  that  the  solar  system 
was  once  a  ball  of  gas  expanded  to  the  limit  of  the  orbit  of  the 

1  CLARKE,  F.  W. :  Chemical  Denudation.     Smithsonian  Misc.  Coll.  vol. 
56,  No.  5,  1910. 

BECKER,  G.  F.:  Age  of  the  Earth.     Idem,  No.  6,  1910. 

2  This  estimate  was  obtained  by  dividing  the  amount  of  sodium  in  the 
•ocean  by  the  amount  discharged  by  rivers  annually.     Several  factors,  the 
values  of  which  are  more  or  less  uncertain,  are  applied  as  corrections  for  the 
resulting  estimate.     The  method  was  first  suggested  by  Halley.     The  size 
of  the  ocean  was  considered  constant.     The  contributions  of  sodium  in 
juvenile  waters  are  mentioned  by  Clarke,  but  no  correction  is  made  for 
them.     An  estimate  has  been  made  also  by  calculating  the  amount  of  sedi- 
ments washed  to  the  ocean  annually  by  rivers  and  estimating  the  amount  of 
matter  in  sedimentary  rocks.     Results  obtained  by  such  methods  are  of 
course  but  rude  approximations. 


288      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

outermost  planet;  as  it  cooled  it  threw  off  rings  and  these  subse- 
quently were  gathered,  each  to  become  a  rotating  ball  of  gas;  on 
cooling  they  became  liquid,  and  later  portions  or  all  became  solid. 
Those  who  accepted  this  hypothesis  generally  regarded  the  earth 
as  a  liquid  ball  solidified  from  the  surface;  volcanism,  or  igneous 
injection,  was  believed  to  be  due  to  ruptures  in  the  crust  which 
permitted  portions  of  the  partly  liquid  interior  to  rise  to  the 
surface. 

The  nebular  hypothesis,  because  it  encounters  insuperable 
physical  and  mathematical  difficulties,  has  now  been  abandoned 
by  many  students  of  earth  problems.1  Chamberlin  has  proposed 
an  alternate  explanation  of  the  earth's  origin,  the  planetesimal 
hypothesis.2  This  meets  the  objections  to  which  the  nebular 
hypothesis  is  open  and  therefore  appears  to  be  more  probable. 
It  assumes  that  the  sun  and  planets  were  once  a  system  of  solid 
or  fluid  matter  or  both,  with  knots  or  nebulae  revolving  about  a 
central  mass,  the  whole  resembling  the  spiro-nebulse,  which  may 
now  be  observed  in  the  sky.  The  central  mass  became  the  sun 
and  the  knots  or  nodes  became  the  planets.  The  larger  of  the 
revolving  bodies,  having  greater  power  of  attraction,  captured 
the  smaller  ones  when  the  latter  came  within  their  paths,  and 
thus  each  of  the  planetary  bodies  grew  by  accretion  of  other 
planetesimal  matter.  The  earth  is  such  a  planetary  body.  It 
may  have  been  made  up  of  solid  particles  with  occluded  gases 
gathered  together  by  infall  and  capture.  It  may  or  may  not 
have  been  all  liquid.  Whatever  hypothesis  appears  most  prob- 
able to  the  student,  it  is  highly  improbable  that  the  interior  of 
the  earth  is  a  molten  mass.  Certainly  it  is  very  hot  toward  the 
center,  yet  its  great  elasticity  as  shown  by  the  rate  and  nature 
of  the  transmission  of  earthquake  shocks  indicates  great  rigidity. 
It  remains  solid  because  of  the  great  pressures  that  exist  at  great 
depths. 

The  earth  is  regarded  as  an  essentially  solid  body,  its  heat 
being  due  partly  to  self -compression  by  its  own  gravity.3  The 

1  CHAMBERLIN,  T.  C.:  An  Attempt  to  Test  the  Nebular  Hypothesis  by 

the  Relations  of  Masses  and  Momenta.     Jour.  Geol,  vol.  8,  pp.  58-73,  1900. 

MOULTON,  E.  R.:  An  Attempt  to  Test  the  Nebular  Hypothesis  by  an 

Appeal  to  the  Laws  of  Dynamics.     Astrophys.  Jour.,  vol.  11,  pp.  103-130, 

1900. 

2 CHAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:  "Geology,"  vol.  2,  Earth 
History,  pp.  38-78,  1905. 

3 CHAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:  "Geology,"  vol.  1,  p.  657, 
1904. 


THERMAL  METALLIFEROUS  WATERS          289 

heat  would  depend  on  the  intensity  of  internal  pressure.  There 
are  reasons  to  suppose  that  the  flow  of  heat  from  the  interior 
to  the  middle  zone  would  be  greater  than  from  the  middle  zone 
to  the  surface,  and  that  the  middle  zone  would  for  that  reason 
become  hotter.  Locally  materials  of  that  zone  would  melt, 
especially  those  materials  which  have  the  lowest  melting  points. 
Gases,  particularly  the  "  mineralizers  "  or  materials  that  make  for 
fluidity  and  lower  the  melting  points  of  rocks,  would  aid  in  solu- 
tion, and  the  places  where  they  were  abundant  would  melt  first. 
The  molten  mass,  by  fusing  or  fluxing  its  way,  would  rise  to  the 
surface,  where  pressures  are  lower.  As  magmas  come  into  regions 
of  lower  pressure  the  temperatures  necessary  for  liquefaction 
fall,  a  change  which  enables  the  cooling  magmas  still  to  remain 
liquid.  Thus,  according  to  Chamberlin  and  Salisbury,  the  more 
fusible  material  may  thread  its  way  upward  toward  the  surface, 
and  when  it  arrives  at  the  fracture  zone  it  enters  any  fractures 
or  other  openings  that  are  available.  Material  highly  heated 
and  liquid  is  carried  up  from  the  middle  zone  to  the  exterior, 
and  this  itself  tends  to  keep  the  earth  solid  by  limiting  the  size 
of  bodies  of  molten  material  accumulated  within.  The  earth 
is  subjected  continually  to  tidal  stresses.1  There  is  not  only 
an  ocean  tide,  but  the  solid  lithosphere  is  subjected  to  tidal  move- 
ments great  enough  to  be  detected  and  computed.  As  a  result 
of  these  stresses  the  earth  is  continually  kneaded,  and  the  stresses 
aid  in  the  expulsion  of  material  to  the  surface,  where  pressures 
are  lower.  The  conception  may  be  illustrated  by  recalling  the 
common  practice  of  freeing  gold  amalgam  from  quicksilver  in 
a  buckskin  sack.  More  quicksilver  is  present  than  unites  with 
the  gold  to  form  amalgam.  On  being  gently  pressed  and  kneaded 
the  excess  quicksilver  is  squeezed  out  of  the  solid  amalgam  and 
presses  outward  through  the  pores  of  the  sack. 

The  genesis  of  magmas  injected  within  geologic  time  and  of  ore 
deposits  related  to  such  magmas  would  probably  be  essentially 
similar,  whether  the  earth  was  formed  as  assumed  by  the  nebular 
hypothesis  or  as  assumed  by  the  planetesimal  hypothesis. 

1  CHAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:  Op.  cit.,  p.  579. 

MICHELSON,  A.  A. :  Preliminary  Results  of  Measurements  of  the  Rigidity 
of  the  Earth.     Jour.  Geol,  vol.  22,  pp.  97-130,  1914. 


CHAPTER  XXII 
IRON 


Mineral 

Per  cent.  Fe 

Composition 

Limonite  '.  

59.8 

2Fe2O3.3H2O 

Goethite 

62.9 

Fe2O3.H2O 

Turgite  

66.3 

2Fe2O3.H2O 

Hematite  

70.0 

Fe2O3 

Magnetite  

72.3 

Fe304 

Siderite  

48.3 

FeCO3 

Melanterite  

20.1 

FeSO4.7H2O 

Pyrite  

46.6 

FeS2 

Marcasite 

46.6 

FeS2 

Pyrrhotite  

60.4 

Fe7S8 

Olivine  

5.0to30.0 

(Mg,  Fe)2Si04 

Greenalite  

25.0  + 

Hydrous  silicate  of  iron 

Glauconite  

25.0  + 

Hydrous  silicate  of  iron  and  potash. 

Actinolite  

15.  0± 

Ca(Mg,  Fe)3(SiO3)4 

Ilmenite  

36.8 

FeTiO3 

Franklinite  

45.0  + 

(Fe,  Mn,  Zn)  (FeMn)2O4 

Ore  Minerals  of  Iron. — Iron  is  an  abundant  metal,  constituting 
4.5  per  cent,  of  the  earth's  exterior  shell.1  Iron  ores  are  widely 
distributed  and  are  formed  under  many  geologic  conditions.  Iron 
is  found  in  large  amounts  in  many  igneous  rocks,  some  of  which 
are  rich  iron  ores.  Some  sedimentary  rocks  also  are  rich  enough 
to  mine  for  iron,  and  many  rocks,  sedimentary  and  igneous,  upon 
weathering  yield  high-grade  iron  ores. 

Because  iron  ores  are  common  and  because  they  are  easily 
beneficiated  iron  is  cheap.  Iron  ores  in  the  United  States  carry- 
in  general  35  to  65  per  cent,  of  iron,  and  the  larger  portion  of  the 
iron  ore  now  mined  carries  more  than  40  per  cent.  Where  its 
impurities  are  easily  removed,  or  where  the  ore  is  "self -fluxing," 
or  where  silica  can  be  separated  from  the  iron  at  low  cost,  ore 
that  carries  30  per  cent,  of  iron  or  less  may  be  valuable.  Iron 

1  CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.  U.  S.  Geol.  Survey 
BuU.  616,  p.  34, 1916. 

290 


IRON 


291 


ores  sell  at  $1  to  $6  a  ton,  the  price  depending  on  location  and 
composition.  Bessemer  ores  bring  higher  prices  than  non- 
•  Bessemer  ores  because  they  are  more  cheaply  treated.  A 
Bessemer  ore  is  one  that  will  make  pig  iron  with  phosphorus 
content  not  more  than  0. 1  per  cent. 

Of  the  iron-bearing  minerals  hematite  is  by  far  the  most  im- 
portant. At  present  it  supplies  over  90  per  cent,  of  the  iron  ore 
mined  in  the  United  States;  limonite,  magnetite  and  siderite 
supply  nearly  all  the  remainder.  The  other  iron  minerals  are 
of  interest  chiefly  as  protores  or  material  from  which  workable 
iron  ore  may  be  formed  by  superficial  alteration  or  weathering. 
These  minerals  are  nevertheless  important  because  much  iron 
ore  has  been  derived  from  them  through  superficial  alteration. 

Genesis  of  Iron-ore  Deposits. — The  ores  of  iron  are  in  the 
main  syngenetic.  Some  valuable  deposits,  including  those  of 
the  Kiruna  district,  Sweden,  and  some  of  the  magnetites  of  the 


FIG.  127. — Cross-section  of  iron-ore  deposit  formed  by  leaching  valueless 
material  from  a  ferruginous  sedimentary  protore.  (Ideal  section  of  Mesabi 
range,  Minnesota.) 

Adirondacks,  New  York,  have  been  formed  by  magmatic 
segregation.  A  great  many  deposits  in  the  United  States  are 
sedimentary.  These  include  the  Lake  Superior  hematites,  the 
Clinton  iron  ores,  the  black-band  carbonate  ores,  and  the  Tertiary 
ores  of  Texas.  The  rich  hematites  of  Minas  Geraes,  Brazil,  and 
the  limonite-carbonate  ores  of  Luxemburg,  Lorraine,  and  the 
Cleveland  district,  England,  and  many  other  regions  are  also 
sedimentary  beds.  Magnetite  ores  in  several  districts  in  Mon- 
tana, Colorado,  California,  New  Mexico,  and  Utah  are  of  contact- 
metamorphic  origin  (see  page  38).  Other  deposits  have  been 
formed  in  the  deep-vein  zone. 

Many  of  the  hematite  and  limonite  ores  in  the  United  States 
are  weathered  products  of  ferruginous  carbonate  or  ferruginous 
silicate  protores.  Some  limonite  deposits  are  oxidation  products 
of  sulphide  deposits — for  example,  the  limonites  of  Ducktown, 
Tenn.  (pages  164  and  165)  and  of  the  Gossan  lead,  Virginia. 


292      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

At  some  places  in  the  West  and  in  Mexico  the  gossan  ores  are 
rich  enough  in  iron  to  be  mined  for  flux. 

All  ferruginous  materials  in  the  presence  of  air  and  water  tend' 
to  change  to  the  hydrous  oxide;  a  steel  rail,  an  igneous  rock,  a 
sulphide  ore,  or  a  ferruginous  sedimentary  rock  will  all  yield 
limonite.  Oxidation  or  weathering  almost  invariably  results  in 
the  concentration  of  iron  near  the  surface  (Figs.  127,  128).  The 
materials  other  than  iron  are  removed  more  rapidly  than  iron, 
and  because  of  their  removal  the  iron  remains  in  a  more  concen- 


FIG.  128. — Cross-section  of 
iron-ore  deposit  formed  by  leach- 
ing valueless  material  from  a 
ferruginous  sedimentary  protore. 
(Ideal  section  of  Cuyuna  range, 
Minnesota.) 


FIG.  129. — Cross-section  of  iron-ore 
deposit  formed  by  leaching  valueless 
material  from  an  igneous  protore. 


trated  state.  Many  deposits  are  valuable  only  after  enrichment 
by  weathering.  Thus  the  Lake  Superior  iron-bearing  forma- 
tions are  all  of  too  low  grade  to  work  except  where  superficial 
alteration  has  taken  place.  Ores  of  the  "lateritic"  type  like 
those  of  Cuba  (page  128)  have  resulted  from  the  thorough  decom- 
position and  leaching  of  iron-rich  igneous  rocks,  other  abundant 
constituents  having  been  removed  in  part  or  altogether  by  long- 
continued  action  of  air  and  water  (see  page  125).  Such  ores 
genetically  bear  a  relation  to  the  igneous  rocks  like  that  of  the 
gossan  to  the  sulphide  ore:  in  the  lateritic  deposits  the  protore 
is  a  basic  ferruginous  igneous  rock  (Fig.  129) ;  in  the  gossan  the 
protore  is  composed  of  iron  sulphides  and  other  minerals  (Fig. 
130). 

Although  limonite  is  in  general  an  end  product  of  weathering 
of  all  ferruginous  rocks,  bodies  of  nearly  pure  hematite  and  mag- 
netite weather  much  more  slowly  than  the  ferruginous  carbonates 


IRON 


293 


and  silicates.  Nevertheless  hematite  and  magnetite  ores  near 
the  surface  will  generally  carry  some  limonite.  On  the  other 
hand,  limonite  may  become  dehydrated  and  change  to  hematite. 
In  a  moist  climate  the  more  highly  hydrated  minerals  will  result 
from  the  weathering  of  the  outcrop  of  a  ferruginous  material; 
in  a  dry  climate  the  less  highly  hydrated  minerals,  such  as  turgite 
or  hematite,  will  form.  Some  of  the  largest  and  most  valuable 
deposits  of  iron  ore  in  the  world  are  workable 
in  their  original  unweathered  state.  Ex- 
amples are  some  magnetites  of  New  York 
and  Pennsylvania,  the  rich  magnetites  of 
the  Kiruna  district,  in  Sweden,  and  the  rich 
sedimentary  hematite  ores  of  Minas  Geraes, 
in  Brazil. 

The  Lake  Superior  ores  have  been  formed 
mainly  by  weathering  of  sedimentary  protores; 
nevertheless  they  are  principally  hematite. 
They  have  doubtless  become  dehydrated 
by  exposure  under  arid  conditions,  or,  in 
some  localities,  by  removal  of  water  during 
periods  in  which  they  were  buried  under 
heavy  bodies  of  later  rocks.  •  deposit  formed  by 


In  migrating  slowly  under  conditions  of 
weathering  iron  resembles  gold  and  lead,  ing  iron  sulphides. 
rather  than  copper  and  zinc.  During 
weathering,  however,  some  iron  is  dissolved  and  precipitated. 
The  cementation  of  fractures  by  iron  oxide,  the  replacement  of 
soluble  carbonates,  and  the  development  of  crusts,  stalactites, 
and  stalagmites  of  limonite,  etc.,  attend  the  weathering  of  most 
ferruginous  materials. 

Under  some  conditions  great  quantities  of  iron  are  transported 
in  solution1  and  deposited  in  swamps,  in  lakes,  and  in  the  sea. 
In  the  presence  of  vegetation  iron  is  reduced  and  tends  to  remain 
in  the  ferrous  state.  Ferrous  salts  are  more  soluble  than  ferric 
salts.  From  these  iron  may  be  precipitated  by  oxidation  or  by 
other  chemical  processes,  or  through  the  action  of  minute  organ- 
isms known  as  iron  bacteria.  Bog  and  lake  iron  deposits  are  com- 
monly produced  by  the  weathering  of  iron-bearing  rocks  in  moist 
countries,  and  locally  such  deposits  are  mined  for  iron.  Many  of 

1  HARDER,  E.  C.  :  Iron-depositing  Bacteria  and  Their  Geologic  Rela- 
tions. U.  S.  Geol.  Survey  Prof.  Paper  (in  preparation). 


294      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

them  are  formed  in  marshes,  swamps,  and  lakes,  and  they  are 
abundant  in  many  glaciated  regions  where  drainage  has  not  yet 
reached  adjustment.  Small  deposits  have  been  laid  down  within 
the  memory  of  man.  The  bog  and  lake  iron  ores  are  not  very 
important  commercially,  but  they  are  of  great  interest  because 
they  throw  light  upon  the  genesis  of  sedimentary  iron  ores  formed 
in  more  remote  geologic  periods. 

In  the  weathering  of  rocks  containing  iron  some  of  the  iron 
is  dissolved  and  carried  away  in  solution,  chiefly  as  ferrous  acid 
carbonate.  Some  iron  may  be  carried  in  solution  also  as  sulphate 
or  as  salts  of  various  organic  acids.  While  the  iron  is  being 
carried  away  in  solution  there  is  a  tendency  for  oxidation  to 
take  place  and  for  ferric  oxide  to  be  deposited  by  hydrolysis. 
In  dilute  solutions  some  of  these  iron  compounds  will  form  col- 
loids, and  much  of  the  iron  may  be  transported  in  colloidal  form 
to  lakes,  bogs,  or  other  bodies  of  water  that  happen  to  receive 
the  drainage  from  an  area  of  weathering  ferruginous  rocks. 
Much  of  the  iron,,  however,  remains  in  solution  until  it  reaches 
quiet  waters. 

When  iron-bearing  solutions  reach  quiet  bodies  of  water  pre- 
cipitation may  take  place  in  various  ways,  being  governed  by 
the  form  in  which  the  iron  exists  in  the  solution  and  by  other 
constituents  that  may  be  present.1  Most  commonly  iron  is 
probably  carried  as  bicarbonate  in  the  presence  of  carbon  dioxide. 
Upon  the  removal  of  excess  carbon  dioxide  the  iron  may  be  pre- 
cipitated under  reducing  conditions  as  ferrous  carbonate  or  by 
oxidation  and  hydrolysis  as  ferric  hydroxide.  If  decaying  organic 
matter  is  present  in  the  precipitate  ferrous  carbonate  will  remain 
as  such,  while  any  ferric  hydroxide  present  may  be  reduced  and 
form  ferrous  carbonate.  On  the  other  hand,  where  oxidizing 
conditions  prevail,  ferrous  carbonate  is  readily  oxidized  to  form 
ferric  hydroxide.  The  precipitation  of  ferric  hydroxide  may 
take  place  by  simple  chemical  reactions,  but  in  nature,  accord- 
ing to  Harder,  iron  bacteria  are  practically  always  active  in  its 
formation  and  accumulation  at  the  surface. 

Some  iron  is  doubtless  carried  in  solution  in  the  form  of  ferric 
salts  of  organic  acids,  from  which  it  may  be  precipitated  as  ferric 
hydroxide  or  as  insoluble  basic  salts  of  organic  acids.  These 
reactions  may  be  those  of  simple  hydrolysis,  or  it  may  be  that 
iron  bacteria  are  involved  also,  as  in  the  precipitation  of  iron 

1  HABDEK,  E.  C. :  Op.  cit. 


IRON  295 

from  ferrous  bicarbonate  solutions.  In  the  sea  reactions  with  salt 
water  probably  favor  precipitation  under  certain  conditions.  If 
the  solutions  react  with  lime  carbonate,  iron  carbonate  or  iron 
oxide  may  be  deposited  and  lime  carbonate  be  dissolved.  The 
precipitation  of  ferric  oxide  from  iron  sulphate  solutions  is 
treated  on  page  154. 

Iron  is  not  dissolved  readily  under  arid  conditions.  Beds  of 
salt  and  gypsum  are  commonly  associated  with  shales  and  sand- 
stones stained  red  with  hematite.1  The  soluble  salts  of  iron  are 
the  ferrous  salts  rather  than  the  ferric  salts.  In  the  absence  of 
organic  products  that  supply  reducing  agents  the  iron  is  likely 
to  be  oxidized  to  the  insoluble  ferric  condition.  Thus,  in  an  arid 
region,  the  iron  may  remain  as  residuary  masses,  or  if  it  is  re- 
moved it  may  be  transported  mechanically  as  the  ferric  oxide 
rather  than  in  solution  as  ferrous  salt. 

Age  of  Iron-ore  Deposits  of  the  United  States. — Many  sedi- 
mentary rocks  carry  noteworthy  percentages  of  iron.  Nearly 
all  limestones,  sandstones,  and  shales  are  more  or  less  ferrugin- 
ous. Nodules  and  layers  of  iron  carbonate,  sulphide,  or  oxide 
are  common  in  limestone  and  in  shale,  and  from  these  rocks 
iron  is  frequently  concentrated  by  weathering,  forming  iron  ores. 
There  is  evidence  that  in  past  geologic  time  much  larger  deposits 
have  formed  in  the  sea  than  are  known  to  be  in  process  of  for- 
mation today.  In  several  epochs  of  pre-Cambrian  time  hundreds 
of  feet  of  highly  ferruginous  material  were  deposited  over  large 
areas,  and  in  the  Clinton  (Silurian)  epoch  iron  was  deposited  in 
great  quantities  over  a  considerable  part  of  the  eastern  United 
States.  Much  iron  was  deposited  also  in  the  Devonian 
period,  especially  in  the  Oriskany  epoch,  and  iron  largely  in 
the  form  of  carbonate  was  deposited  in  Carboniferous  time.  In 
the  Tertiary  period  extensive  iron-bearing  rocks  were  formed  in 
Texas. 

Some  geologic  periods  appear  to  have  been  more  favorable  for 
the  production  of  iron  ores  than  others.  It  is  commonly  sup- 
posed that  the  controlling  conditions  were  principally  physio- 
graphic, and  that  base-leveled  regions  that  permit  long  periods 
of  leaching  and  chemical  denudation  are  more  favorable  to  the 
development  of  iron  deposits  than  more  rugged  regions  where 

1  TOMLINSON,  C.  W. :  The  Origin  of  Red  Beds.  Jour.  Geol.,  vol.  24, 
pp.  153-179,  238-253,  1916. 


296      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  denudation  is  largely  mechanical.  It  is  believed  also  that 
certain  periods  which  were  marked  by  the  extravasation  of  igneous 
rocks  were  peculiarly  favorable  to  the  development  of  iron  ores.1 
Many  such  rocks  carry  much  iron,  and  hot  waters  of  magmatic 
origin,  released  during  volcanic  eruptions,  are  thought  by  some 
to  have  contributed  iron  to  the  general  run-off. 

The  deposits  now  being  formed  in  lakes  and  seas,  so  far  as  is 
known,  do  not  include  iron-bearing  formations  comparable  to 
those  that  were  laid  down  in  past  geologic  time.  Nodules  of 
iron  ore  are  found  by  dredging  the  deep  sea,  and  sediments  that 
are  somewhat  ferruginous  are  evidently  being  deposited  today, 
yet  there  are  no  indications  that  great  geologic  formations  rich  in 
iron  are  being  accumulated.  Geologic  conditions  vary  from  time 
to  time:  one  period  may  be  marked  by  the  deposition  of  phos- 
phate rock;  another  is  more  favorable  for  the  formation  of 
coal;  still  another  may  be  more  favorable  for  the  formation  of 
iron  ore. 

Valuable  deposits  of  iron  ore  have  been  formed  by  magmatic 
segregation  in  several  geologic  periods  when  igneous  agencies 
were  active  (see  page  9).  Those  of  pre-Cambrian  age  are 
noteworthy.  Contact-metamorphic  deposits  of  iron  ore  have 
been  formed  in  many  periods,  especially  in  the  Mesozoic  and  the 
early  part  of  the  Tertiary. 

Production. — The  iron  ores  of  the  United  States  are  of  varied 
genesis,  and  nearly  all  the  important  classes  are  represented.  In 
1915  the  United  States  produced  over  55  million  tons  of  iron 
ore,  valued  at  $101,283,984.  This  ore  yielded  30,384,486  tons 
of  pig  iron  valued  at  $401,409,604.  The  greatest  item  in  the 
production  is  hematite  from  the  districts  of  the  Lake  Superior 
region.  Second  in  rank  are  the  southern  Appalachian  districts — 
Birmingham,  Ala.,  and  Chattanooga,  Tenn.  These  regions  yield 
hematite  and  subordinate  amounts  of  limonite.  The  Adirondack 
region,  New  York,  and  the  districts  of  southeastern  Pennsylvania 
and  northern  New  Jersey  yield  magnetite.  The  most  productive 
iron-producing  district  in  the  United  States  is  the  Mesabi  range 
in  Minnesota. 

1  VAN  HISE,  C.  R.,  and  LEITH,  C.  K. :  Geology  of  the  Lake  Superior  Region. 
U.  S.  Geol.  Survey  Mon.  52,  p.  513,  1911. 


IRON  297 

IRON  ORE  MINED  IN  THE  UNITED  STATES  IN  GROSS  TONS  IN  1915° 


District 

Hematite 

Brown 
ore 

Magne- 
tite 

Carbon- 
ate 

Total 

Marquette  (Michigan)  
Menominee    (Michigan   and   Wis- 
consin)   

3,817,892 
4,665,465 

Gogebic  (Michigan  and  Wisconsin) 
Vermilion  (Minnesota)  
Mesabi  (Minnesota)  
Cuyuna  (Minnesota)  

4,996,237 
1,541,645 
30,802,409 
1,120,606 

46  944  254 

4,213,597 

535,332 

4,748  929 

340,481 

198,543 

539  024 

699,213 

699,213 

Northern  New  Jersey  and  south- 

(b) 

^655,493 

644  493 

Other  districts 

728,992 

754,000 

464,130 

3,455 

1,950  577 

52,227,324 

1,488,709 

1,807,002 

3,455 

55,526,490 

0  B  ORCHARD,  E.   F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part   1,  pp.  285,  295, 
1916. 
fc  Some  brown  ore  included  with  magnetite. 

LAKE  SUPERIOR  IRON-ORE  DEPOSITS 

The  iron-ore  deposits  of  the  Lake  Superior  region  are  the  most 
productive  in  the  United  States.     They  occur  in  the  main,  in 


FIG.  131. — Sketch  of  Lake  Superior  region  showing  mining  districts  and 
ports.     (After  Van  Hise  and  Leith,  U.  S.  Geol  Survey.) 


298      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


long  and  relatively  narrow  belts, 
the  so-called  iron  "ranges."  The 
country  is  hilly,  but  the  relief  is 
not  great;  the  ranges  and  peaks 
rise  only  a  few  hundred  feet  or 
less  above  the  surrounding  coun- 
try. The  ores  are  carried  by 
rail  to  upper  Lake  ports  and 
shipped  by  boats  to  lower  Lake 
ports,  from  which  they  are  dis- 
tributed to  iron  furnaces.  The 
positions  of  the  principal  dis- 
tricts and  their  ports  are  shown 
in  Fig.  131. 

Practically  all  the  ores  mined 
in  the  Lake  Superior  region  are 
superficially  enriched  products  of 
pre-Cambrian  sedimentary  pro- 
tores.  The  principal  iron-bear- 
ing formations  are  the  Soudan 
(Keewatin),  of  the  Vermilion 
range;  the  Negaunee  (middle 
Huronian),  of  the  Marquette 
range;  and  the  upper  Huronian 
or  Animikie  formations,  of  several 
other  ranges.  The  upper  Hu- 
ronian is  the  most  productive, 
having  yielded  over  73  per  cent, 
of  the  iron  ore  of  the  Lake  Su- 
perior region  to  the  end  of  1909. l 

The  principal  upper  Huronian 
iron-bearing  members  in  the 
different  districts  are  named  as 
follows:  in  the  Mesabi  range, 
the  Biwabik;  in  the  Penokee- 
Gogebic,  the  Iron  wood;  in  the 
Cuyuna,  the  Deerwood;  in  the 

1  VAN  HISE,  C.  R.,  and  LEITH,  C. 
K:  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Man.  52, 
p.  461,  1911. 


IRON  299 

Menominee,  Crystal  Falls,  Iron  River,  and  Florence  districts, 
the  Vulcan.  Some  ore  is  mined  from  the  Bijiki  schist  (upper 
Huronian)  of  the  Marquette  region.  The  principal  ores  of  that 
district,  however,  are  in  the  Negaunee  (middle  Huronian). 

The  iron-bearing  formations  are  stratified  sedimentary  rocks 
composed  chiefly  of  iron  oxide,  silica,  iron  carbonate,  and  iron 
silicates.  Such  rocks  are  called  jasper,  ferruginous  chert,  taconite, 
greenalite,  cherty  iron  carbonate,  etc.  By  weathering  and  en- 
richment they  become  ore.  The  process  is  chiefly  solution  and 
removal  of  silica  and  carbon  dioxide,  although  locally  iron  may 
be  dissolved  and  precipitated  as  oxide  by  ground  water. 

In  all  the  ranges  the  iron-bearing  formations  have  been  tilted, 
and  in  some  of  them  closely  folded  (Fig.  132).  The  ore-bearing 
formations  have  been  exposed  to  weathering  through  many 
geologic  periods.  At  some  places  removal  of  silica  and  concentra- 
tion of  iron  began  in  pre-Cambrian  time.  In  the  Vermilion  range 
parts  of  the  ore-bearing  formation  were  weathered  and  meta- 
morphosed to  schists  in  the  pre-Cambrian.  In  the  Marquette 
district  the  Negaunee  formation  was  altered  by  weathering 
before  upper  Huronian  time.  All  phases  of  the  iron-bearing 
formation  shown  in  this  region,  except  specular  hematite,  had 
developed,  for  they  are  represented  by  pebbles  in  the  upper 
Huronian.  The  specular  hematite  was  developed  by  deep- 
seated  metamorphism  of  portions  of  the  iron-bearing  formation 
already  weathered  and  enriched.  This  metamorphism,  which 
gave  a  secondary  cleavage  to  the  iron  ore,  was  accomplished  by 
deformation  that  took  place  after  the  deposition  of , the  upper 
Huronian  sediments.1  In  the  Mesabi  range  concentration  by 
weathering  had  taken  place  before  the  Cretaceous  period,  for 
pebbles  of  weathered  ore  are  found  in  the  conglomerate  at  the 
base  of  the  Cretaceous.  During  long  geologic  ages  large  parts 
of  the  Lake  Superior  region  have  been  land;  for  much  of  this 
time  the  land  surface  has  been  relatively  low,  a  condition  favoring 
extensive  chemical  denudation  and  deep  weathering. 

The  weathered  parts  of  the  ore-bearing  formations  are  found 
in  various  positions  with  respect  to  the  geologic  structure.  On 
the  Mesabi  range  the  ore  deposits  are  for  the  most  part  blankets 
that  lie  below  the  mantle  of  drift.  Here  and  there  the  ores  ex- 
tend down  the  dip  of  the  beds  below  interstratified  lean  beds  or 

1  VAN  HISE,  C.  R.,  and  LEITH,  C,  K.:  Op,  cit.,  p.  278. 


300      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


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IRON  301 

below  Virginia  slate.  Some  of  the  deposits  are  due  to  leaching 
along  enlarged  joints. 

In  some  of  the  iron  ranges  the  rocks  associated  with  the  ores 
are  complexly  folded;  the  ores  are  in  places  found  in  pitching 
troughs  where  ground-water  circulation  has  been  controlled  by 
impermeable  beds  or  intrusive  rocks,  or  by  both.  Circulating 
ground  waters  conducted  along  restricted  paths  through  long 
periods  have  leached  out  silica  to  depths  far  below  the  surface. 
Locally  also  they  have  deposited  iron  oxide,  cementing  the  ore 
and  further  enriching  it.  The  concentration  of  iron  oxide  by 
weathering  is  treated  on  page  125. 

Mesabi  Range,  Minnesota. — The  iron  deposits  of  the  Mesabi 
range1  are  formed  by  local  concentration  in  a  ferruginous  sedimen- 


FIG.  133. — Sketch  showing  trend  of  Biwabik  iron-bearing  formation  and 
township  lines,  Mesabi  range,  Minnesota. 


tary  formation,  which  extends  from  a  point  about  12  miles  south- 
west of  Pokegama  Lake  to  Birch  Lake,  a  distance  of  over  100 
miles  (Fig.  133).  The  rocks  strike  about  N.  73°E.  and  have  low 
dips  toward  the  south.  Running  parallel  to  the  iron-bearing 
formation  and  north  of  it  is  a  ridge  called  the  Giants  Range.  In 
the  western  part  of  the  district  this  ridge  is  only  1,400  feet  above 
sea  level  and  not  appreciably  higher  than  the  surrounding  country. 
From  Virginia  eastward  it  is  locally  about  400  feet  above  the 
country  on  each  side.  The  lower  areas  are  covered  with 

1  WINCHELL.  H.  V. :  The  Mesabi  Iron  Range.  Minn.  Geol.  and  Nat. 
Hist.  Survey  Twentieth  Ann.  Rept.,  pp.  111-180,  1893. 

WINCHELL,  N.  H.,  GRANT,  U.  S.,  and  SPURR,  J.  E.:  Minn.  Geol.  and  Nat. 
Hist.  Survey  Final  Report,  vol.  4. 

LEITH,  C.  K:  The  Mesabi  Iron-bearing  District  of  Minnesota.  U.  S. 
Geol.  Survey  Mon.  43,  1903. 

SPURR,  J.  E. :  The  Iron-bearing  Rocks  of  the  Mesabi  Range  in  Minne- 
sota. Minn.  Geol.  and  Nat.  Hist.  Survey  Bull.  10,  pp.  1-268,  1894. 

WOLFF,  J.  E.:  Recent  Geologic  Developments  on  the  Mesabi  Iron 
Range,  Minnesota.  Am.  Inst.  Min.  Eng.  Bull.  118,  pp.  1763-1788,  1916; 
Eng.  and  Min.  Jour.,  July  17-Aug.  7,  1914. 

VAN  BARNEVELD,  C.  E. :  Iron  Mining  in  Minnesota.  Minn.  Univ. 
School  of  Mines  Exper.  Sta.  Bull.  1,  1913. 


302      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

drift.  The  distribution  of  the  iron-bearing  formation  has  been 
determined  principally  by  drilling.  The  following  succession  of 
formations  is  given  bv  Leith. 

Quaternary  system : 

Pleistocene  series Glacial  drift. 

Unconformity. 
Cretaceous  system. 
Unconformity. 
Algonkian  system: 

Keweenawan  series Great  basal  gabbro  (Duluth  gabbro) 

and  granite  (Embarrass  granite), 
intrusive  in  all  older  formations. 
Huronian  series: 
Unconformity. 

Acidic  and  basic  intrusive  rocks. 


Upper  Huronian  (Animikie 


group). 


Virginia  slate. 

Biwabik  formation  (iron-bearing). 


Pokegama  quartzite. 
Unconformity. 

Giants   Range   granite,  intrusive  in 

T  -jji     TT        •  lower  formation. 

Lower-middle  Huronian...     01  ,  ,  ,  , 

blate-graywacke-conglomerate  f  o  r  - 

mation. 
Unconformity. 
Archean  system: 

Lauren tian  series Granites  and  porphyries. 

Keewatin  series Greenstone,  green  schists,  and  por- 
phyries. 

The  oldest  rocks  of  the  district  are  Archean  greenstones,  green 
schists,  and  porphyries  of  the  Keewatin  series.  These  are  asso- 
ciated with  and  probably  cut  by  Laurentian  granites  and  por- 
phyries. The  Archean  rocks  are  exposed  here  and  there  in  the 
central  part  of  the  district.  Some  but  not  all  of  the  Archean 
rocks  have  a  well-developed  cleavage  which  is  nearly  vertical 
and  strikes  about  parallel  to  the  axis  of  the  range. 

The  rocks  of  the  Algonkian  system  rest  unconformably  upon 
the  Archean  rocks  and  are  not  so  greatly  metamorphosed.  The 
oldest  member  of  this  system  is  a  sedimentary  series  of  con- 
glomerates, graywackes,  and  slates,  of  lower-middle  Huronian 
.age.  The  most  extensive  exposure  is  from  Eveleth  eastward  to 
Biwabik. 

The  lower-middle  Huronian  beds  strike  approximately  with 
the  axis  of  the  range  and  stand  nearly  vertical.  Their  thickness 


IRON 


303 


is  probably  between  3,000  and  5,000  feet.     These  rocks  are 
intruded  by  the  Giants  Range  granite. 

The  upper  Huronian  is  composed  of  (1)  the  Pokegama  quartz- 
ite,  consisting  mainly  of  quartzite  but  containing  also  con- 
glomerate at  its  base;  (2)  the  Biwabik  formation,  which  rests 
upon  the  Pokegama  and  consists  of  ferruginous  cherts,  iron  ores, 
slates,  greenalite  rocks,  and  carbonate  rocks,  with  a  small  amount 
of  coarse  detrital  material  at  its  base;  and  (3)  the  Virginia  slate. 


Formations  Oldei 

than  Pokegama 

Quartzite 


mi 

am*  Qu. 


Biwabik  Iron  Bearing 
Formation 


Virginia 

Slates 


FIG.  134. — Map  of  central  part  of  Mesabi  range,  Minnesota,  with  cross- 
sections.     (Based  on  map  by  C.  K.  Leith,  U.  S.  Geol.  Survey.) 


Between  the  Pokegama  quartzite  and  the  Biwabik  formation 
a  slight  break  in  deposition  is  indicated  by  conglomerate.  Some 
acidic  and  basic  intrusive  igneous  rocks  are  associated  with  the 
upper  Huronian  sediments.  All  the  upper  Huronian  rocks  were 
formed  after  the  close  folding  which  affected  the  lower  and 
middle  Huronian  sedimentary  rocks.  They  are  not  on  edge,  but 
dip  at  low  angles  (Fig.  134). 

The  Pokegama  quartzite,  the  lowest  member  of  the  Animikie 
group,  at  many  places  forms  the  base  upon  which  the  iron-bearing 
formation  rests.  Its  thickness  ranges  from  a  few -feet  to  200  feet, 
and  for  so  thin  a  formation  it  is  fairly  persistent.  From  Iron 
Mountain  westward  to  the  end  of  the  iron-bearing  district  it 


304      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

forms  a  continuous  belt  from  a  few  feet  to  half  a  mile  or  more  wide. 
East  of  Iron  Mountain  it  is  nearly  continuous  to  Embarrass 
Lake. 

The  Biwabik  iron-bearing  formation  extends  along  the  entire 
length  of  the  range.  Its  average  thickness  is  about  620  feet,  but 
owing  to  the  prevailing  low  dips,  the  width  exposed  ranges  from 
a  quarter  of  a  mile  to  3  miles.  The  formation  is  generally  cov- 
ered with  glacial  drift,  which  ranges  in  thickness  from  20  to  200 
feet.  At  most  places  where  it  is  not  too  thick  the  drift  is  re- 
moved by  stripping,  and  the  ore  is  loaded  into  cars  with  steam 
shovels.  As  a  rule,  the  iron-bearing  formation  rests  upon  the 
Pokegama  quartzite,  but  where  that  is  lacking  it  is  in  uncon- 
formable  contact  with  the  older  Huronian  or  the  Archean  rocks. 
At  the  east  end  of  the  district  the  Embarrass  granite  lies 
between  the  iron-bearing  formation  and  the  older  rocks. 

Everywhere  along  the  productive  part  of  the  district  the  iron- 
bearing  formation  south  of  its  outcrop  is  capped  by  the  Virginia 
slate,  but  east  of  Trimble  to  Birch  Lake  the  Duluth  gabbro  lies 
above  the  iron-bearing  rocks.  Farther  east  the  formation  is  cut 
off  by  the  Duluth  gabbro;  on  the  west  it  probably  thins  out,  the 
Pokegama  quartzite  and  Virginia  slate  coming  together. 

The  bulk  of  the  Biwabik  formation  exposed  is  ferruginous  chert, 
with  which  are  varying  amounts  of  amphibole,  some  lime  and  iron 
carbonate,  and  bands  and  irregular  deposits  of  iron  ore.  Asso- 
ciated with  the  slaty  layers  in  the  iron-bearing  formation  or 
closely  adjacent  to  the  overlying  Virginia  slate  are  green  rocks 
made  up  of  small  granules  of  a  ferrous  silicate  called  greenalite. 
The  greenalite  has  at  some  localities  been  replaced  by  cherty 
quartz,  magnetite,  hematite,  limonite,  and  other  minerals,  and 
associated  with  the  greenalite  rocks  are  small  quantities  of  lime 
and  iron  carbonates. 

At  the  east  end  of  the  range,  near  Birch  Lake,  the  iron-bearing 
formation  has  been  metamorphosed  by  intrusion  of  gabbro.  It 
is  greatly  indurated  and  changed  to  a  rock  composed  of  mag- 
netite, amphibole,  olivene  and  quartz.  There  are  large  quantities 
of  this  material.  Experiments  at  the  School  of  Mines  Experiment 
Station  of  the  University  of  Minnesota  have  shown  that  it  may 
easily  be  concentrated  to  a  high-grade  Bessemer  product  in  the 
Davis  magnetic  concentrator.  It  is  not  yet  exploited  on  a  com- 
mercial scale,  but  in  the  future,  as  iron-ore  reserves  decrease,  it 
may  become  an  important  economic  asset. 


IRON 


305 


The  Biwabik  formation  may  be  subdivided  into  four  members 
(Figs.  135,  136).  These  are,  from  bottom  to  top,  the  lower 
cherty  member,  the  lower  slaty  member,  the  upper  cherty 
member,  and  the  upper  slaty  member.  The  principal  ore  bodies 
are  in  the  cherty  members  and  in  the  lower  slaty  member. 

Only  a  small  part  of  the  iron-bearing  formation  is  iron 
ore. 

The  Virginia  slate  rests  on  the  Biwabik  formation.  At  the 
east  end  of  the  district,  however,  the  slate  is  not  exposed,  and 


ssH 


FIG.  135. — Generalized  cross-section  showing  relation  of  Biwabik  iron- 
bearing  formation  to  associated  rocks,  in  the  Mesabi  range,  Minn.     (After 


the  Biwabik  is  capped  by  the  Duluth  gabbro.  Near  the  contact 
with  the  gabbro  the  mineral  composition  of  the  slate  is  changed 
and  typical  heavy  silicate  minerals  are  developed.  At  its  base 
the  slate  grades  into  the  underlying  iron-bearing  formation, 
which  is  itself  slaty  in  places. 


FIG.  136. — Generalized  cross-section  showing  subdivisions  of  the  Biwabik 
iron-bearing  formation,  Mesabi  range,  Minn.     (After  Wolff.) 

Flat-lying  conglomerates  and  shales  of  Cretaceous  age,  cap 
the  Algonkian  and  Archean  formations  here  and  there.  The 
basal  beds  of  the  Cretaceous  locally  carry  detrital  iron  ores  derived 
from  the  weathered  iron-bearing  formation. 

The  lower-middle  Huronian  beds  and  also  the  Archean 
formations  were  subjected  to  close  folding  before  the  upper 
Huronian  sediments,  including  the  Pokegama,  the  iron-bearing 
formation,  and  the  Virginia  slate,  were  deposited.  Consequently 
the  dips  of  the  older  beds  are  at  few  places  in  accord  with  the 


306      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

dips  of  the  iron-bearing  series,  which  range  in  general  from  5° 
to  20°,  toward  the  south  or  southeast.  In  the  vicinity  of  Virginia, 
Eveleth,  and  Gilbert  the  beds  are  gently  folded  and  make  a  broad 
loop.  The  dips  of  the  beds  are  generally  normal  to  the  contact 
of  the  ore-bearing  formation  with  underlying  rocks.  West  of 
Virginia  the  dips  are  nearly  everywhere  toward  the  south,  but 
at  Virginia  the  beds  make  a  sharp  swing,  and  between  Virginia 
and  Eveleth  the  dip  is  northwestward.  About  a  mile  south  of 
Virginia  the  iron-bearing  beds  dip  west  of  north  at  an  angle  of 
10°.  At  Eveleth  the  beds  make  a  sharp  turn,  swinging  back 
again,  and  in  some  of  the  pits  northwest  of  Eveleth  the  iron- 
bearing  formation  dips  nearly  due  west  at  low  angles.  East 
of  Eveleth  the  beds  resume  their  northeast  strike,  and  in  the 
mines  between  Eveleth  and  Gilbert  they  dip  nearly  southeast. 
Dips  toward  the  southeast  prevail  from  Gilbert  to  Biwabik  and 
eastward  to  the  end  of  the  range.  This  loop  and  a  section  across 
it  are  shown  in  Fig.  134. 

The  iron  ores  are  formed  by  local  concentration  in  the  iron- 
bearing  formation.  This  formation  contains  conglomerate  and 
quartzite  layers  near  the  base  and  here  and  there  thin  layers  of 
slate  or  other  sedimentary  rocks.  The  ferruginous  layers  grade 
laterally  into  slate  bands,  and  upward  the  formation  grades 
into  the  Virginia  slate.  The  formation  as  a  whole  is  extensive 
laterally  and  it  has  a  comparatively  uniform  thickness  like  a  bed 
deposited  in  water. 

The  iron-bearing  formation  is  not  a  common  type  of  sedi- 
mentary rock.  It  is  thought  by  some  that  the  waters  which 
deposited  the  iron  protore  were  derived  from  volcanic  centers  and 
that  the  iron  has  been  leached  out  of  igne6us  rocks,  possibly  by 
hot  waters,  and  contributed  to  the  sea.1 

Only  small  proportions  of  the  Biwabik  formation  are  rich 
enough  to  constitute  iron  ore.  These  are  patches  here  and  there 
along  the  eroded  surface  of  the  iron-bearing  formation.  The 
workable  deposits  are  due  to  secondary  concentration.  The  ore 
rarely  extends  to  depths  of  more  than  400  feet  below  the  bed- 
rock surface,  although  at  some  places  it  is  thicker. 

The  development  of  the  ore  from  protore  has  been  worked  out 
quantitatively  by  Van  Hise,  Leith,  and  Mead.2 


HISE,  C.  R.,  and  LEITH,  C.  K:    Geology  of  the  Lake  Superior 
Region,  U.  S.  Geol.  Survey  Mon.  p.  169,  1911. 

1  VAN  HISE,  C.  R.,  and  LEITH,  C.  K.:  Op.  cit.,  p.  194. 


IRON 


307 


In  the  Stevenson  mine  four  samples  were  taken  from  a  layer 
in  which  the  ore  grades  into  taconite.  No.  1  is  fresh  or  but 
slightly  altered  protore;  Nos.  2  and  3  are  intermediate,  partly 
altered;  No.  4  is  the  leached  protore  or  low-grade  iron  ore.  The 
chemical  analyses  and  the  volume  composition  are  given  in 
the  table  below: 


ANALYSES  OP  FERRUGINOUS  MATERIAL  OF  THE  MESABI  RANGE  IN  VARIOUS 
STAGES  OF  ALTERATION 


i 

2 

3 

4 

Chemical  composition: 
Fe 

29  470 

33  010 

35  260 

48  880 

Si02     . 

52  890 

50  080 

43  440 

25  030 

P  

0  016 

0  016 

0  013 

0  015 

A12O3  

0  620 

0  350 

0  400 

0  210 

Loss  on  ignition  
Volume  composition: 
Pore  space 

2.920 
8  000 

1.650 
16  500 

4.480 
26  300 

3.830 
52  700 

Hematite  and  limonite 

32  350 

31  250 

33  510 

30  810 

Quartz  
Kaolin  •  

57.900 
1.740 

51  .  400 
0.930 

39.300 
0.920 

16.180 
0.340 

The  original  greenalite  is  altered  to  taconite  (ferruginous  chert), 
which  in  turn  is  altered  to  iron  ore  by  the  loss  of  silica.  Through 
the  leaching  process  magnesia,  small  amounts  of  lime,  and  alkalies 
are  also  dissolved  out,  and  these  aid  in  the  solution  of  silica.  The 
net  result  is  to  concentrate  the  iron  and  to  develop  correspond- 
ing pore  space,  which,  however,  is  decreased  by  the  slumping 
of  the  cellular,  porous,  weak  iron  ore.  Evidence  of  such  slump- 
ing is  found  in  many  of  the  mines,  where  the  ore  beds  dip  toward 
the  thicker  and  richer  parts  of  the  ore  body,  which  in  general  are 
near  the  center. 

Concentration  of  this  nature  in  places  where  water  solutions 
have  found  more  ready  access  has  been  going  on  through  long 
geologic  periods.  That  it  was  well  advanced  in  Cretaceous  time 
is  shown  in  the  detrital  ore  of  the  Cretaceous,  which  carries 
abundant  iron  ore  in  the  form  of  polished  pebbles. 

Many  of  the  ore  bodies  have  been  formed  where  the  iron-bear- 


308      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

A -Tension  Cracfcs  in  Iron-Formation  on" 'Axs  of  an  Anticline. 


B-Ore  forming  by  alteration  of  Taconite  along  Fissuresi  Bedding-Plants. 


0-  Present  Condition   of   Average  Trough  Orebody. 


FIG.  137. — Cross-sections  showing  stages  in  development  of  a  trough  ore 
body  on  the  axis  of  a  gentle  anticline.     (After  Wolff.) 


IRON  309 

ing  formation  has  been  fractured,  especially  along  axes  of  folds. 
Where  there  are  only  a  few  closely  spaced  fractures,  a  vein-like, 
vertical  mass  of  ore  will  be  formed.  On  anticlines  the  brittle 
iron-bearing  formation  is  crossed  by  numerous  cracks.  Waters 
circulating  along  these  cracks  have  developed  great  ore  bodies,1 
in  the  manner  shown  diagrammatically  by  Fig.  137.  Section 
A  shows  fissures  along  the  crest  of  a  gentle  anticline.  Section 
B  shows  that  iron  ore  has  formed  along  the  fractures.  The 
slaty  layer  near  the  center  of  the  iron-bearing  formation  is  not 
readily  leached,  but  leaching  may  take  place  in  the  iron-bearing 
formation  below  it,  and  slumping  of  the  slate  develops  a  trough- 
like  structure  in  the  anticline.  Sections  C  and  D  show  that 
the  slate  has  altered  to  " paint  rock"  which  has  sagged  down 
owing  to  shrinkage  in  the  lower  ore  body. 

Cuyuna  Range,  Minn. — The  Cuyuna  range,2  southwest  of  the 
Mesabi  range,  extends  from  Aitkin  through  Deerwood  and 
Brainerd  to  a  point  beyond  Fort  Ripley.  As  now  defined  it  is 
about  65  miles  long  in  a  N.  50°  E.  direction  and  ranges  in  width 
from  1  or  2  miles  to  12  miles.  This  range  is  a  comparatively 
late  discovery  and  has  recently  entered  the  producing  stage. 
It  was  discovered  by  drilling  areas  showing  strong  magnetic 
attraction.  The  surface  is  hilly,  and  the  ore-bearing  forma- 
tion is  overlain  by  a  thicker  mantle  of  drift  than  that  in  the 
Mesabi  range.  The  geologic  succession  is  as  follows: 

Quaternary  system : 

Pleistocene  series Glacial  drift,  15  to  400  feet  thick. 

Cretaceous  system Sediments,  small  areas. 

Algonkian  system: 

Keweenawan  (?)  series.  ..  .Igneous    rocks,     extrusive     and    intrusive, 
basic  and  acidic. 

1  WOLFF,   J.   E. :  Recent   Geologic   Developments  on  the   Mesabi  Iron 
Range,  Minnesota.     Am.  Inst.  Min.  Eng.  Bull.  118,  pp.  1763-1787,  1916. 

2  HARDER,  E.  C.,  and  JOHNSTON,  A.  W. :  Preliminary  Report  on  the  Geol- 
ogy of  East-central  Minnesota,   Including  the  Cuyuna  Iron-ore  District. 
Minn.  Geol.  Survey  Bull.  15,  1917. 

LEITH,  C.  1C:  The  Geology  of  the  Cuyuna  Iron  Range,  Minnesota. 
Econ.  Geol.,  vol.  2,  pp.  145-152,  1907. 

ZAPFFE,CARL:  The  Cuyuna  Iron-ore  District  of  Minnesota.  Supplement 
to  the  Brainerd  (Minn.)  Tribune,  1910,  pp.  32-35  (with  map). 

ADAMS,  F.  S.:  The  Iron  Formation  of  the  Cuyuna  Range.  Econ.  Geol., 
vol.  5,  pp.  729-740,  1910;  vol.  6,  pp.  60-70,  156-180,  1911. 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K. :  The  Geology  of  the  Lake  Superior 
Region:  U.  S.  Geol.  Survey  Man.  52,  p.  211,  1911. 


310      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Virginia  slate:  Chloritic,  sericitic,  quartz- 
ose,  and  carbonaceous  slates  and  schists, 
with  small  amounts  of  interbedded  gray- 
wackes,  quartzite,  and  limestone.  Thick- 
ness great.  Locally  metamorphosed  basic 


Huronian  series: 
Upper    Huronian 
(Animikie  group) : 


igneous  rocks  are  found  interlayered  with 
the  schists  and  slates. 
Deerwood  iron-bearing  member  of  Virginia 


slate.  Contained  originally  much  iron  car- 
bonate but  is  largely  altered  to  amphi- 
bole-magnetite  rocks,  magnetitic  slate, 
ferruginous  chert  and  slate,  and  iron  ore. 
It  is  found  in  lenses  in  the  Virginia  slate, 
presumably  near  the  base. 

The  sedimentary  rocks  strike  in  general  about  N.  50°  E.  and 
are  folded  closely,  so  that  they  dip  at  high  angles,  chiefly  to  the 
southeast.  East,  south  and  southwest  of  the  district,  micaceous, 
garnetiferous,  staurolitic,  and  hornblendic  schists  crop  out  in 
many  places,  .and  are  associated  with  acidic  and  basic  igneous 
rocks,  many  of  which  are  intrusive  into  the  schists. 

The  iron-bearing  formation  occurs  in  eight  or  ten  northeast- 
ward-trending, discontinuous  belts  of  varying  length.  The  dip 
of  the  beds  is  usually  steep,  and  the  prevailing  dip  is  southeast. 
The  formation  is  inclosed  between  walls  of  sericitic,  chloritic, 
or  quartzoze  schist  or  slate.  Carbonaceous  slate  also  is  a  com- 
mon associate.  In  many  places  lenses  of  chloritic  schist  are 
interlayered  with  the  iron-bearing  beds.  The  formation  con- 
sists mainly  of  ferruginous  chert,  but  ferruginous  slate  is 
abundant,  and  in  parts  of  the  district  the  metamorphosed  phases, 
such  as  magnetitic  slate  and  amphibole-magnetite  rock,  are 
prominent. 

The  ore  bodies  are"  irregularly  lens-shaped  and  lie  within  the 
iron-bearing  layers;  the  longer  diameter  is  usually  parallel  to 
the  bedding  of  the  iron-bearing  formation.  They  may  be  in- 
closed within  ferruginous  chert  or  slate  or  other  phases  of  the 
iron-bearing  formation.  Many  of  them  are  also  bounded  on  one 
or  both  sides  by  schist  or  slate  wall  rocks.  In  places  original 
iron-bearing  rocks  such  as  cherty  and  slaty  iron  carbonate  have 
been  encountered  at  varying  depths  below  the  ore  and  the  asso- 
ciated altered  phases  of  the  iron-bearing  formation. 

The  ore  bodies  have  a  maximum  width  of  several  hundred 
feet,  and  some  are  known  to  extend  along  the  strike  for  nearly  a 
mile.  The  ores  are  soft  and  hard  and  are  in  the  main  non- 


IRON  311 

Bessemer.  The  principal  ore  is  a  much  fractured  dark-brown 
hydrated  hematite  ore  containing  50  to  60  per  cent,  of  iron. 
Locally,  however,  there  are  deposits  of  high-grade  dark  reddish- 
purple  hematite,  much  of  which  is  soft,  and  elsewhere  yellow- 
ish-brown medium-soft  ocherous  limonite  forms  valuable  deposits. 
The  ores  are  in  the  main  superficial  alteration  products  of  various 
types  of  iron-bearing  rocks.  Most  of  them  probably  do  not 
extend  to  depths  of  more  than  a  few  hundred  feet  below  the 
bedrock  surface.  Many  of  the  iron  deposits  are  highly  manga- 
niferous  (see  page  510). 

Penokee-Gogebic  Range,  Wisconsin  and  Michigan. — The 
Penokee-Gogebic  range  is  south  of  Lake  Superior  in  northern 
Michigan  and  Wisconsin.  The  range  trends  N.  30°  E.  and  is 
well  defined  for  a  distance  of  80  miles.  The  principal  mines 
are  near  Hurley,  Wis.,  and  near  Iron  wood,  Wakefield,  and 
Bessemer,  Mich.  (Fig.  138). 

The  succession  of  formations  in  the  district  is  as  follows:1 


Cambrian  system Lake  Superior  sandstone. 

Unconformity. 
Algonkian  system: 

Keweenawan  series Gabbros,  diabases,  conglomerates,  etc. 

Unconformity. 

Huronian  series: 

Greenstone  intrusives  and  extrusives. 


Upper    Huronian 
(Animikie  group) 


Tyler  slate. 

Ironwood  formation  (iron-bearing). 


Palms  formation. 

Unconformity. 

T           __  /  Bad  River  limestone. 

Lower  Hutoman (  gunday  quartzite 

Unconformity. 
Archean  system: 

Laurentian  series Granite  and  granitoid  gneiss. 

Eruptive  unconformity. 

Keewatin  series Greenstones  and  green  schists. 


1  IRVING,  R.  D.,  and  VAN  HISE,  C.  R.:  The  Penokee-Gogebic  Iron-bearing 
Series  of  Michigan  and  Wisconsin.  U.  S.  Geol.  Survey  Mon.  19,  pp.  1-534, 
1892. 

CHAMBERLIN,  T.  C.:  "Geology  of  Wisconsin,"  vol.  1,  pp.  3-300,  1883. 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K:  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  52,  pp.  225-250,  1911. 


312      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


IRON  313 

In  a  broad  way,  the  iron-bearing  rocks  correspond  to  the 
Animikie  iron-bearing  series  of  the  Mesabi  range,  Minnesota. 
The  Palms  formation,  the  lowest  member,  is  made  up  of  con- 
glomerate, clay  slate,  and  quartzite;  the  quartzite  forms  the  foot 
wall  of  the  ore  bodies  at  many  places.  The  Ironwood  formation 
corresponds  in  age  and  character  to  the  Biwabik  formation  of 
the  Mesabi  range.  It  is  succeeded  by  the  Tyler  slate,  corre- 
sponding to  the  Virginia  slate  on  the  Mesabi  range.  On  the 
Mesabi  the  ore-bearing  series  dips  southeast  at  low  angles;  on 
the  Penokee-Gogebic  the  series  dips  north  or  northwest  at  high 
angles.  Low  dips  on  the  Mesabi  favor  the  development  of  the 
broad,  shallow  deposits  that  are  worked  by  open  cuts.  Steep 
dips  and  narrower  outcrops  on  the  Penokee-Gogebic  range  make 
it  necessary  to  do  the  mining  mainly  underground.  In  both 
districts  the  sedimentary  Animikie  rocks  have  a  comparatively 
simple  structure,  but  on  the  Penokee-Gogebic  range  they  are 
intruded  by  many  dikes,  mainly  basic  in  composition.  On  the 
Mesabi  range  the  protore  is  greenalite  rock  with  some  iron  car- 
bonate; on  the  Penokee-Gogebic  the  protore  is  chiefly  cherty 
iron  carbonate,  associated  with  a  little  greenalite. 

In  the  Penokee-Gogebic  district  the  Archean  rocks  include  the 
Keewatin  greenstones  and  schists  and  the  Laurentian  granites 
and  gneisses;  the  Laurentian  rocks  are  intrusive  into  the  Kee- 
watin. The  lower  Huronian  is  represented  only  by  the  Sunday 
quartzite  and  the  Bad  River  limestone,  but  at  most  places  these 
formations  are  not  present  and  the  Animikie  or  upper  Huronian 
rests  directly  on  the  Archean.  The  Laurentian  rocks  were 
eroded  nearly  to  base-level  before  the  lower  Huronian  was 
deposited;  consequently  the  contact  between  them  is  a  nearly 
plane  surface. 

The  upper  Huronian  comprises  the  Palms,  Ironwood,  and 
Tyler  formations,  a  conformable  series  which  constitutes  a 
northward-dipping  monocline.  The  Palms  formation  consists 
of  three  members:  the  lowest  is  a  thin  conglomerate,  above  this 
is  a  clay  slate,  and  above  the  slate  is  a  quartzite.  The  formation 
is  unconformable  with  the  underlying  Bad  River  limestone, 
although  there  is  no  considerable  discordance  in  bedding. 

The  Ironwood  formation  is  above  the  Palms  and  conforms 
with  it  in  strike  and  dip.  Its  outcrop  is  1,000  feet  wide,  or  less 
at  most  places,  but  locally,  probably  where  folded  or  faulted, 
it  is  broader.  In  the  eastern  part  of  the  district  volcanic  action 


314      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

prevailed  during  its  deposition,  and  there  the  iron  beds  are  thin 
and  impure.  The  greater  part  of  the  formation  carries  more  than 
25  per  cent,  of  iron,  and  considerable  portions  average  37  per 
cent.  The  ore  bodies  are  altered  by  superficial  processes  and 
are  much  richer  in  iron.  The  Ironwood  formation  consists  of 
slaty,  cherty  iron  carbonate,  with  some  greenalite  and  ferro- 
dolomite;  ferruginous  slates  and  cherts;  actinolitic  and  magnetitic 
slates,  formed  largely  through  metamorphism  by  Keweenawan 
intrusive  rocks;  and  black  carbonaceous  fragmental  slates.  The 
iron-bearing  carbonates  are  found  usually  near  the  upper  part 
of  the  formation,  where  they  have  been  protected  from  alteration 
by  the  Tyler  slate.  The  ferruginous  slates  and  ferruginous 
cherts  are  characteristic  of  the  central  iron-producing  part  of 
the  district,  and  the  actinolitic  and  magnetitic  slates  are  charac- 
teristic of  the  western  and  eastern  parts  of  the  district.  The 
latter  also  form  a  belt  20  to  300  feet  wide  bordering  the  Kewee- 
nawan rocks  on  the  north.  Thin  black  slates  are  intercalated  in 
the  iron-bearing  formation.  The  Ijonwood  passes  gradually 
into  the  overlying  Tyler  slate,  which  is  7,000  to  11,000  feet  thick. 
It  consists  of  mica  schists  and  slates,  graywackes,  and  clay  slates. 

Keweenawan  basic  effusive  and  intrusive  rocks  rest  on  the 
upper  Huronian  unconformably,  but  there  is  no  great  discord- 
ance in  bedding  between  the  Keweenawan  flows  and  the  upper 
Huronian  metamorphosed  sediments.  In  the  central  part  of 
the  district  the  Keweenawan  rocks  rest  on  the  Tyler  slate.  At 
the  east  and  west  ends  the  Keweenawan  diagonally  crosses 
the  eroded  slates  and  rests  on  the  Ironwood. 

The  ore  bodies  are  the  portions  of  the  Ironwood  formation 
that  have  been  enriched  by  surface  agencies.  Only  a  small  area 
of  the  iron-bearing  formation  is  workable,  and  the  ores  are  found 
only  in  the  central  part  of  the  range,  in  a  belt  about  26  miles  long 
lying  between  Potato  River,  Wisconsin,  and  a  point  east  of 
Sunday  Lake,  Michigan. 

Oxidation  extends  to  a  depth  of  more  than  2,000  feet.  At 
this  depth  in  the  Newport  mine  the  iron-bearing  formation  is  as 
thoroughly  oxidized  and  leached  as  it  is  near  the  surface.  The 
downward  circulation  has  been  controlled  by  pitching  troughs, 
and  thus  oxidation  has  been  carried  to  extraordinary  depths  in 
channels  where  downward-moving  waters  concentrated.  The 
quartzite  of  the  Palms,  which  forms  the  foot  wall  of  the  iron- 
bearing  formation,  strikes  from  east  to  northeast  and  dips  about 


IRON 


315 


65°  N.  The  Tyler  slate,  above  the  Ironwood  formation,  is  rel- 
atively impervious.  The  Huroniari  rocks  are  cut  by  many  basic 
(greenstone)  dikes  of  Keweenawan  age.  These  cross  the  forma- 
tions, and  many  of  them  dip  southeast  so  that  they  form  troughs 
with  the  quartzite,  the  bottoms  of  the  troughs  pitching  eastward. 
Some  troughs  are  formed  also  by  westward-pitching  dikes  inter- 
secting eastward-pitching  dikes,  and  by  dikes  intersecting  slate 
bands  in  the  ore-bearing  formation.  These  relations  are  shown 
by  Fig.  139.  Several  ore  bodies  may  be  formed  in  troughs 
one  below  another. 


FIG.  139. — Longitudinal  section  of  Ashland  mine,  Penokee-Gogebic  dis- 
trict, Michigan.  Shows  ore  bodies  lying  on  dikes  one  above  the  other. 
(AfUr  Van  Hise  and  Leith,  U.  S.  Geol  Survey  from  plate  furnished  by  Olcott.) 


Menominee  District,  Michigan.1 — The  Menominee  district 
is  in  Michigan  not  far  west  of  Escanaba,  but  outlying  areas 
that  may  properly  be  included  in  this  district  are  the  Florence 
area,  in  Wisconsin,  and  the  Crystal  Falls,  Iron  River,  and  Met- 
ropolitan areas,  in  Michigan.  The  rocks  in  the  Menominee 
district  proper  are  the  following: 

1  BAYLEY,  W.  S. :  The  Menominee  Ironbearing  District  of  Michigan.  U. 
S.  Geol.  Survey  Mon.  46,  1904. 

VAN  HISE,  C.  R.,  and  BAYLEY,  W.  S.:  U.  S.  Geol.  Survey  Geol  Atlas, 
Menominee  Special  Folio  (No.  62),  1900. 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K. :  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  52,  pp.  329-354,  1911. 

HOTCHKISS,  W.  O.:  Mineral-land  Classification  of  Wisconsin.  Wis. 
Geol.  and  Nat.  Hist.  Survey  Bull.  44,  1915. 

ALLEN,  R.  C. :  The  Iron  River  Iron-bearing  District  of  Michigan.  Mich. 
Geol.  and  Biol.  Survey  Pub.  3,  Geol.  series  2,  1910. 

HOTCHKISS,  W.  O.:  "Geology  of  the  Florence  District,  Wisconsin,"  1914. 

CLEMENTS,  J.  M.,  and  SMYTH,  H.  L. :  The  .Crystal  Falls  Iron-bearing  District 
of  Michigan  with  a  chapter  on  the  Sturgeon  River  Tongue  by  W.  S. 
BAYLEY.  TJ.  S.  Geol.  Survey  Mon.  36,  1899. 


316      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Pleistocene  series 

Cambro-Ordovician 

Cambrian  system 

Unconformity. 

Algonkian  system: 

Keweenawan  series 

Huronian  series: 


Upper  Huronian 
(Animikie  group) , 


Unconformity. 
Middle  Huronian. 

Unconformity. 
Lower  Huronian . . 

Unconformity. 
Archean  system: 

Laurentian  series . . . 


Keewatin  series . . . 


Glacial  deposits. 
Hermansville  limestone. 
Lake  Superior  sandstone. 


Granite  (?). 

Quinnesec  schist  and  other  green  schists  rep- 
resenting surface  eruptions  overlying  and 
interbedded  with  Michigamme  slate. 

Michigamme  ("Hanbury")  slate,  including 
iron-bearing  beds. 

Vulcan  formation,  subdivided  into  Curry 
iron-bearing  member,  Brier  slate  member, 
and  Traders  iron-bearing  member. 

Quartzite  generally  not  separated  from  Rand- 
ville  dolomite. 

I  Randville  dolomite. 
\  Sturgeon  quartzite.- 


Granites  and  gneisses  cut  by  granite  and 
diabase  dikes. 
Green  schists. 


The  iron  ores  are  in  the  Vulcan  and  Michigamme  formations 
of  the  Animikie  group.  The  Traders  iron-bearing  member  of 
the  Vulcan  consists  of  conformable  beds  of  ferruginous  conglomer- 
ates, ferruginous  quartzites,  heavily  ferruginous  quartzose  slates, 
and  iron-ore  deposits.  The  conglomerates  and  quartzites  are 
usually  at  the  base  of  the  member,  resting  upon  the  Randville 
dolomite.  These  rocks  contain  fragments,  usually  of  quartzite, 
jaspilite,  white  quartz,  and  rocks  that  make  up  the  Archean 
complex.  In  many  places  the  conglomerate  contains  enough 
iron  to  mine.  Most  of  this  ore  is  schistose  and  carries  scales  of 
specularite.  The  conglomerates  and  quartzites  pass  upward 
into  the  ferruginous  slates.  The  Brier  slate,  black  and  ferrugi- 
nous, rests  conformably  above  the  Traders  iron-bearing  member. 
The  Curry  iron-bearing  member  consists  of  interbedded  jaspilites 
and  ferruginous  quartzose  slates  and  iron-ore  deposits.  It  has 
resulted  from  the  alteration  of  greenalite  rock  like  that  in  the 
Mesabi  district  and  of  iron  carbonate  like  that  in  the  Penokee- 


IRON  317 

Gogebic  district.1  Greenalite  and  iron  carbonate  are  not  now 
present,  but  pseudomorphs  of  both  are  abundant. 

The  Vulcan  formation  and  the  overlying  Michigamme  slate 
are  conformable,  and  the  contact  is  usually  sharp.  The  Michi- 
gamme formation  is  composed  of  black  and  gray  clay  slates,  gray 
calcareous  slates,  graphitic  slates,  graywackes,  thin  beds  of 
quartzite,  local  beds  of  ferruginous  dolomite  and  siderite,  and 
rarer  bodies  of  ferruginous  chert  and  iron  oxide. 

The  formations  that  carry  the  ore  deposits  are  all  closely 
folded.  The  larger  deposits  N 

rest    Upon     relatively     imper-  Traders  Ore-Bearing  Bed 

vious  formations  whose  folds 
form  pitching  troughs.  Such 
a  trough  may  be  made  by 
the  Randville  dolomite,  un- 
derlying the  Traders  iron- 
bearing  member;  by  a  slate 
constituting  the  lower  part  of 
the  Traders  member:  or  by  *IG-  140.— Vertical  section  through 
A.  _.  .  .  .  J  Aorway-Aragon  region,  Menommee 

the  Brier  slate  member,  be-  district,  Michigan.  Ore  is  developed 
tween  Traders  and  Curry  ^trough.  (After  Bayley,  U.  S.  Geol. 
iron-bearing  members.  The 

dolomite  or  quartzite  formation  is  likely  to  furnish  an  imper- 
vious basement,  especially  where  its  upper  portion  has  been  trans- 
formed into  a  talc  schist  (Fig.  140). 

Marquette  District,  Michigan.2 — The  Marquette  district  ex- 
tends from  Marquette,  Mich.,  on  Lake  Superior,  westward 
nearly  40  miles,  to  Lake  Michigamme.  The  principal  towns  in 
the  district  are  Marquette,  Ishpeming,  Negaunee,  Champion, 
and  Republic.  The  outcrops  of  the  Algonkian  rocks  range  in 
width  from  about  1  mile  to  more  than  6  miles.  From  the  west- 
ern part  of  the  main  Algonkian  area  the  Republic  trough  pro- 
jects southeastward  and  the  Western  trough  projects  southward. 
The  formations  are  as  follows: 


1  VAN  HISE,  C.  R.,  and  LEITH,  C.  K.:  The  Geology  of  the  Lake  Superior 
Region.     U.  S.  Geol.  Survey  Mon.  52,  p.  327,  1911. 

2  VAN  HISE,  C.  R.,  BAYLEY,  W.  S.,  and  SMYTH,  H.  L. :  The  Marquette 
Iron-bearing  District  of  Michigan.     U.  S.  Geol.  Survey  Mon.  28,  1897 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K.:    The  Geology  of  the  Lake  Superior 
Region.     U.  S.  Geol.  Survey  Mon.  52,  pp.  251-283,  1911. 


318      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Quaternary  system: 

Pleistocene  series Glacial  deposits. 

Cambrian  system: 

Upper  Cambrian  sandstone  (Potsdam  sandstone). 
Unconformity. 
Algonkian  system: 

Keweenawan  series Not  identified  but  probably  represented  by 

part  of  intrusives  in  upper  Huronian. 
Huronian  series: 

[  Greenstone  intrusives  and  extrusives. 

Michigamme  slate   (slate  and  mica  schist), 

Upper  Huronian       I    locally  largely  replaced  by  volcanic  Clarks- 
(Animikie  group) .  )     burg  formation. 

Bijiki  schist  (iron-bearing). 
Goodrich  quartzite  (iron-bearing). 
Unconformity. 

[  Negaunee  formation  (chief  productive  iron- 

'      f°rmation)' 


Middle  Huronian. .  •>  „.  ,   , 

Siamo  slate. 

[  Ajibik  quartzite. 
Unconformity. 

f  Wewe  slate. 
Lower  Huronian  . . .  {  Kona  dolomite. 

[  Mesnard  quartzite. 
Unconformity. 
Archean  system: 

Laurentian  series grfnite'  syenite'  Peridotite' 

Palmer  gneiss. 

Kitchi  schist  and  Mona  schist,  both  green 

-j^.  schists,   the    Mona  banded  and  in  a  few 

places  containing  narrow  bands  of  non-pro- 
ductive iron-bearing  formation. 

Basic  igneous  dikes  and  bosses  intrude  all  the  Archean  and 
Huronian  formations.1 

The  principal  iron-bearing  formation  is  the  Negaunee  (middle 
Huronian),  but  the  Bijiki  schist  (upper  Huronian)  also  is  iron- 
bearing.  The  Goodrich  quartzite,  which  overlies  the  Negaunee 
unconformably,  includes  a  basal  conglomerate,  derived  from  the 
underlying  Negaunee  rocks,  that  locally  contains  iron  ore.  As 
the  Marquette  district  is  structurally  a  synclinorium,  the 
Negaunee  formation  is  complexly  folded  so  that  it  crops  out  over 
a  large  area,  especially  in  the  east  end  of  the  district  (Fig.  141). 

The  protore  in  the  Negaunee  formation  was  originally  iron 
carbonate  and  greenalite  interbedded  with  more  or  less  slate  and 

1  VAN  HISE,  C,  R.,  BAYLEY,  W.  S.,  and  SMYTH,  H.  L. :  Op.  eft. 


IRON 


319 


v_ 


UB13JUOSIV 


320      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

containing  much  detrital  ferric  oxide  at  the  base  of  the  forma- 
tion. The  alteration  was  accomplished  by  oxidation  and  hydra- 
tion  of  the  iron  minerals  in  place,  leaching  of  silica,  and  intro- 
duction of  secondary  iron  oxide  and  iron  carbonate  dissolved 
from  other  parts  of  the  formation.  The  flow  of  water  concen- 
trating the  ore  moved  principally  in  planes  parallel  to  the  bedding 
and  was  especially  effective  in  pitching  troughs.  The  pitching 
troughs  in  this  district  are  formed  where  a  basic  dike  or  boss  cuts 
an  impervious  bed,  where  two* igneous  bodies  are  joined,  or  where 


FIG.  142. — Generalized  section  in  Marquette  district,  Michigan,  showing 
relations  of  all  classes  of  ore  deposits  to  associated  formations.  On  the  right 
is  soft  ore  resting  in  a  V-shaped  trough  between  the  Siamo  slate  and  a  dike 
of  soapstone.  In  the  lower  central  part  of  the  figure  the  more  common 
relations  of  soft  ore  to  vertical  and  inclined  dikes  cutting  the  jasper  are 
shown.  The  ore  may  rest  upon  an  inclined  dike,  between  two  inclined 
dikes,  and  upon  the  upper  of  the  two,  or  be  on  both  sides  of  a  nearly  vertical 
dike.  In  the  upper  central  part  of  the  figure  are  seen  the  relations  of  the 
hard  ore  to  the  Negaunee  formation  and  the  Goodrich  quartzite.  At  the 
left  is  soft  ore  resting  in  a  trough  of  soapstone  which  grades  downward  into 
greenstone.  (After  Van  Rise,  Bayley  and  Smyth,  U.  S.  Geol.  Survey.) 

an  impervious  bed  is  folded.  Ores  of  the  Negaunee  formation 
are  found  near  the  base,  in  the  middle  zone  of  the  formation,  and 
near  the  top  (Fig.  142). 

The  ores  of  the  lower  and  middle  portions  of  the  Negaunee  are 
mainly  soft,  but  the  ores  near  the  top  are  hard.  The  middle 
Huronian,  including  the  Negaunee,  was  raised  above  sea 
level,  weathered,  and  partly  eroded  before  the  upper  Huronian 
sediments  were  deposited.  The  weathered  Negaunee  rocks  near 
the  ancient  surface  were  doubtless  enriched  by  the  removal  of 
material  other  than  iron.  After  subsidence  and  deposition  of  the 
overlying  upper  Huronian,  the  rocks  were  metamorphosed  and 
deformed  by  pressure.  .The  enriched  ores  were  thereby  changed 
to  specular  hematite  of  high  grade.  In  the  iron-bearing  forma- 
tion of  the  middle  and  lower  horizons  of  the  Negaunee,  silica  and 
alkaline  earths  had  not  then  been  removed  by  weathering,  and 
the  rock  was  not  oxidized  during  the  early  stage.  The  concen- 
tration of  the  iron  at  these  horizons  was  brought  about  by  sur- 
face waters  at  a  later  period,  after  the  ores  in  the  upper  part  of 


IRON 


321 


the  formation  had  been  rendered  hard  and  schistose  by  dynamic 
agencies. 

The  Vermilion  Range,  Minnesota. — The  Vermilion  Range,1  in 
northeastern  Minnesota,  extends  from  a  point  near  the  west 
end  of  Vermilion  Lake  about  20°  north  of  east  to  Gunflint  Lake, 
on  the  Canadian  boundary.  It  is  about  100  miles  long  and  from 
5  to  15  miles  wide. 

The  stratigraphic  succession,  as  stated  by  Clements,  is  as 
follows : 


Quaternary  system: 

Pleistocene  series 

Unconformity. 
Algonkian  system: 

Keweenawan  series 

Unconformity. 
Huronian  series: 

Upper  Huronian 

(Animikie  group) . 
Unconformity. 

Lower-middle 

Huronian 

Unconformity. 


Archean  system : 

Laurentian  series. , 


Keewatin  series. 


Glacial  drift. 


Duluth  gabbro  and  Logan  sills. 


Rove  slate. 

Gunflint  formation  (iron-bearing). 

Intrusive  rocks :  Granites,  granite  porphyries, 
dolerites. 
Knife  Lake  slate. 
Agawa  formation  (iron-bearing). 
Ogishke  conglomerate. 


Granite  of  Basswood  Lake  and  other  intru- 
sive rocks. 

f  Soudan  formation  (iron-bearing). 
{  Ely  greenstone,  a  basic  igneous  and  largely 
I    volcanic  rock. 


The  Ely  greenstone  is  the  oldest  and  the  most  extensive  forma- 
tion in  this  district.  It  consists  mainly  of  altered,  basic  igneous 
rocks,  probably  in  the  main  surface  flows.  At  many  places  these 
rocks  are  highly  schistose.  The  Soudan  formation,  which  was 
deposited  above  the  Ely  greenstone,  is  the  oldest  iron-bearing 
formation  in  the  district.  It  is  made  up  chiefly  of  beds  of  jasper 
and  contains  also  some  slates  and  conglomerates.  Some  beds 

1  WINCHELL,  N.  H.,  GRANT,  U.  S.,  and  WINCHELL,  H.  V. :  Minn.  Geol. 
and  Nat.  Hist.  Survey  Final  Kept.,  vol.  4,  1898. 

CLEMENTS,  J.  M. :  The  Vermilion  Iron-bearing  District  of  Minnesota. 
U.  S.  Geol.  Survey  Mon.  45,  p.  463,  1903. 

VAN  HISE,  C.  R.,  and  LEITH,  C.  K. :  The  Geology  of  the  Lake  Superior 
Region.     U.  S.  Geol.  Survey  Mon.  52,  pp.  118-143,  1911. 


322      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

are  composed  largely  of  iron  oxides  or  iron  carbonates.  The  Ely 
greenstone  and  the  Soudan  formation  are  intruded  by  granite, 
felsite,  and  porphyries,  which  locally  are  metamorphosed  to 
schists.  These  and  the  rocks  they  intrude  were  deformed  and 
otherwise  altered  and  in  places  were  eroded  before  the  sediments 
of  the  next  system,  the  Algonkian,  were  deposited. 

The  lowest  of  the  Algonkian  rocks  are  of  lower-middle  Huronian 
age.  The  oldest  member  of  this  series  is  the  Ogishke  conglomer- 
ate, which  locally  has  a  thickness  of  over  1,000  feet.  At  some 
places,  however,  it  is  lacking.  Above  the  Ogishke,  in  the  eastern 
part  of  the  district,  is  the  Agawa  formation,  which  contains  beds 
of  slate,  jasper,  iron  oxides,  and  iron  carbonates.  This  formation 
reaches  a  thickness  of  50  feet  and  contains  some  unimportant 
iron  ores. 

Above  the  Agawa  is  the  Knife  Lake  slate,  probably  several 
thousand  feet  thick.  The  Knife  Lake  slate  and  older  formations 
are  intruded  by  the  Giants  Range,  Snowbank,  and  Cacaquabic 
granites. 

The  upper  Huronian  rocks  that  contain  the  iron  ores  in  the 
Mesabi  range  are  of  little  value  in  the  Vermilion  district,  although 
they  occur  in  a  small  area  west  of  Gunflint  Lake. 

The  iron  ores  of  the  Vermilion  range  are  almost  exclusively  in 
the  Soudan  formation  and  in  places  are  2,000  feet  below  the  sur- 
face. The  principal  developments  are  at  Ely  and  Tower.  After 
the  iron-bearing  formation  was  subjected  to  close  folding,  erosion 
removed  the  upper  parts  of  many  folds.  Consequently  the 
remnants  of  the  formation  are  found  at  many  places  in  pitching 
troughs,  which  as  a  rule  are  bottomed  by  the  Ely  greenstone  or 
by  the  soapstone  or  paint-rock — in  general  the  altered  phases  of 
the  Ely  greenstone — or  of  certain  aluminous  or  magnesian  por- 
tions of  the  Soudan  formation.  The  paint-rock  or  soapstone 
and  associated  rocks  are  comparatively  impervious  to  water. 
The  Soudan  formation  consisted  originally  of  cherty  iron  car- 
bonate and  probably  some  banded  chert  and  iron  oxide.  Surface 
waters  circulating  down  the  tilted  troughs  leached  out  the  silica, 
leaving  the  iron  oxide  concentrated.  Some  of  this  concentration 
took  place  before  the  lower  Huronian  sediments  were  deposited, 
as  is  indicated  by  the  fact  that  the  lower  Huronian  conglomerates 
contain  detrital  iron  ores.  Close  folding  after  the  lower  Huronian 
deposition  rendered  the  ores  hard,  anhydrous,  and  crystalline. 
At  Ely  much  concentration  has  taken  place  also  since  the 


IRON  323 

deposition  of  the  lower  Huronian.  Because  the  ore  formations 
in  the  Vermilion  district  were  closely  folded  after  some  concentra- 
tion by  ground  water  had  taken  place,  the  enriched  ores  are 
locally  deep-lying. 

CLINTON  HEMATITE  DEPOSITS 

Hematite  deposits  are  found  at  many  places  along  the  outcrops 
of  the  Clinton  (Silurian)  formation  from  New  York  to  Alabama 
but  are  workable  only  here  and  there.  Valuable  deposits  of  the 
Clinton  type  have  been  found  in  New  York,  Pennsylvania, 
Virginia,  Tennessee,  Georgia,  Alabama  and  Wisconsin. 

The  deposits  are  lenses  in  sandstone  and  shale  and  occur  at 
several  horizons  in  the  formation.  At  some  places  there  are 
three  or  four  beds,  generally  less  than  10  feet  thick,  although 
some  are  much  thicker.  At  Birmingham,  Ala.,  the  "Big  seam" 
is  16  to  40  feet  thick.  Two  types  of  ore  are  noteworthy — the 
fossil  ore,  made  up  of  fossil  fragments,  mainly  those  of  lime- 
secreting  organisms  replaced  by  iron  oxide,  and  oolitic  ore,  made 
up  of  small  spherules.  Some  of  the  ore  is  soft  and  some  is  hard. 
The  soft  ores  are  the  weathered  parts  of  the  seams;  they  form  the 
outcrops  and  extend  downward  for  varying  distances.  Below 
them  the  hard  ore  is  found.  The  soft  ore  carries  about  50  per 
cent,  of  iron  and  12  per  cent,  of  silica.  The  hard  ore  is  of  lower 
grade  and  carries  more  lime.  Its  composition  is  approximately 
as  follows:  Iron  35  per  cent.,  silica  25  per  cent.,  lime  20  per  cent. 
By  mixing  hard  and  soft  ores  a  self-fluxing  furnace  charge  is 
obtained;  thus  the  ores  are  cheaply  beneficiated. 

The  reserves  of  the  Clinton  ores  are  large.  In  the  Birmingham 
region  358,470,000  tons  was  estimated  as  available  in  1909. x 
In  the  region  tributary  to  Chattanooga,  Tenn.,  nearly  100,000,000 
tons  was  estimated  as  available;  in  New  York  about  30,000,000 
tons;  and  in  Dodge  County,  Wisconsin,  about  the  same  tonnage 
as  in  New  York.  The  structural  conditions  in  these  regions  in- 
dicate that  these  estimates  are  low. 

Birmingham  Region,  Alabama. — In  the  Birmingham  district, 
Alabama,2  the  sedimentary  rocks,  including  the  Clinton  beds,  are 

1  HAYES,  C.  W. :  Iron  Ores  in  the  United  States.     U.  S.  Geol.  Survey 
Bull.  394,  p.  90,  1909. 

2  BURCHARD,  E.  F.,  BUTTS,  CHARLES,  and  ECKEL,  E.  C. :  Iron  Ores,  Fuele, 
and  Fluxes  of  the  Birmingham  District,  Alabama.     U.  S.  Geol.  Survey 
Butt.  400,  1910. 


324      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

folded  and  faulted,  and  the  Clinton  ores  crop  out  as  long,  narrow 
strips  striking  northeast  (Fig.  143).  The  section  includes  beds 
ranging  from  the  Cambrian  to  Carboniferous,  and  because  these 
beds  are  folded  and  eroded,  their  products,  iron  ore,  coal,  and 
limestone  flux,  are  brought  near  together.  The  region,  to  use 
an  engineering  phrase, may  be  termed  "self-contained,"  for  all 
the  materials  necessary  for  the  manufacture  of  steel  are  abun- 
dantly present  within  a  radius  of  a  few  miles.  The  ores,  though 
of  lower  grade  than  the  Lake  Superior  hematites,  can  compete 
with  them,  because  good  coke  is  made  in  their  vicinity  and  be- 
cause they  command  the  Southern  market  for  steel.  They  carry 
considerable  phosphorus  and  are  non-Bessemer.  The  develop- 
ment of  the  open-hearth  process  of  steel  making  has  greatly 
enhanced  their  value. 

A  generalized  section  of  the  rocks  of  the  Birmingham  district 
is  shown  below. 


Present  in  extreme  southwestern  part 
of  district. 


Floyd  shale .  .  . 


Tertiary:  ?Lafayette  formation. 
Cretaceous:  Tuscaloosa  formation. 
Carboniferous : 

Pennsylvanian :  Pottsville    formation     ("Coal    Meas-  Feet 

ures") 2,600  to  7,000 

Unconformity. 
Mississippian : 

Parkwood  formation 

Pennington  shale  (30-300  feet). 
Bangor    limestone     (670    feet); 
includes     Hartselle     sandstone 
member  (100  +  feet). 

Fort  Payne  chert 

Unconformity. 
Devonian : 

Chattanooga  shale. 
Frog  Mountain  sandstone.    / 
Unconformity. 

Silurian:  Clinton  (Rockwood)  formation 

Unconformity. 

Ordovician:  Chickamauga  (Pelham)  limestone 

Unconformity. 

Cambro-Ordovician :  Knox  dolomite  (includes  at  base  Ke- 

tona  dolomite  member,  600  feet) 

Cambrian: 

Conasauga  (Coosa)  limestone 

Rome  (Montevallo)  formation  (great  thickness). 


0  to  2,000 


1,000  ± 


200  to  250 


Ito25 


250  to  500 


200  to  1,000 


3,300 
1,000  + 


7,351  to  16,075 


IRON 


325 


.,  V1  Outcrop  of  possibly      Outcrop  probably  containing 

nn  ore  workable  iron  ore      little  or  no  workable  iron  ore 

s      ip      is      zp      is      ip  Miles 


FIG.  143. — Map  of  region  near  Birmingham,  Alabama.     (After  Burchard, 
U.  S.  Geol.  Survey.) 


326 


PRINCIPLES  OF  ECONOMIC  GEOLOGY 


11 


tl 

!! 


The  principal  fold  in 
the  district  is  an  anticline, 
with  a  shallow  syncline 
near  its  center  :  by  the  fold- 
ing the  outcrops  of  the 
Clinton  ore  are  repeated, 
forming  several  parallel 
belts  (Figs.  143,  144).  The 
trend  of  the  folds  and 
therefore  of  the  rock  beds 
also  is  northeast. 

The  ore  bodies  dip  with 
the  rocks  at  moderately 
low  angles  (Fig.  145). 
Where  the  ore  beds  are 
weathered,  lime  carbonate 
is  dissolved  out  of  them, 
thereby  increasing  the  pro- 
portion of  iron,  silica,  and 
other  constituents.  Such 
altered  ore  is  termed  "soft 
ore,"  for  it  is  usually 
porous  and  friable,  com- 
pared with  the  unaltered 
material,  which  is  termed 
"hard  ore."  The  altera- 
tion extends  from  the  out- 
crop for  distances  of  a  few 
feet  to  400  feet.  The  soft 
ore  is  generally  more  ac- 
cessible and  of  higher 
grade  than  the  hard  ore, 
and  in  consequence  much 
of  it  has  already  been 
mined. 

Ore  occurs  at  four  hori- 
zons in  the  Clinton  forma- 
tion. (1)  The  Hickory 
Nut  seam,  the  highest  one, 
is  of  too  poor  a  grade  to 
work  under  present  con- 


IRON 


327 


ditions.  It  is  a  sandy,  ferruginous  bed  and  is  named  from 
the  presence  of  a  fossil  brachiopod  which  looks  like  a 
hickory  nut  in  a  partly  opened  hull.  (2)  The  Ida  seam 
is  of  low  grade  but  workable  locally.  It  is  2  to  6  feet  thick 
and  is  siliceous  and  associated  with  sandstones.  (3)  The  Big 
seam,  20  to  50  feet  below  the  Ida  and  from  16  to  30  feet  thick,  is 
persistent  along  the  strike  and  of  good  grade,  although  rarely 


FIG.  145. — Section  showing  Clinton  ore  in  Birmingham  region,  Alabama. 
Sc,  Clinton  (Rockwood)  formation;  Oc,  Chickamauga  limestone;  Cfp,  Fort 
Payne  chert.  (After  Burchard,  U.  S.  Geol.  Survey.) 

more  than  10  or  12  feet  of  it  is  worked.  (4)  The  Irondale  seam, 
the  lowest  one,  is  thin  but  locally  of  excellent  grade.  A  typical 
section  of  the  Clinton  showing  the  Big  and  Irondale  seams  is 
given  below. 

CHARACTER  OF  BIG  AND  IRONDALE  SEAMS  1  MILE  NORTHEAST  OF  RED  GAP, 
NEAR  IRONDALE,  ALA. 


Strata 

Thick- 
ness 

Character 

Sandstone. 

Big  seam: 

Ft.  in. 

Ore,  sandy  

1    8 

Unweathered  ore:   Metallic  iron,   16-20  per 

Ore,  lean,  with  fine  quartz  peb- 

5 

cent.;   insoluble,  40±  per  cent.;  lime,  18  ± 

bles. 

per  cent. 

Ore,       massive,      cross-bedded, 

7 

Hard  ore,  averages  metallic  iron,  36  per  cent.  ; 

mined. 

insoluble,  26  per  cent.;  lime,  20  per  cent. 

Ore,   similar   in   appearance   to 

6 

Percentage  of  iron  grades  down  from  35  at  top 

above,  but  not  mined  at  pres- 

to less  than  20  at  bottom;  insoluble  rises  to 

ent. 

more  than  60  per  cent. 

Sandstone,  ferruginous,  lean  ore, 

20 

and  shale,  in  thin  strata. 

Shale  

0  to  6 

Sandstone,  very  hard  

3 

"Gouge,"  calcareous  ,  

6 

Irondale  seam:  Ore,  mined  

5 

Semihard  ore,  averages  metallic  iron,  37  per 

cent.  ;  insoluble,  29  per  cent.;  lime  carbonate, 

14.25  per  cent. 

Shale,  hard. 

The  ores  are  sedimentary.     Below  the  zone  of  surface  leaching 
they  extend  thousands  of  feet  without  material  change. 


328      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  occurrence  of  fragments  of  the  ore  in  overlying  beds  of 
limestone  in  the  Clinton  formation,  as  described  by  Smyth1 
points  to  the  fact  that  the  ore  had  been  formed  prior  to  the 
deposition  of  this  limestone. 

Smyth,  Burchard,  and  Eckel  agree  that  the  ores  are  of  marine 
origin.  There  is  reason  to  suppose  that  the  iron  replaced  the 
lime  carbonate  before  the  beds  were  deeply  buried,  possibly  be- 
fore they  were  consolidated,  or  even  while  they  were  exposed  to 
sea  water.  Sea  water  carries  iron,  and  iron  replaces  lime  car- 
bonate, as  the  iron  salts,  either  carbonate  or  oxide,  are  less 
soluble  than  those  of  lime. 

Eastern  Tennessee  Iron  Deposits. — In  eastern  Tennessee, 
extending  from  Chattanooga  northeastward  across  the  State  to 
Middleboro,  Ky.,  and  beyond,  a  distance  of  about  150  miles, 
Silurian  beds  crop  out  almost  continuously.  In  this  belt,  which 
is  practically  in  the  strike  of  the  Clinton  beds  of  Birmingham, 
Ala.,  and  also  in  some  outlying  areas,  there  are  valuable  bodies 
of  iron  ore  in  the  "Rock wood"  formation,  which  corresponds 
to  the  Clinton  in  Alabama  and  New  York. 

The  rocks  of  this  area  (Fig.  146)  are  of  sedimentary  origin  and 
range  from  the  Cambrian  to  the  Carboniferous.  They  are 
folded,  generally  in  open  anticlines  and  synclines,  and  are  faulted 
along  reverse  or  thrust  faults  that  generally  dip  at  low  angles. 
Locally,  however,  the  folding  is  close,  and  overturned  anticlines 
are  developed.2 

The  "Rockwood"  ore  crops  out  along  the  foot  of  the  Cumber- 
land escarpment  from  a  point  near  Chattanooga  to  the  northern 
border  of  the  State  at  Cumberland  Gap  and  in  several  separate 
areas  in  the  Tennessee  Valley.  There  are  strips  of  outcrop  that 
extend  continuously  15  to  20  miles.  The  normal  dip  of  the  rocks 
along  the  escarpment  is  northwest,  but  in  many  places  where 
closely  folded  the  ore  beds  dip  to  the  southeast,  or  away  from  the 
mountain. 

As  in  the  Birmingham  district,  the  ore  occurs  in  lenticular 
sedimentary  beds.  It  is  mainly  of  two  types,  fossil  ore  and 
granular  ore.  The  fossil  ore  consists  of  aggregates  of  fossils, 
including  bryozoans,  crinoids,  corals,  brachiopods,  and  trilo- 

1  SMYTH,  C.  H.,  JR.,  in  BAIN,  H.  F.,  and  others:  "Types  of  Ore  Deposits," 
pp.  33-52,  1911.     See  also  Am.  Jour.  Sci.,  3d  ser.,  vol.  43,  pp.  487-496,  1892. 

2  BURCHARD,  E.  F. :  The  Red  Iron  Ores  of  East  Tennessee.     Tenn.  Geol. 
Survey  Bull.  16,  p.  27,  1913. 


IRON 


329 


bites.  These  fossils  were  originally  calcium  carbonate.  There 
are  calcareous  streaks  in  the  ore  in  which  many  of  the  fossil  frag- 
ments are  still  composed  mainly  of  calcite,  but  in  other  places 
the  fossil  forms  are  composed  partly  or  wholly  of  ferric  oxide, 
some  of  which  probably  filled  cavities  from  which  the  lime  has 
been  dissolved.  The  fossil  material,  much  of  which  consists  of 
broken  and  water-worn  fragments,  was  evidently  gathered  by  the 


LEGEND 


FIG.  146. — Sketch  map  of  iron-ore  district  near  Chattanooga,  Tennessee. 
(After  Bur  chard,  Tennessee  Geol.  Survey.) 

action  of  waves  and  currents  into  beds  and  subsequently  cemented 
together  by  calcium  carbonate  and  ferric  oxide.  Some  clay 
material  has  been  included  in  the  beds  during  their  formation; 
this  commonly  appears  as  thin  seams  of  shale  and  argillaceous 
nodules  and  lenses. 

The  granular  or  oolitic  ore  consists  of  aggregates  of  flat  grains 
with  rounded  edges  like  flaxseeds  cemented  by  ferric  oxide  and 
calcium  carbonate.  The  flat'  grains  generally  have  minute 
nuclei  of  quartz,  about  which  iron  oxide  has  been  deposited.  The 


330      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


fossil  ore  generally  predominates  in  a  bed,  but  in  some  localities 
the  fossil  fragments  and  the  granular  bodies  are  mixed  in  varying 
proportions. 

ANALYSES  AND  SPECIFIC  GRAVITY  TESTS  OF  HARD,  SEMIHARD,  AND  SOFT 
"ROCKWOOD"  IRON  ORE  FROM  CHAMBERLAIN,  TENN.° 


i 

2 

3 

4 

SiO2  
A1203  
Fe203  
FeO 

5.00 
2.82 
36.44 
2  20 

7.92 
3.07 
50.60 
2  44 

7.63 
3.64 
67.60 
4  47 

7.62 
4.31 
74.96 
0  10 

MgO 

1  63 

1  71 

0  50 

0  47 

CaO. 

24  84 

13  77 

1  68 

0  40 

Na2O  
K2O  

0.10 
0.22 

0.10 
0.25 

0.12 
0.33 

0.13 
0  30 

TiO2 

0  11 

0  10 

0  26 

0  12 

CO2 

19  89 

12  29 

3  04 

0  32 

P2O6  

0  99 

1  31 

1  69 

1  22 

S  
Mn  
H2O- 

0.05 
0.30 
0  89 

0.05 
0.33 
0  59 

0.07 
0.58 
0  84 

0.02 
0.31 
0  66 

H20  +  

4.72 

5.52 

8.15 

9.35 

Fe    (from    Fe2O3   and 
FeO) 

100.20 
27  22 

100.05 
37  32 

100.29 
52  55 

100.60 
50  79 

Specific  gravity  

3.05 

3.09 

2.49 

2.59 

0  BURCHARD,  E.  F.:  The  Red  Iron  Ores  of    East  Tennessee.    Tenn.  Geol.  Survey  Bull. 
16.  p.  76,  1913. 

1.  Hard  ore,  large  lump,  remote  from  line  of  division  between  hard  and  soft  ore. 

2.  Semihard,  small  slab,  near  line  of  division  between  hard  and  soft  ore. 

3.  Soft,  small  slab,  near  line  of  division  between  hard  and  soft  ore. 

4.  Soft  ore,  large  lump,  remote  from  line  of  division  between  hard  and  soft  ore. 

Where  weathered  by  surface  waters  the  lime  carbonate  is  dis- 
solved out  of  the  beds,  thereby  increasing  the  content  of  iron 
oxide  and  silica.  As  in  the  Birmingham  district,  such  altered  ore 
is  termed  "soft  ore,"  and  the  unaltered  material  is  termed  "hard 
ore."  The  alteration  of  the  ore  beds  extends  from  the  outcrop 
down  the  dip  for  a  few  feet  to  more  than  500  feet,  the  distance 
depending  on  the  attitude  of  the  beds  and  the  thickness  and 
permeability  of  their  cover.  Conditions  favoring  fairly  deep 
leaching  of  the  ore  beds  are  a  rather  steep  dip  and  impervious 


IRON 


331 


shale  at  the  top  and  bottom  of  the  ore,  which  should  crop  out  in 
a  ridge  high  above  the  ground-water  level.  Below  the  ground- 
water  level  the  ore  is  generally  hard. 

Chemical  analyses  and  specific  gravity  tests,  by  George 
Steiger,  of  four  specimens  collected  by  Burchard  illustrate  the 
gradation  from  hard  to  soft  ore.  Solution  is  attended  by  an 
increase  in  percentages  of  iron  oxide,  silica,  and  alumina  and  a 
decrease  in  lime.  With  the  decrease  in  lime  the  minor  insoluble 
impurities,  such  as  manganese,  phosphorus,  and  sulphur,  tend 
to  increase  proportionally. 


Oolitic  ore  Fossil  ore 

FIG.  147. — Clinton  iron  ores  from  Clinton,  N.  Y.     (After  Burchard,  Bulls 
and  Eckel,  U.  S.  Geol.  Survey.) 

Clinton  Region,  New  York. — The  Clinton  formation  is  ex- 
tensively exposed  in  western  and  central  New  York.1  Its  out- 
crops are  nearly  parallel  to  the  south  shore  of  Lake  Ontario,  and 
the  beds  dip  gently  to  the  south.  Developments  are  most  ex- 
tensive in  Wayne  County  near  Lake  Ontario,  but  mines  have 
also  been  opened  near  Clinton  and  elsewhere  in  central  New 

1  NEWLAND,  D.  H.,  and  HARTNAGEL,  C.  A. :  Iron  Ores  of  the  Clinton 
Formation  in  New  York  State.  N.  Y.  State  Mus.  Bull.  123,  1908. 


332      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

York.1  As  in  Alabama,  the  ores  are  fossiliferous  or  oolitic 
(Fig.  147),  and  from  one  to  four  beds  are  known.  Owing  to  the 
nearly  flat  dips  the  ores  can  be  mined  for  considerable  distances 
down  the  dip  by  stripping.  The  workable  beds  are  thin,  how- 
ever, compared  to  those  of  Alabama. 

Eastern  Wisconsin. — Deposits  of  sedimentary  iron  ore  of  the 
Clinton  type  are  found  in  Dodge  County,  Wisconsin,2  above  the 
Maquoketa  shale  and  below  the  Niagara  limestone.  They  dip 
east  at  low  angles,  and  their  horizon  is  not  more  than  800  feet 
below  the  surface  at  Lake  Michigan,  35  miles  east  of  their  out- 
crop. The  ore  beds  vary  in  thickness,  reaching  a  maximum  of 
55  feet.  They  have  been  followed  400  feet  down  the  dip  and  are 
explored  at  greater  depths  by  drilling.  At  many  places,  according 
to  Thwaites,  the  ore  lenses  thin  out. 

The  ore  is  hydrated  iron  oxide  with  29  to  54  per  cent,  of  iron, 
averaging  perhaps  45  per  cent.,  and  is  high  in  phosphorus.  Cal- 
cite  is  an  abundant  gangue  mineral.  The  iron  oxide  is  granular 
or  oolitic,  and  the  granules  lie  with  their  flat  sides  parallel  to  the 
bedding  of  the  ore.  Some  of  the  granules  or  aggregates  of 
granules  are  water-worn.  At  the  top  of  the  iron-bearing  member, 
grading  into  the  oolitic  ore,  is  a  bed  of  hard  hematite  6  inches 
or  less  in  thickness,  which  is  richer  in  iron  than  the  average  ore. 
The  iron  minerals  were  probably  deposited  as  ferric  oxide  in 
shallow  seas;  the  worn  particles  of  hematite  suggest  shore 
conditions. 

TERTIARY   ORES  OF  NORTHEASTERN  TEXAS 

In  northeastern  Texas3  iron  ores  occur  over  an  extensive  area 

1  NEWLAND,  D.  H. :  The  Clinton  Iron-ore  Deposits  of  New  York  State. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  40,  pp.  165-183,  1910. 

2CHAMBEELiN,  T.  C. '.  "  Geology  of  Wisconsin,"  vol.  2,  pp.  328-334,  1877. 

VAN  HISE,.  C.  R.,  and  LEITH,  C.  K:  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  52,  p.  567,  1913. 

THWAITES,  F.  T. :  Recent  Discoveries  of  "Clinton"  Iron  Ore  in  Eastern 
Wisconsin.  U.  S.  Geol.  Survey  Bull.  540,  pp.  338-342,  1914. 

3  BURCHARD,  E.  F. :  Iron  Ore  in  Cass,  Marion,  Morris,  and  Cherokee 
Counties,  Texas.  U.  S.  Geol.  Survey  Bull.  620,  pp.  60-109,  1916. 

PENROSE,  R.  A.  F.,  JR.:  The  Iron  Ores  of  East  Texas.  Texas  Geol. 
Survey  First  Ann.  Rept.,  pp.  65-84,  1890. 

PHILLIPS,  W.  B.:  The  Iron  Resources  of  Texas.  Eng.  Soc.  Western 
Pennsylvania  Proc.,  March,  1902,  pp.  64-70. 

DUMBLE,  E.  T.,  KENNEDY,  WILLIAM,  and  others:  Reports  on  the  Iron-ore 
District  of  East  Texas.  Texas  Geol.  Survey  Second  Ann.  Rept.  (for  1890), 
pp.  7-326,  1891. 


IRON 


333 


embracing  all  or  parts  of  21  counties.  In  this  area  the  surface 
formations  consist  chiefly  of  sand,  clay,  gravel,  and  silt,  pre- 
dominantly soft  or  little  consolidated.  The  most  recent  de- 
posits— those  that  form  the  bars,  flood  plains,  and  stream  ter- 
races— are  Quaternary,  but  the  great  masses  of  sand  and  clay 
with  which  the  iron-ore  deposits  are  associated  are  Eocene  (Fig. 
148). 

The  principal  deposits  of  brown  ore  are  in  two  Eocene  forma- 
tions, the  Mount  Selman  formation  and  the  Cook  Mountain 


10  FEET 


Clay 


Glauconitic 

sandyclay      carbonate 


FIG.  148. — Section  showing  associations  of  iron  ore  near  Linden,    Cass 
County,  Texas.     (After  Bur  chard,  U.  S.  Geol.  Survey.) 

formation  of  the  Claiborne  group.  The  ore  consists  chiefly  of 
limonite  and  iron  carbonate.  The  principal  iron-ore  deposits 
are  residual,  unconsolidated  deposits  of  limonite;  nodular,  geodal, 
and  concretionary  masses  of  limonite  and  iron  carbonate;  and 
laminated  beds  of  limonite.  The  nodular  and  geodal  masses 
of  both  the  brown  ore  and  the  iron  carbonate  are  segregated  in 
glauconitic  sand  and  clay  in  thin  lenses  and  irregular  ledges. 
Unconsolidated  material,  residual  from  the  breaking  down  of 
such  masses,  is  found  in  many  places  at  the  surface.  The  brown 
ore  occurs  also,  particularly  in  central  Cherokee  County,  in  a 
rather  persistent  laminated  bed,  1^  to  4  feet  thick. 

IRON  CARBONATE  ORES  OF  EASTERN  UNITED  STATES 

Bedded  iron  carbonate  ores  are  found  in  the  coal  measures  of 
western  Pennsylvania,  northern  West  Virginia,  eastern  Ohio, 
and  northeastern  Kentucky.  Once  extensively  exploited,  they 
are  now  mined  only  in  southeastern  Ohio,  on  a  small  scale. 


334      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

These  ores  occur  in  all  the  formations  from  upper  Mississippian 
to  Permian,  but  the  most  valuable  beds  are  in  the  Pottsville  and 
Alleghany  formations  of  the  lower  part  of  the  Pennsylvanian. 

According  to  Harder,1  the  ores  are  of  four  classes — limestone 
ores,  block  ores,  kidney  ores,  and  black-band  ores.  The  lime- 
stone ores,  which  are  the  most  valuable,  occur  just  above  or 
very  near  the  top  of  a  limestone  stratum  at  several  horizons. 
The  principal  bed  is  that  just  above  the  Vanport  or  "Ferrif- 
erous" limestone,  in  the  lower  part  of  the  Alleghany  formation. 
At  this  horizon  ore  occurs  in  many  places.  The  Vanport  lime- 
stone is  as  much  as  8  or  10  feet  thick,  and  the  ore  above  it  ranges 
from  a  few  inches  to  several  feet.  These  variations  occur  within 
short  distances,  giving  the  ore  a  pockety  character.  The  ore  is 
altered  to  limonite  along  the  outcrop,  and  it  is  mainly  the  altered 
portion  which  has  been  mined.  The  limonite  ore  is  usually 
brown  or  red  and  carries  40  to  50  per  cent,  of  iron.  The 
carbonate  ore  carries  from  25  to  40  per  cent. 

Block  ores,  so  called  because  they  cleave  into  angular  blocks 
when  mined,  occur  in  irregular  beds  in  the  Pottsville  and  Alle- 
ghany formations.  The  beds  are  more  persistent  than  those  of 
limestone  ore,  but  the  ores  are  leaner  and  less  uniform  in  grade. 
Along  the  outcrop  they  are  altered  to  limonite. 

Kidney  ore  is  iron  carbonate  which  occurs  in  rounded  masses 
scattered  through  certain  beds  of  clay  and  shale.  Such  masses  are 
abundant  in  the  upper  part  of  the  Alleghany  formation,  occurring 
at  several  stratigraphic  horizons.  They  alter  to  limonite  near 
the  weathered  surface. 

Black-band  iron  carbonate  ore  usually  occurs  in  beds  inter- 
layered  with  bituminous  shale.  As  a  rule  it  contains  much  car- 
bonaceous material.  Individual  beds  of  black-band  ore  are 
generally  only  a  few  inches  thick,  but  in  places  interbedded 
bituminous  shale  and  ore  occupy  zones  10  or  15  feet  thick. 
Black-band  ore  is  richer  than  the  other  varieties  of  iron  carbonate. 

BROWN  ORES  OF  EASTERN  UNITED  STATES 

The  Appalachian  valley2  from  Vermont  to  Alabama  is  bordered 
on  the  east  by  iron-bearing  crystalline  rocks  which  have  been 
exposed  to  weathering  for  long  periods.  In  the  southern  por- 

1  HARDER,  E.  C. :  U.  S.  Geol.  Survey.     Mineral  Resources,  1908,  part  1, 
p,  92,  1909. 

2  ECKEL,  E.  C.:  U.  S.  Geol.  Survey  Bull.  400,  p.  145,  1910. 


IRON 


335 


"Monterey"  (Oriskany)  sandstone.. 
Lewlstown  limestone 


J  Oriskany  brown  ore. 


Clinton  (Rockwood)  formation 


1  Clinton  fossil  hematite. 


L..{ 


Liberty  Hall  limes 


ty  Hall  limestone..] 

[chickamftuga  . . 
t  limestone J 


Limestone  magnetite. 


Natural  Bridge  limestone.  .{Nolichucky 


.JNolichiick 
(Moniker 


Valley  brown  ore  (Blue  Rldgo  district). 


Buena  Vista"  shale fWatauga) . 


Sherwood  limestone  (Shady). 


Lower  Cambrian  quartz! te 


Valley  brown  ore  (New  Klver  district). 
Mountain  brown  ore. 


Lower  CambKan  quartzite  and  shale. . . . 


2000 


Siliceous  specular  hematite. 


2000  Feet 


FIG.  149.— Generalized  section  showing  stratigraphic  position  of  several 
classes  of  iron  ore  in  the  Appalachian  region  of  Virginia.  (After  Harder, 
U.  S.  Geol.  Survey.) 


336      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tion  of  the  valley,  from  southeastern  Pennsylvania  to  Alabama, 
there  are  thick  deposits  of  residual  material  with  which  are 
associated  many  deposits  of  brown  ore.  North  of  the  glacial 
limit  ice  erosion  has  removed  iron  deposits,  and  those  remaining 
are  merely  the  basal  portions  of  deposits  that  were  once  more 
extensive.  Most  of  these  deposits  are  in  masses  of  residual 
material  partly  or  entirely  inclosed  in  solid  rock.  By  far  the 
majority  are  alteration  products  of  iron  carbonate  or  pyrite, 
which  in  turn  originated  through  the  replacement  of  limestone. 
These  ores  have  a  wide  geologic  distribution  (Fig.  149).  Many 
of  th  brown-ore  deposits  appear  to  have  been  formed  rather 
recently,  most  of  them  probably  during  Tertiary  time.  Some  are 
associated  with  residual  material  derived  from  Cambro-Ordo- 
vician  sediments;  others  associated  with  later  rocks  may  be  epi- 


FIG.  150. — Structure  section  near  Sweetwater,  Tennessee,  showing  posi- 
tion of  residual  iron  ore  and  relation  to  Tellico  ore  beds  and  ferruginous 
Holston  marble  to  the  ore  deposit.  -COk,  Knox  dolomite;  Oc,  Chickamauga 
limestone;  OKI,  Holston  marble;  Ot,  Tellico  sandstone.  (After  Burchard, 
Tennessee  Geol.  Survey.) 

genetic.  According  to  Eckel,  most  of  the  deposits  were  formed 
after  all  the  folding  of  the  region  had  been  accomplished,  at  a 
period  when  the  topography  was  substantially  similar  to  that 
now  existing.  Evidence  of  the  age  of  a  few  of  the  deposits  is 
afforded  also  by  Tertiary  fossils  included  in  or  associated  with 
them.  At  some  places  the  siliceous  weathered  limonitic  material 
has  been  concentrated  naturally  by  washing. 

At  Sweetwater,  Tenn.,  hematite  and  limonite  have  accumulated 
as  residual  deposits  of  limestones  and  associated  sedimentary 
beds  (Fig.  150). 

At  the  Gossan  lead,  Virginia,  and  at  Ducktown,  Tenn.,1  rich 
limonite  ore  has  been  formed  through  the  weathering  of  iron 
sulphide. 

Small  deposits  of  limonite  and  hematite  in  the  Appalachian 

1  EMMONS,  W.  H.,  and  LANEY,  F.  B. :  A  Preliminary  Report  on  the  Mineral 
Deposits  of  Ducktown,  Tenn.  U.  S.  Geol.  Survey  Bull.  470,  p.  170,  1910. 


IRON  337 

region  have  been  formed  also  as  fissure  fillings  and  by  the  re- 
placement  of  limestone,    quartzite,   and   shale   along   fracture 


MAGNETITE  ORES  OF  PENNSYLVANIA 

At  Cornwall,  Dillsburg,  and  several  other  places  in  eastern 
Pennsylvania,2  large  bodies  of  magnetic  iron  ores  are  developed. 
The  region  is  underlain  by  sedimentary  rocks,  which  are  intruded 
by  Triassic  diabase  and  covered  locally  by  basalt  flows.  The 
ores  occur  at  the  contact  of  the  diabase  with  the  sediments. 
The  principal  deposits  lie  along  the  northern  edge  of  the  Mesozoic 
Newark  belt,  where  the  diabase  is  intruded  into  limestones  and 
limy  shales  of  Cambro-Ordovician  age,  but  locally  small  deposits 
are  found  at  the  contact  of  the  intrusive  masses  with  Triassic 
sandstones  and  shales.  In  places  garnet,  pyroxene,  epidote,  and 
other  heavy  silicates  form  the  gangue  of  the  magnetite  ore.  It  is 
believed  that  the  ores  were  deposited  by  solutions  that  emanated 
from  the  diabase  and  replaced  the  sedimentary  rocks. 

MAGNETITE  ORES  OF  NEW  YORK  AND  NEW  JERSEY 

In  the  Adirondack  region,  New  York,3  great  deposits  of  mag- 
netic non-titaniferous  ores  are  associated  with  metamorphosed 
sediments  and  gneisses.  The  ores  occur  as  rudely  tabular  bodies 
parallel  to  the  general  structure,  though  some  are  elongated  and 
some  irregular.  They  lie  along  the  planes  of  foliation  in  the 
inclosing  rocks.  These  ores  are  now  mined  on  a  large  scale  and 
concentrated. 

In  New  Jersey  magnetite  iron  ores  which  occur  in  gneisses  are 
believed  to  be  due  to  magmatic  segregation.4 

1  HARDER,  E.  C.:  The  Iron  Ores  of  the  Appalachian  Region  in  Virginia. 
U.  S.  Geol.  Survey  Bull.  380,  p.  235,  1909. 

2 SPENCER,  A.  C.:  Magnetite  Deposits  of  the  Cornwall  Type  in  Pennsyl- 
vania. U.  S.  Geol.  Survey  Bull.  359,  1908. 

HARDER,  E.  C. :  Structure  and  Origin  of  the  Magnetite  Deposits  of  Dills- 
burg,  Pa.  Econ.  Geol.,  vol.  5,  p.  599,  1910. 

3  NEWLAND,  D.  H.,  and  KEMP,  J.  F.:  Geology  of  the  Adirondack  Magnetic 
Iron  Ores.     N.  Y.  State  Mus.  BuU.  119,  1908. 

4  BAYLEY,  W.  S.:  Iron  Mines  and  Mining  in  New  Jersey.     N.  J.  Geol. 
Survey  Final  Rept.,  vol.  7,  1910. 


338      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

HEMATITES  AND  MAGNETITES  OF  WESTERN  UNITED 
STATES 

Hartville,  Wyoming.— The  Hartville  iron-bearing  district1  is 
in  the  Hartville  uplift,  a  broad,  low  domical  mountain  mass  in 
east-central  Wyoming.  The  rocks  of  the  Hartville  iron  range 
include  steeply  dipping  pre-Cambrian  rocks,  flat-lying  or  gently 
dipping  Carboniferous  and  Mesozoic  rocks,  and  rocks  of  Tertiary 
and  Recent  age  encircling  the  older  rocks  and  filling  depressions 
of  erosion  extending  into  them.  The  pre-Cambrian  rocks  com- 
prise metamorphosed  sedimentary  rocks  and  igneous  rocks  and 
their  mashed  equivalents.  The  oldest  rocks  are  an  interbedded 
series  of  siliceous  limestones.  The  pre-Cambrian  sedimentary 
rocks  have  been  folded  into  a  complex  synclinorium  whose  axis 
trends  east.  Above  the  pre-Cambrian  is  the  Guernsey  (Carbon- 
iferous) sedimentary  formation,  and  above  that  the  Hartville 
formation.  These  formations  are  encircled  by  outcrops  of  the 
Tertiary  rocks  of  the  plains. 

*  The  most  valuable  deposits  are  lenses  of  hematite  that  occur 
in  schist  on  a  limestone  foot  wall.  The  ore  largely  replaces  the 
schist,  although  in  part  it  fills  cavities  in  the  schist  which  are  due 
to  jointing,  faulting,  and  brecciation.  Detrital  ores  of  secondary 
derivation  from  these  deposits  are  situated  at  the  base  of  the 
Guernsey  formation,  at  the  base  of  the  Hartville  formation,  in 
the  Tertiary  lake  deposits,  and  in  the  Pleistocene  and  Recent 
stream  deposits. 

Iron  Springs,  Utah. — Large  deposits  of  magnetic  iron  ore  are 
developed  at  Iron  Springs,  Utah.2  The  area  is  occupied  by  tilted 
sedimentary  rocks  including  Carboniferous,  Cretaceous,  and 
Tertiary  limestones.  These  are  intruded  by  small  bodies  of 
diorite  porphyry.  The  magnetite  replaces  limestone  near  the 
igneous  contacts  and  fills  fractures  in  the  intrusive  rock.  The 
gangue  minerals  include  garnet,  diopside,  hornblende,  and  other 
heavy  silicates.  The  ore  available  is  estimated  at  40,000,000 
tons. 

The  intrusive  bodies  are  regular  in  form,  and  the  sediments 
surround  them  in  concentric  rings.  In  most  places  the  Car- 
boniferous limestone  is  in  contact  with  the  diorite  porphyry, 

1  BALL,  S.  H. :  The  Hartville  Iron-ore  Range,  Wyoming.     U.  S.  Geol. 
Survey  Bull.  315,  p.  190,  1907. 

2  LEITH,  C.  K,  and  HARDER,  E.  C. :  The  Iron  Ores  of  the  Iron  Springs  Dis- 
trict, Southern  Utah.     U.  S.  Geol.  Survey  Butt.  338,  1908. 


IRON  339 

but  locally  the  diorite  breaks  through  and  lies  against  the 
Cretaceous  rocks.  The  principal  ore  deposits  are  found  here  and 
there  around  the  peripheral  parts  of  the  intrusives,  which  stand 
up  as  mountains  above  the  surrounding  desert.  The  sediments 
bordering  them  occupy  the  lower  slopes.  The  ores  are  irregular 
lenses,  the  longer  diameters  of  which  are  generally  parallel  to 
the  intrusive  contacts.  Some  ore  bodies  are  directly  at  the  con- 
tact; others  are  separated  from  the  diorite  porphyry  by  narrow 
bands  of  siliceous  contact  rock.  The  deposits  are  found  princi- 
pally in  the  Carboniferous  limestone,  but  some  occur  in  the 
Cretaceous  rocks  where  they  are  in  contact  with  the  diorite 
porphyry.  The  contact  silicates  are  associated  with  the  ore, 
in  some  places  between  the  ore  bodies  and  the  sediments  and 
elsewhere  between  the  ore  bodies  and  the  diorite  porphyry.  In 
many  places  around  the  intrusive  masses  contact  minerals  are 
lacking,  and  unaltered  sediments,  such  as  limestone,  occur  at  the 
contact. 

Eagle  Mountain,  Calif. — The  Eagle  Mountain  region,  Cali- 
fornia,1 is  an  area  of  ancient  gneiss,  schist,  and  quartzite,  upon 
the  eroded  surface  of  which  was  deposited  quartzite  with  lenses 
of  dolomite.  These  rocks  are  intruded  by  great  sills  of  monzo- 
nite,  and  the  intrusion  was  accompanied  by  an  arching  up  of  the 
sediments  into  an  elongated  dome,  in  the  center  of  which  are 
the  older  rocks  with  the  associated  monzonite.  The  iron  ores  are 
best  developed  in  the  sediments  on  the  north  and  west  sides  of 
the  dome. 

The  ores  are  mainly  irregular  deposits  formed  through  replace- 
ment of  the  limestone  by  solutions  emanating  from  the  igneous 
rocks  during  or  soon  after  their  crystallization.  The  ore  is  pre- 
dominantly hematite,  probably  less  than  10  per  cent,  being  mag- 
netite. A  considerable  proportion  of  it  is  very  pure  and  of  high 
grade,  containing  between  62  and  67  per  cent,  of  iron  and  less 
than  0.06  per  cent,  of  phosphorus.  The  principal  gangue  minerals 
are  serpentine,  mica,  amphibole,  garnet,  epidote,  pyroxene,  and 
titanite,  which  are  present  in  different  proportions  and  occur  as 
large  and  small  irregular  masses  scattered  through  the  iron- 
bearing  rock.  According  to  Harder,  there  is  probably  more 
than  60,000,000  tons  of  ore  in  this  area. 

Hanover    (Fierro)   District,  New  Mex.— The   Hanover  dis- 

1  HARDER,  E.  C. :  Iron-ore  Deposits  of  the  Eagle  Mountains,  California. 
U.  S.  Geol.  Survey  Bull.  503,  1912. 


340      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

trict,  New  Mexico,1  is  2  miles  northwest  of  Santa  Rita,  occupy- 
ing a  narrow  basin  along  Hanover  Creek.  Iron  ores  from  the 
district  are  smelted  at  Pueblo,  Colo.  The  sedimentary  rocks 
consist  mainly  of  Carboniferous  limestone,  which  is  intruded  by 


Sedimentary  Contact  rock  Diorite  porphyry 

ChieflyCarboniferous   Garnet,  epidote. 
limestones  pyroxene 


Magnetite. 


FIG.  151.  —  Map  showing  relations  of  iron-ore  deposits  to  contact,  Hanover 
district,  New  Mexico.     (After  Paige,  U.  S.  Geol.  Survey.) 

a  stock  of  quartz  diorite  porphyry.  The  relations  are  shown  by 
Fig.  151.  Here  and  there  along  the  contacts  of  the  limestone 
with  the  porphyry  thick  garnet  zones  have  been  developed. 

1  PAIGE,  SIDNEY:  The  Hanover  Iron-ore  Deposits,  New  Mexico.  U.  S. 
Geol.  Survey  Bull.  380,  p.  199,  1909. 

LINDGREN,  WALDEMAR,  GRATON,  L.  C.,  and  GORDON,  C.  H.:  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Prof.  Paper  68,  p.  313,  1910. 


IRON  341 

These  zones  contain  epidote,  garnet,  and  augite,  with  quartz,  cal- 
cite,  pyrite,  magnetite,  and  zinc  blende.  The  boundary  between 
the  contact  rock  and  the  sediments  is  usually  sharp;  on  the  outer 
edge  of  the  zone  west  of  the  intrusive  mass  pyroxene  and  garnet 
rock  change  abruptly  to  pure  limestone.  Metamorphism  is  not 
confined  to  the  intruded  rocks;  the  diorite  porphyry  has  been 
altered  in  less  degree  by  changes  of  the  same  nature  as  those  that 
have  affected  the  limestones. 

The  ores  are  arranged  about  the  periphery  of  the  intrusive, 
nearly  everywhere  practically  at  the  contact.  They  are  in  the 
main  irregular  masses  of  magnetite,  with  some  hematite.  At 
the  Union  mine  they  are  long  compared  with  their  depth  and 
width.  Locally  the  ore  carries  considerable  chalcopyrite.  Such 
ore  is  left  in  the  mines  exploited  for  iron,  but  some  has  been  mined 
for  flux  and  shipped  to  Arizona  smelters.  According  to  Paige 
the  ores  were  formed  by  contact  metamorphism  and  related 
processes,  which  acted  principally  on  limestones. 

Iron  Age  Deposit,  Dale,  Calif. — The  Iron  Age  deposit,  near 
Dale,  San  Bernardino  County,  California,1  illustrates  a  type  of 
ore  not  yet  described.  The  country  rock  is  an  intrusive  mass  of 
soda  granite  and  granite  porphyry.  The  ores  are  chiefly  hema- 
tite with  subordinate  magnetite  and  occur  as  veins  cutting  the 
intrusive  granite  and  granite  porphyry  (Fig.  152).  Garnet  and 
epidote  are  locally  associated  with  the  ore  and  rocks.  The  prin- 
cipal iron-ore  veins  occur  over  an  area  about  half  a  mile  square; 
the  larger  veins,  on  account  of  their  resistance  to  erosion,  form 
the  summit  of  a  hill. 

Iron  Mountain  and  Pilot  Knob,  Mo. — In  St.  Francis  County, 
southeastern  Missouri,2  knobs  of  pre-Cambrian  rocks,  including 
porphyry,  are  surrounded  by  Cambrian  sediments.  At  Iron 
Mountain  and  Pilot  Knob  considerable  iron  ore  has  been  mined, 
the  total  production  being  nearly  6,000,000  tons.  Iron  Mountain 
was  originally  thickly  covered  with  loose  boulders  of  iron,  and  it 
was  at  first  supposed  that  the  entire  hill  was  ore;  but  the  mining 
operations  disclosed  the  porphyry  below  the  veneer  of  rich  sur- 
face material. 

1  HARDER,  E.  C.,  and  RICH,  J.  L. :  The  Iron  Age  Iron-ore  Deposit  near 
Dale,  San  Bernardino  County,  California.     U.  S.  Geol.  Survey  Bull.  430,  p. 
231,  1910. 

2  CRANE,  G.  W. :  The  Iron  Ores  of  Missouri.      Mo.  Bur.  Geol.  and  Mines, 
vol.  10,  2d  ser.,  pp.  107  et  seq.,  1912. 


342      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  structure  of  Iron  Mountain  is  domical.  The  crest  of  the 
hill  is  a  porphyry  knob  which  is  flanked  by  Cambrian  sediments 
that  rest  unconformably  upon  the  porphyry  and  dip  away  from 
it.  The  ore  occurs  as  huge  irregular  veins  cutting  the  porphyry, 
as  iron  conglomerate  at  the  base  of  the  sedimentary  series,  and 


Fia.  152. — Map  of  Iron  Age  iron-ore  deposits  near  Dale,  California. 
Harder  and  Rich,  U.  S.  Geol.  Survey.) 


(After 


as  residual  boulders  embedded  in  the  clay  that  once  covered  the 
mountain.  The  veins  are  in  places  60  feet  wide  and  are  the 
sources  of  the  superficial  boulder  ore  and  of  the  conglomerate 
at  the  base  of  the  Cambrian.  The  ore  is  hard  hematite  of  good 
grade,  in  part  specular.  Some  magnetite  is  present,  with  quartz, 
tremolite,  and  apatite.  The  iron  content  ranges  from  55  to  67 
per  cent. 

Pilot  Knob,  near  Iron  Mountain,  is  a  small  mass  of  porphyry 
almost  surrounded  by  Cambrian  sediments.     The  iron  ore  is  a 


IRON  343 

rudely  tabular  body  which  dips  at  low  angles  in  the  porphyritic 
rocks  (see  Fig.  153).  Basal  Cambrian  conglomerates  have 
accumulated  near  the  deposits.  These  also  are  worked  for  iron 
ore.  Boulder  ore  from  the  older  deposits  has  accumulated  at 
the  present  surface. 

Porphyry  Breccia 

Cl.y 


Conglomerate  Ore 


FIG.  153. — Cross-section  through  Pilot  Knob,  Missouri,  showing  relations 
of  ore  at  three  horizons  to  structure.  (Based  on  drawing  by  Potter,  Missouri 
Bur.  Geology  and  Mines.) 

TITANIFEROUS  IRON  ORES 

At  many  places  in  the  United  States  there  are  large  deposits 
of  titaniferous  magnetite  ore.1  These  are  not  exploited  at 
present,  but  if  certain  difficulties  in  smelting  them  are  overcome 
they  will  probably  become  valuable.  Most  of  these  deposits 
have  been  formed  by  magmatic  segregation. 

Iron  Mountain,  Wyoming. — The  region  of  Iron  Mountain, 
southeastern  Wyoming,2  about  40  miles  northwest  of  Cheyenne, 
contains  huge  deposits  of  titaniferous  iron  ore,  which,  however, 
are  not  exploited.  The  pre-Cambrian  complex  near  the  large 
dike  of  iron  ore  at  Iron  Mountain  consists  of  granular  igneous 
rocks  of  three  varieties — anorthosite  (essentially  labradorite), 
the  iron  ore,  and  granite.  The  anorthosite  is  the  oldest  of  these 
and  is  cut  by  dikes  and  lenticular  masses  of  iron  ore  and  granite. 
The  deposit  of  iron  ore  is  a  dike  \Y±  miles  long  and  40  to  300 
feet  wide.  At  several  places  it  is  almost  cut  in  two  by  wedge- 
like  masses  of  granite.  Through  practically  its  whole  length 
it  is  bordered  on  both  sides  by  anorthosite.  The  contact  between 

1  SINGEWALD,  J.  T.,  JR.  :  The  Titaniferous  Iron  Ore  of  the  United  States. 
U.  S.  Bur.  Mines  Bull.  64,  1913. 

2  BALL,  S.  H. :  Titaniferous  Iron  Ore  of  Iron  Mountain,  Wyoming.     U.  S. 
Geol.  Survey  Butt.  315,  p.  206,  1907. 

LINDGREN,  WALDEMAR:  Science,  new  ser.,  vol.  16,  pp.  984-985,  1902. 
KEMP,   J.   F.:  A  Brief  Review  of  Titaniferous   Magnetites.     School  of 
Mines  Quart.,  vol.  20,  pp.  352-355,  1899. 
SINGEWALD,  J.  T.,  JR.:  Op.  cit. 


344      THE  PRINCIPLES -OF  ECONOMIC  GEOLOGY 

the  anorthosite  and  the  ore,  where  exposed,  is  sharp,  neither  rock 
having  notable  gradational  borders.  The  iron  ore  is  a  black 
granular  holocrystalline  igneous  rock,  whose  constituent  grains 
range  from  %  to  %  inch  in  diameter.  The  greater  portion  of  the 
iron  ore  is  titaniferous  magnetite  free  from  mechanical  impurities, 
but  biotite,  olivine,  and  feldspar  are  sporadically  distributed 
throughout  its  mass. 

Adirondack  Region,  New  York. — The  titaniferous  iron  ores  of 
the  Adirondack  region,  New  York,  are  closely  associated  with 
gabbros  and  nearly  related  rocks.  The  principal  area  of  these 
rocks  is  in  Essex  County  and  southern  Franklin  County,  where 
they  occupy  about  1,200  square  miles.1  The  largest  deposits 
are  in  anorthosites — rocks  composed  essentially  of  labradorite. 
The  iron  ores  contain  magnetite,  ilmenite,  a  little  augite  or 
hypersthene,  and  garnet;  as  a  rule  plagioclase,  olivine,  horn- 
blende,  and  apatite  are  present.  The  deposits  are  intruded 
locally  by  igneous  dikes  and  together  with  the  gabbro  have  been 
subjected  to  dynamic  metamorphism. 

1  SINGE WALD,  J.  T.,  JR.:  The  Titaniferous  Iron  Ores  of  the  United  States. 
U.  S.  Bur.  Mines  Bull.  64,  pp.  47-49,  1913. 

KEMP,  J.  F.:  The  Titaniferous  Iron  Ores  of  the  Adirondacks.  U.  S. 
Geol.  Survey  Nineteenth  Ann.  Rept.,  part  3,  pp.  397-399,  1899. 

NEWLAND,  D.  H. :  Geology  of  the  Adirondack  Magnetitic  Iron  Ores. 
N.  Y.  State  Mus.  Bull.  119,  part  3,  pp.  146-170,  1908. 

KEMP,  J.  F.,  and  RUEDEMANN,  RUDOLPH:  Geology  of  the  Elizabethtown 
and  Port  Henry  Quadrangles.  N.  Y.  State  Mus.  Bull.  138,  pp.  137-149, 
1910. 


CHAPTER  XXIII 


COPPER 


Mineral 

Per  cent, 
of  copper 

Composition 

Chalcanthite 

25  4 

CuSCh  5H2O 

Brochantite  
Atacamite  
Malachite  
Azurite 

56.2 
59.5 
57.4 
55  3 

Cu4SO4(OH)6  or  4CuO.S03.3H2O 
Cu2Cl(OH)3 
Cu2(OH)2CO3  or  2CuO.COs.H2O 
Cu3(OH)2(CO3)2  or  3CuO  2CO2  HaO 

Chrysocolla 

36  0 

CuSiOs  2H2O  or  CuO  SiOz  2H2O 

Cuprite  ... 

88  8 

Cu2O 

Tenorite  

79  9 

CuO 

Melaconite  
Chalcocite  
Covellite 

79.9 
79.8 
66  5 

CuO 

Cu2S 
CuS 

Bornite 

55  5 

CusFeSs 

Chalcopyrite  
Enargite  
Tetrahedrite  
Tennantite  

34.6 
48.3 
52.1 
57.5 

CuFeS2  or  Cu2S.Fe?Ss. 
Cu3AsS4  or  3Cu2S.As2S6. 
Cu8Sb2S7  or  4Cu2S.Sb2S3 
Cu8As2S7  or  4Cu2S.As2S3. 

Mineral  Composition  of  Copper  Deposits. — Mineralogically 
the  principal  copper  deposits  may  be  grouped  in  two  classes — 
(1)  deposits  composed  of  sulphides  with  various  gangue  minerals 
and  alteration  products,  and  (2)  zeolitic  native  copper  ores. 
In  the  majority  of  sulphide  deposits  chalcopyrite  is  by  far  the 
most  common  primary  copper  mineral.  In  some  deposits, 
however,  enargite  is  the  principal  primary  mineral.  Other 
deposits  contain  one  or  more  of  the  minerals  chalcocite,  covel- 
lite,  bornite,  and  tetrahedrite,  as  primary  constituents.  Copper 
sulphide  deposits  are  readily  oxidized ;  near  the  surface  malachite, 
azurite,  chrysocolla,  cuprite,  chalcanthite,  brochantite,  and 
native  copper  are  formed.  The  principal  secondary  sulphides 
in  copper  deposits  are  chalcocite  and  covellite,  although  secondary 
bornite  and  chalcopyrite  are  not  uncommon.  As  a  rule  the 
secondary  sulphides  are  found  below  the  zone  of  oxidation  and 
their  zone  grades  downward  into  the  primary  ores.  Zeolitic 
native  copper  ores  alter  very  slowly.  Near  the  surface,  how- 

345 


346      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


ever,  some  of  them  contain  some  copper  as  oxides,  carbonates, 
and  silicates. 

Of  the  metallic  sulphides  that  are  associated  with  the  copper 
minerals  in  sulphide  deposits,  pyrite  is  practically  everywhere 
present  in  the  primary  ores.  Many  deposits  contain  sphalerite 
also,  intergrown  with  pyrite  and  chalcopyrite.  Pyrrhotite  is 
associated  with  chalcopyrite  in  some  copper  deposits.  Galena, 
in  subordinate  amount,  is  common. 

Of  the  gangue  minerals,  some  quartz  is  generally  present. 
Nearly  all  copper  sulphide  deposits  that  replace  igneous  rocks 
carry  sericite.  Ores  replacing  limestone  generally  contain  cal- 
cite.  Most  contact-metamorphic  deposits  contain  heavy  sili- 
cates, such  as  tremolite,  actinolite,  garnet,  and  epidote,  inter- 
grown  with  chalcopyrite  and  other  primary  sulphides.  Copper 
ores  with  tourmaline  gangue  are  found  in  the  San  Francisco 
region,  Utah,  and  in  some  localities  in  Chile.  The  great  majority 
of  the  copper  deposits  in  the  United  States  do  not  contain  sul- 
phates in  the  gangue  of  the  primary  ore,  but  anhydrite  is  a  gangue 
mineral  in  the  Cactus  mine  of  the  San  Francisco  region,  Utah, 
and  barite  is  present  in  the  copper  ores  of  Shasta  County,  Cali- 
fornia. A  little  barite  occurs  also  in  ores  of  Butte,  Mont. 

COPPER  PRODUCED  IN  THE  PRINCIPAL  DISTRICTS  OF  THE  UNITED  STATES 
TO  END  OP  1915° 


District 

Date 
pro- 
duction 
began 

Quantity 
(pounds) 

Percent- 
age of 
total  pro- 
duction 

Rank 

Butte,  Montana  

1868 

6,680,500,000 

31.24 

1 

Lake  Superior,  Michigan 

1845 

5,759,983,236 

26  93 

2 

Bisbee,  Arizona  

1880 

2,025,860,000 

9.47 

3 

Morenci-Metcalf,  Arizona  

1873 

1,221,296,000 

5.71 

4 

Bingham,  Utah  

1896 

1,149,500,000 

5.37 

5 

Jerome,  Arizona 

1883 

753  440,000 

3  52 

6 

Globe,  Arizona  . 

1881 

683,200,000 

3.20 

7 

Shasta  County,  California  

1897 

474,300,000 

2.22 

8 

Ely,  Nevada....           

1908 

441,080,000 

2.06 

9 

Ducktown,  Tennessee 

1850 

305,410,000 

1  43 

10 

Santa  Rita,  New  Mexico 

1880  (?) 

298,911,000 

1.40 

11 

Mineral  Creek  (Ray),  Arizona.  .  . 

219,978,000 

1.03 

12 

Foothill  belt,  California  

1862 

125,700,000 

0.59 

13 

Tintic,  Utah  

1880  (?) 

124,500,000 

0.58 

14 

0  BUTLER,  B.  S.:  U.  S.  Geol  Survey,  Mineral  Resources,  1916,  part  1,  p.  667,  1916. 


COPPER 


347 


COPPER  PRODUCED  IN  THE  PRINCIPAL  DISTRICTS  OF  THE  UNITED  STATES 
IN  1915° 


District  or  region 

Approximate 
mine  output, 
pounds 

Percent- 
age of 
total  pro- 
duction 

Rank 

Butte   Montana 

267  000  000 

17  94 

1 

Lake  Superior,  Michigan 

238,956,000 

16  06 

2 

Bingham,  Utah  
Bisbee,  Arizona  

165,000,000 
164,600,000 
98  500  000 

11.09 
11.06 
6  62 

3 

4 
5 

Alaska  (all  districts) 

70,695,000 

4  75 

6 

Santa  Rita,  New  Mexico  
Ely,  Nevada  
Mineral  Creek  (Ray),  Arizona  

65,000,000 
64,638,000 
60,338,000 

4.37 
4.34 
4.05 

7 
8 
9 

53,260,000 

3.58 

10 

Morenci-Metcalf,  Arizona  
Shasta  County,  California  
Ducktown,  Tennessee  

51,096,000 
30,500,000 
18,205,000 
6,399,000 

3.43 
2.05 
1.22 
0.43 

11 
12 
13 
14 

Tintic  Utah 

5,350,000 

0.36 

15 

Alder  Creek,  Idaho  
Big  Bug,  Arizona  

4,700,000 
4,285,000 
3,890,000 

0.32 
0.29 
0.26 

16 
17 
18 

3,815,000 

0.26 

19 

Calaveras  County,  California  
Southwestern  Colorado  ' 
Pima   Arizona           .              

3,700,000 
3,600,000 
3,164,000 

0.25 
0.24 
0.21 

20 
21 
22 

3,000,000 

0.20 

23 

Ophir,  Utah  

2,212,000 

0.15 

24 

Courtland  (Turquoise),  Arizona  

2,048,000 
1,900,000 

0.14 
0.13 

25 
26 

1,900,000 

0.13 

27 

Uinta-Summit  (Park  City),  Utah.  
Leadville,  Colorado  
New  Placers,  New  Mexico  
Goldfield,  Nevada  

1,812,000 
1,800,000 
1,700,000 
1,670,000 

0.12 
0.12 
0.11 
0.11 

28 
29 
30 
31 

Burro  Mountain,  New  Mexico  
Magdalena,  New  Mexico  

1,400,000 
1,400,000 

0.09 
0.09 

32 
33 

All  others                            

1,407,533,000 
80,467,000 

Grand  total  

1,488,000,000 

BUTLER,  B.  S.:  U.  S.  Geol.  Survey,  Mineral  Resources,  1915,  part  1,  p.  667,  1916. 


348      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Minerals  found  in  zeolitic  copper  ores  include  native  copper, 
native  silver,  laumontite,  delessite,  orthoclase,  calcite,  thom- 
sonite,  quartz,  prehnite,  analcite,  apophyllite,  natrolite,  .and 
many  others.  In  some  deposits  sulphides  are  sparingly  asso- 
ciated with  these  minerals. 

Copper  Deposits  in  the  United  States. — In  point  of  value 
copper  ranks  second  only  to  iron  among  the  metals  produced  in 
the  United  States,  and  the  output  is  greater  than  the  combined 
output  of  gold  and  silver.  Operations  of  mining  and  beneficiat- 
ing  copper  ores  are  generally  more  centralized  than  those  of 
mining  and  smelting  iron  ores.  Most  of  the  copper  ores  carry 
much  lower  percentages  of  metal  than  iron  ores,  and  it  is  profit- 
able to  concentrate  them  near  the  producing  centers. 

Of  the  copper  ores  the  sulphide  deposits  are  the  richest.  The 
Lake  Superior  region,  however,  produces  copper  almost  exclu- 
sively from  native  metal  ores,  which  yield  an  average  of  about 
1  per  cent,  of  copper.  Sulphide  smelting  ores  (not  concentrated) 
generally  carry  3  to  5  per  cent,  or  more,  and  concentrating  ores 
from  1  to  3  per  cent.  At  Ducktown,  Tenn.,  however,  ores  with 
less  than  2  per  cent,  of  copper  are  smelted.  Gold  and  silver 
are  valuable  by-products  of  many  copper  ores,  and  many  gold 
and  silver  ores  carry  appreciable  amounts  of  copper. 

Genesis  of  Copper  Deposits. — All  the  larger  copper  deposits 
of  the  United  States  are  epigenetic,  thus  differing  from  the  larger 
iron  deposits,  which  are  in  the  main  of  syngenetic  origin.  Out- 
side of  the  United  States,  however,  there  are  valuable  syngenetic 
copper  deposits.  The  nickel  ores  of  Sudbury,  Ontario,  which 
carry  as  much  copper  as  some  copper  ores,  were  formed  by  mag- 
matic  segregation,  and  the  famous  copper  deposits  at  Mansfeld, 
Germany,  are  probably  of  sedimentary  origin.  The  great 
majority  of  copper  deposits,  however,  the  world  over,  are 
epigenetic  (see  Fig.  40,  page  91). 

The  deposits  of  copper  in  the  United  States  belong  to  several 
genetic  groups.  Many  of  them  replace  limestone,  and  of  these 
some  have  been  formed  under  contact-metamorphic  conditions 
by  solutions  emanating  from  igneous  rocks.  Some  of  the  de- 
posits of  Morenci,  Ariz.,  and  Bingham,  Utah,  are  examples. 
Other  deposits  that  replace  limestone  are  essentially  without 
contact-metamorphic  minerals.  As  a  rule  the  replacement 
deposits  in  limestone  do  not  follow  closely  well-defined  fissures, 
but  many  of  them  are  irregular  masses  or  chambers  (see  page 


COPPER  349 

201).  Some,  however,  are  rudely  tabular,  especially  those  in 
which  the  rock  has  been  replaced  along  bedding  planes.  Many 
copper  deposits  are  veins  of  the  replacement  type  (see  page  242). 
In  these  a  fissure  has  become  a  channel  of  mineralizing  solutions 
which  have  replaced  the  wall  rock  near  the  fissure  and  also  filled 
any  openings  available.  Many  such  deposits  are  contained  in 
granites,  quartz  monzonites,  monzonites,  or  diorites.  As  a  rule 
these  bodies  are  more  nearly  tabular  than  those  which  replace 
limestone.  Examples  of  replacement  veins  of  copper  ore  are 
found  at  Butte,  Mont.,  at  Globe  and  Morenci,  Ariz.,  and 
elsewhere.  Primary  copper  deposits  have  formed  also  by  de- 
position from  cold  solutions  (page  74). 

The  deposits  of  disseminated  copper  ores  in  porphyry  and  in 
schists  have  recently  become  highly  productive.  Popularly 
these  are  termed  the  "porphyry"  ores.  They  include  huge 
deposits  developed  at  Bingham,  Utah,  Ely,  Nev.,  and  Santa  Rita 
(Chino),  N.  M.,  and  some  of  the  deposits  at  Morenci  and 
Ajo,  Ariz.,  and  at  Cananea,  Sonora,  Mexico,  as  well  as  many 
similar  deposits  in  the  Southwest.  At  Miami  and  Ray,  Ariz., 
disseminated  copper  ores  occur  in  siliceous  mica  schists.  The 
disseminated  ores  in  porphyry  and  in  schists  include  more  than 
half  the  developed  reserves  of  copper  ore  within  the  United 
States.  The  bodies  of  disseminated  ore  contain  many  small, 
irregular,  closely  spaced  veins  and  veinlets.  The  country  rock 
between  the  veins  is  hydrothermally  altered  and  carries  numerous 
small  particles  of  copper  ore,  and  the  entire  mass  is  mined  and 
sent  to  the  concentrators.  Nearly  all  the  disseminated  or  por- 
phyry ore  has  been  greatly  enriched  by  superficial  alteration. 
An  outline  of  the  stages  involved  in  the  genesis  of  a  disseminated 
copper  deposit  in  porphyry  is  given  below. 


1.  Intrusion  of  a  magma  and  its  solidification,  forming  diorite,  monzonite, 
or  granite  porphyry. 

2.  Development  of  great  bodies  of  fractured  or  shattered  porphyry. 
As  such  bodies  are  not  everywhere  developed  in  the  porphyry  they  are 
probably  fractured  by  stresses  rather  than  by  shrinkage  due  to  cooling. 
Some  of  the  fractures,  however,  may  be  cooling  cracks. 

3.  Primary  metallization  of  porphyry  by  ascending  hot  waters  emanating 
from  deep-seated  sources,  probably  from  a  related  igneous  body  not  yet 
completely  solidified;  formation  of  primary  protore  consisting  of  pyrite, 
chalcopyrite,  and  other  metallic  minerals,  accompanied  by  the  development 
of  sericite  from  feldspars  and  other  minerals  of  porphyry  and  by  the  deposi- 


350      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tion  of  quartz.  The  sulphides  fill  the  minute  and  closely  spaced  cracks  and 
are  deposited  by  replacement  as  shots  and  small  masses  between  cracks. 
Copper  content  of  primary  ore  rarely  equals  1  per  cent,  and  is  generally  not 
over  0.5  per  cent. 

4.  Top  of  metallized  porphyry  body  is  exposed,  by  erosion,  to  weathering. 
Surface  waters  dissolve  the  copper  from  the  rock  near  the  surface,  carry  it 
deeper  and  deposit  it  in  the  reducing  zone,  where  oxygen  is  excluded.    The 
copper  is  generally  deposited  as  chalcocite,  although  some  bornite,  covellite, 
and  chalcopyrite  may  be  deposited.     The  acid  waters  alter  some  of  the 
feldspar  and  sericite  to  kaolin  and  other  secondary  minerals.     The  chalco- 
cite zone  is  a  thick  blanket-like  body  lying  between  the  leached  zone  and 
primary  ore.     It  is  extensive,  undulating,  and  of  irregular  thickness  and 
grade.     The  larger  and  richer  ore  bodies  are  in  fractured  portions  of  the  rock, 
where  downward-moving  waters  converge. 

5.  The  outcrop,  leached  zone,  and  zone  of  secondary  sulphides  move 
gradually  downward;  as  the  country  is  eroded  the  copper  becomes  concen- 
trated more  and  more  in  the  secondary  sulphide  zone.     Pyrite  and  chalco- 
pyrite are  replaced  by  chalcocite  and  associated  copper  sulphides.     Locally 
the  primary  ore,  which  generally  contains  less  than  0.5  per  cent,  of  copper,  is 
converted  to  an  ore  with  1.5  per  cent,  or  more.     The  surface  zone  is  leached 
and  generally  carries  only  a  little  copper;  iron  oxide  is  commonly  present. 
As  iron  is  removed  from  the  deposit  the  leached  zone  and  outcrop  become  less 
ferruginous.     Much  silica  may  become  concentrated  with  the  kaolin  at  the 
surface.     (For  sections  of  disseminated  copper  deposits  see  Figs.  141,  p.  380). 

The  above  outline  with  some  modifications  may  serve  to  illus- 
trate the  origin  of  many  of  the  deposits  of  disseminated  copper 
ores.  If  the  climate  becomes  arid  and  the  water  level  is  rapidly 
depressed  after  the  process  has  gone  far,  the  chalcocite  ore  may 
be  oxidized  largely  to  cuprite  and  native  copper,  or  it  may  be 
altered  to  carbonates  and  silicates  near  the  surface.  If  the 
primary  ore  is  comparatively  rich  in  copper  and  contains  little 
pyrite  it  migrates  less  readily  and  concentrates  more  slowly, 
and  richer  oxidized  ores  may  be  exposed  at  the  outcrop  and  in 
the  oxidized  zone,  as  at  Ajo,  Ariz.  At  Miami  and  Ray,  Ariz., 
the  copper  ores  are  developed  in  siliceous  schists  that  are  in- 
truded by  granite  porphyry.  Their  secondary  concentration  is 
comparable  to  that  of  the  typical  disseminated  ores  in  porphyry. 
In  the  descriptions  of  copper-bearing  districts  that  follow,  other 
exceptional  features  in  several  deposits  are  pointed  out.  Some 
districts  contain  disseminated  ores,  veins  and  contact-meta- 
morphic  deposits  replacing  limestone,  all  in  comparatively  small 
areas.  In  the  Southwest  there  are  many  comparatively  small 
deposits  of  copper  sulphides  that  have  probably  been  deposited 
by  cold  meteoric  waters  (see  page  77). 


COPPER  351 

Age  of  Copper  Deposits  in  United  States.— Deposits  of  copper 
ores  in  the  United  States  show  great  range  in  age.  The  deposits 
of  Lake  Superior,  of  the  Encampment  district,  Wyoming,  of 
Jerome,  Ariz.,  and  of  a  few  other  localities  in  the  West  are  of 
pre-Cambrian  age.  These  deposits  according  to  Butler,1  have 
yielded  about  34  per  cent,  of  the  output  of  the  United  States. 
In  the  Appalachian  region  valuable  deposits  of  copper  were 
formed  in  Paleozoic  time.  These  include  the  copper  ores  of 
Ducktown,  Tenn.,  and  some  other  deposits,  which  together  have 
yielded  about  2  per  cent,  of  the  country's  output.  Copper  de- 
posits were  formed  also  in  the  Mesozoic  era,  probably  at  about 
the  beginning  of  the  Cretaceous.  These  include  the  deposits 
of  Shasta  County  and  possibly  those  of  the  "foothill  belt"  in 
California;  also  the  deposits  of  Bisbee  and  probably  those  of 
Globe,  Ariz.  Possibly  the  deposits  of  Ray,  Ariz.,  and  Ely,  Nev., 
should  also  be  included  in  this  group.  The  ores  of  these  districts 
are  associated  in  the  main  with  intrusive  masses  of  granite, 
diorite,  or  monzonite  and  their  related  porphyries. 

The  greatest  period  of  copper  deposition  was  in  early  Tertiary 
time.  The  deposits  of  this  period  include  those  of  Butte,  Mont., 
Morenci,  Ariz.,  Santa  Rita,  N.  Mex.,  and  probably  those  of  Bing- 
ham,  Utah,  as  well  as  many  minor  deposits.  It  is  noteworthy 
that  the  metalliferous  deposits  of  middle  and  late  Tertiary  age 
in  the  United  States,  although  they  have  supplied  enormous 
quantities  of  gold  and  silver,  have  yielded  comparatively  little 
copper. 

Outcrops  of  Copper  Deposits. — Sulphide  ores  of  copper  are 
almost  invariably  leached  near  the  surface  except  where  the 
former  surface  material  has  been  removed  by  rapid  erosion  or 
by  glaciation.  Many  copper  ores,  however,  contain  other  metals 
that  are  not  so  readily  leached  as  the  copper.  Many  valuable 
deposits  of  copper  sulphide  ore  have  been  discovered  by  down- 
ward exploitation  of  oxidized  gold  and  silver  ores.  Butte, 
Mont.,  was  first  exploited  as  a  gold-placer  district;  later  the 
upper  parts  of  the  veins  were  worked  for  silver,  and  these  work- 
ings about  200  feet  below  the  surface  revealed  great  bodies  of 
rich  copper  ores.  Deposits  at  Bingham,  Utah  (Highland  Boy 
mine),  and  at  Jerome,  Ariz.  (United  Verde  mine),  were  first 

1  BUTLER,  B.  S.:  U.  S.  Geol.  Survey.  Mineral  Resources,  1911,  part  1, 
p.  258,  1912. 


352      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Limonite  Quartz, 

Kaolin 
Malachite 
Azurite 
Chrysocolla 
Tenorite 
Cuprite,  Etc. 


exploited  for  gold.  Underneath  the  oxidized  gold  ores  great 
bodies  of  copper  ore  were  discovered.  The  first  deposit  ex- 
ploited at  Ducktown,  Tenn.,  was  worked  for  iron.  At  Bisbee, 
Ariz.,  the  first  ores  discovered  were  rich  oxidized  copper  ores. 
The  ores  cropped  out  at  one  place,  from  which  they  were  fol- 
lowed downward  on  the  dip,  the  operations  disclosing  enormous 
bodies  of  sulphide  ores,  partly  oxidized  ahd  in  the  main  at  the 
horizon  of  the  ores  first  discovered.  The  deposits  of  Santa 
Rita,  N.  Mex.,  carried  rich  oxidized  copper  ore  near  the  surface 
which  passed  into  sulphide  ore  below.  In  regions  where  copper 
ores  abound  areas  richly  stained  with  iron  are  generally  con- 
sidered worthy  of  exploration  in  a  search  for  copper.  On  the 

other  hand,  deposits  of  copper 
have  been  found  below  outcrops 
that  show  very  little  staining 
by  iron  oxide.  These  outcrops, 
however,  are  generally  silicified 
and  kaolinized. 

Sulphide  Enrichment  of  Cop- 
per Deposits. — Most  of  the  large 
copper  sulphide  deposits  in  the 
United  States  show  three  zones 

FIG.  154.— Ideal  section  showing  —a  leached  zone  near  the  sur- 
distribution  of  ore  minerals  in  a  cop-  face  an  enriched  zone  below 
per  lode  composed  of  chalcopynte, 

bornite,  pyrite,  quartz,  and  sericite,  the  leached  zone,  and  a  zone  ol 
after  superficial  alteration  by  lower-grade  primary  ore  below 
weathering.  •  »  *  /TV  -.^N 

the   enriched    zone    (Fig.    154). 

In  some  deposits  the  oxidized  ores  and  in  some  the  primary  sul- 
phide ores  are  rich  enough  to  work.  In  other  deposits  only  the 
ores  of  the  secondary  sulphide  zone  are  profitable. 

In  the  oxidized  zones  of  sulphide  deposits  the  mineral  waters 
are  sulphuric  acid  and  ferric  sulphate  solutions.  Such  solutions 
dissolve  copper  readily,  and  in  contact  with  copper  compounds 
such  a  system  will  contain  also  copper  sulphate.  The  copper 
sulphate  in  solution  reacts  with  carbonates  or  with  acid  car- 
bonate in  solution,  precipitating  copper  carbonates.  If  chlorides 
are  abundant,  copper  chlorides  may  form.  In  moist  countries 
copper  chlorides  are  unstable.  The  sulphates  chalcanthite  and 
brochantite  also  may  be  precipitated,  and  the  basic  sulphate 
brochantite  once  formed  is  stable.  The  silicates  of  copper  are 
probably  formed  by  copper-bearing  solutions  reacting  on  silicic 


COPPER  353 

acid,  which,  as  shown  by  analyses,  is  commonly  dissolved  in 
mine  waters.  Native  copper,  cuprite,  and  tenorite  are  formed 
by  the  reduction  or  oxidation  of  various  copper  compounds.  All 
the  copper  minerals  mentioned  above  are  formed  in  the  main  in 
the  oxidized  zone,  and  in  sulphide  ore  deposits  their  occurrence 
below  this  zone  is  exceptional.  None  of  them  are  known  to 
be  formed  in  depth  by  deposition  from  hot  ascending  alkaline 
solutions. 

Below  the  oxidized  zone,  where  air  is  excluded,  copper  is 
precipitated  as  sulphides:  chalcocite,  covellite,  bornite,  chalco- 
pyrite,  and  possibly  some  of  the  more  complex  antimony  and 
arsenic  compounds  are  formed  by  these  processes.  Precipita- 
tion may  be  brought  about  by  chemical  interchange  with  pyrite, 
chalcopyrite,  pyrrhotite,  zinc  blende,  galena,  and  probably  with 
some  other  sulphides,  the  process  being  mainly  metasomatic 
replacement.  The  copper  sulphides  are  precipitated  also  by 
hydrogen  sulphide,  which  may  be  generated  by  attack  of  acid 
solutions  on  several  of  these  sulphides.  The  chalcocite  that 
forms  under  these  conditions  may  be  a  fine  dust-like  powder, 
termed  "sooty  chalcocite,"  but  much  of  it  is  fine  massive  glisten- 
ing material.  In  a  reducing  environment  the  copper  sulphides 
are  highly  stable.  Iron  sulphide  is  dissolved  in  acid  even  in  a 
reducing  environment.  The  double  sulphides  of  iron  and  copper 
would  probably  not  be  precipitated  from  acid  solutions  that 
contained  much  copper.  As  the  solutions  descend,  however, 
they  lose  acidity,  and  copper  sulphide  is  precipitated  at  the  ex- 
pense of  iron  sulphide,  the  iron  going  into  solution.  A  decrease 
in  acidity,  a  decrease  in  copper,  and  an  increase  of  iron  in  solu- 
tion bring  about  a  state  of  equilibrium  which  is  increasingly 
favorable  to  the  precipitation  of  double  sulphides,  such  as 
chalcopyrite  and  bornite.1 

In  the  oxidizing  zone  copper  is  much  more  soluble  than  gold, 
and,  unlike  gold,  it  may  be  dissolved  in  the  absence  of  chlorides 
in  sulphate  solutions.  Thus  many  deposits  which  contain  both 
copper  and  gold  show  a  distinct  segregation  of  gold  near  the 
surface,  while  copper  ores  with  subordinate  gold  are  found  lower 
down.  Even  where  the  conditions  for  the  solution  of  gold  are 
most  favorable  it  is  probably  precipitated  mainly  in  the  upper 
part  of  the  chalcocite  zone.  It  would  not  remain  in  solutions 

1  WELLS,  R.  C. :  The  Fractional  Precipitation  of  Sulphides.  Econ.  Geol, 
vol.  5,  pp.  12-13,  1910. 


354      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


that  contained  much  ferrous  sulphate,  and  chalcocitization  is 
probably  attended  by  the  generation  of  abundant  ferrous 
sulphate.  Ferrous  sulphate  precipitates  silver  also,  and  silver 
is  reprecipitated  by  chalcocite  very  readily  in  both  neutral  and 
acid  solutions  (see  page  443). 

In  pyrrhotite  ores  chalcocite  enrichment  is  shallow.  In  de- 
posits of  sphaleritic  copper  ores  without  pyrrhotite  the  chalcocite 
zone  may  be  moderately  extensive  vertically.  The  most  ex- 
tensive chalcocite  zones  are  in  pyrite  and  chalcopyrite  deposits 
that  do  not  contain  pyrrhotite.  Most  of  them  also  contain  little 
or  no  sphalerite  (Fig.  155). 


FIG.  155. — Diagrams  showing  influence  of  mineral  composition  of  copper 
lodes  on  depth  of  enrichment,  a,  Minerals  react  rapidly  with  copper  sul- 
phate solutions  and  deposit  copper  sulphides;  b,  minerals  react  less  rapidly; 
c,  minerals  react  slowly. 


Carbonates  react  with  acid  solutions  and  tend  to  delay  the 
downward  migration  of  copper.  If  there  is  much  lime  carbonate 
in  the  gangue  of  the  ore  or  in  the  wall  rock,  the  downward  migra- 
tion of  metallic  sulphates  may  be  checked. 

A  zone  of  chalcopyrite  enrichment  may  exist  below  a  chalcocite 
zone.  Some  profitable  ores  of  copper  are  found  below  the  zone 
of  chalcopyrite  enrichment,  but  these  are  not  ordinarily  "  bonanza" 


COPPER  MINERALS 

Native  copper  in  zeolite  ores  is  primary  (see  page  395).  In 
sulphide  deposits  it  is  invariably  formed  by  secondary  processes. 
In  some  sulphide  deposits  it  is  a  valuable  constituent  of  the  ore 

1  For  additional  data  relating  to  the  influence  of  gangue  and  ore  minerals 
on  depth  of  secondary  sulphide  zones,  see  EMMONS,  W.  H.:  The  Enrich- 
ment of  Ore  Deposits.  U.  S.  Geol.  Survey  Bull  625,  pp.  172-175,  1917. 


COPPER  355 

bodies  occurring  above  the  chalcocite  zone  or  in  the  upper  part 
of  the  chalcocite  zone. 

Chalcanthite,  blue  vitriol,  is  present  in  many  oxidized  zones 
as  efflorescences  or  stalactites  on  walls  of  open  fissures,  or  as 
veinlets  filling  small  crevices  above  the  upper  limit  of  the  zone 
of  secondary  sulphides. 

Brochantite  has  been  identified  in  only  a  few  deposits  but  is 
probably  not  uncommon.  It  is  frequently  mistaken  for  car- 
bonates. Brochantite  is  unknown  in  the  deeper  levels  of  sulphide 
lodes. 

Atacamite,  the  oxychloride  of  copper,  is  not  stable  in  most 
climates.  In  arid  countries  it  forms  in  the  oxidized  zones.  It 
has  lately  been  shown  that  much  of  the  material  of  Chuquicamata, 
Chile,  formerly  supposed  to  be  atacamite  is  brochantite. 

Malachite  and  azurite  are  abundant  in  the  oxidized  zones  of 
many  cupriferous  deposits,  especially  in  deposits  that  are  inclosed 
in  limestone. 

Chrysocolla  occurs  abundantly  in  the  outcrops  and  near  the 
surface  of  some  copper  deposits;  in  others  it  is  rare  or  absent. 
It  is  a  common  mineral  also  in  the  oxidized  zones  of  some  silver 
and  gold  mines.  It  is  in  places  associated  with  malachite  and 
azurite  and  is  not  known  as  a  deposit  of  ascending  hot  waters. 

Cuprite  is  a  common  mineral  of  the  oxidized  zones  of  deposits 
of  copper  sulphides  and  is  probably  secondary  in  all  its  occur- 
rences. At  Morenci,  Ariz.,  according  to  Lindgren,1  it  is  an  oxida- 
tion product  of  chalcocite. 

Tenorite,  the  crystalline  form  of  the  black  oxide  of  copper,  is 
much  less  abundant  than  cuprite.  The  earthy,  sooty  variety  is 
known  as  melaconite. 

Copper  pitch  ore  is  a  secondary  material  of  complex  character 
and  somewhat  uncertain  composition.  A  sample  from  the 
Detroit  mine  in  the  Morenci  district,  Arizona,2  showed  oxides 
of  copper,  zinc,  and  manganese,  with  considerable  water  and 
silica. 

Chalcocite,  copper  glance,  is  the  most  valuable  copper  mineral. 
In  most  of  its  occurrences  it  is  clearly  of  secondary  origin  (see 
page  155),  foritreplaces  other  minerals  metasomatically or  occurs 
as  veinlets  in  small  cracks  in  the  primary  ore.  Many  examples 

LINDGREN,  WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.     U.  S.  Geol.  Survey  Prof.  Paper  43,  p.  Ill,  1905. 
*Idem,  p.  114. 


356      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

are  known  of  its  replacement  of  pyrite,  zinc  blende,  and  galena. 
In  many  deposits  exploration  extends  below  chalcocite  into  the 
lower-grade  primary  sulphide  ore,  which  generally  consists  of 
chalcopyrite,  pyrite,  and  other  minerals  without  chalcocite. 

The  replacement  of  pyrite  by  chalcocite1  is,  according  to  Stokes, 
effected  as  follows: 

5FeS2  +  14CuSO4  +  12H2O  =  7Cu2S  +  5FeSO4  +  12H2SO4. 

In  a  few  districts  chalcocite  is  primary.2 

Some  chalcocite  deposits  in  sandstone  and  shale,  widely  dis- 
tributed in  the  Southwest,  are  in  areas  remote  from  igneous 
rocks  and  appear  not  to  be  related  genetically  to  igneous  processes. 
They  have  doubtless  been  deposited  by  cold  waters  and  some  of 
them  have  replaced  coal  or  other  organic  material  Although 
these  deposits  are  not  secondary  in  the  sense  that  they  have 
been  formed  generally  at  the  expense  of  older  sulphides,  the 
conditions  under  which  they  were  formed  as  regards  tempera- 
ture, pressure,  and  concentration  of  solution  are  probably  near 
those  which  prevail  in  processes  of  sulphide  enrichment. 

Covellite  is  found  in  small  amounts  in  many  mining  districts 
of  North  America  but  is  not  abundant  in  many  of  the  larger 
deposits.  As  a  rule  it  is  associated  with  chalcocite,  and  it  is 
formed  chiefly  by  the  replacement  of  iron  or  zinc  sulphides. 
As  the  precipitation  of  cupric  sulphide  from  cupric  sulphate  solu- 
tions involves  no  change  of  valence,  some  simple  equations  may 
be  written: 

ZnS  +  CuSO4  =  CuS  +  ZnSO4. 
CuFeS2  +  CuS04  =  2CuS  +  FeSO4. 
FeS  +  CuS04  =  CuS  +  FeSO4. 
H2S  +  CuSO4  =  CuS  +  H2SO4. 

1  STOKES,  H.  N. :  Experiments  on  the  Action  of  Various  Solutions  on 
Pyrite  and  Marcasite.  Econ.  Geol,  vol.  2,  p.  22,  1907. 

SPENCER,  A.  C.:  Chalcocite  Deposition.  Wash.  Acad.  Sci.  Jour.,  vol. 
3,  p.  73,  1913. 

ZIES,  E.  G.,  ALLEN,  E.  T.,  and  MERWIN,  H.  E.:  Some  Reactions  Involved 
in  Secondary  Copper  Sulphide  Enrichment.  Econ.  Geol,  vol.  11,  pp.  407- 
503,  1916. 

2LANEY,  F.  B.:  The  Relation  of  Bornite  and  Chalcocite  in  the  Copper 
Ores  of  the  Virgilina  District  of  North  Carolina  and  Virginia.  Econ.  Geol., 
vol.  6,  pp.  399-411,  1911. 


COPPER  357 

Bornite  is  found  in  associations  that  indicate  its  formation 
under  many  different  geologic  conditions.  It  occurs  in  lodes  that 
were  formed  at  great  depths  and  also  in  some  that  were  formed 
at  moderate  depths,  and  less  abundantly  in  deposits  remote  from 
outcrops  of  igneous  rocks.1  It  is  deposited  on  pyrite  and  other 
sulphides  by  cold  copper  sulphate  waters,  and  in  some  deposits 
it  is  a  valuable  secondary  sulphide.  As  such  it  is  usually  much 
less  abundant  than  chalcocite. 

Chalcopyrite  in  the  greater  number  of  its  occurrences  is  clearly 
primary,  and  in  many  sulphide  deposits  it  is  the  only  important 
primary  copper  mineral  in  the  unaltered  ore.  A  list  of  occur- 
rences of  primary  chalcopyrite  would  include  nearly  all  important 
deposits  of  copper  ore  in  the  United  States.  It  is,  however, 
secondary  in  many  deposits.  In  general  it  forms  at  greater 
depths  than  secondary  chalcocite. 

Enargite  is  an  ore  mineral  of  great  value  at  Butte,  Montana, 
is  present  in  considerable  amounts  at  Tintic,  Utah,  and  occurs 
in  less  abundance  in  several  other  districts.  It  is  in  the  main, 
primary. 

Famatinite  is  not  a  common  ore  of  copper,  and  little  is  known 
as  to  its  origin.  In  view  of  the  primary  origin  of  enargite,  its 
corresponding  arsenic  salt,  it  is  probably  primary. 

Tetrahedrite  is  a  comparatively  common  copper  mineral. 
In  most  of  its  occurrences  it  is  primary. 

Tennantite  is  the  arsenic  salt  corresponding  to  tetrahedrite, 
but  it  is  not  so  common  as  tetrahedrite. 

COPPER-BEARING  DISTRICTS 

Butte,  Mont— The  Butte  district,  in  western  Montana,  is 
the  most  productive  copper  district  in  the  world.  It  has  yielded 
over  7,000,000,000  Ib.  of  copper,  more  than  300,000,000  ounces 
of  silver  and  1,500,000  ounces  of  gold,  also  much  zinc  and  smaller 
amounts  of  arsenic,  lead  and  manganese.  Developments  ex- 
tend to  depths  greater  than  3,000  feet,  where  ores  of  good  grade 
are  found.  The  copper  ore  is  concentrated  or  smelted  directly 
in  the  great  plants  at  Anaconda,  Great  Falls,  and  Butte. 

^LINDGREN,  WALDEMAE,  GRATON,  L.  C.,  and  GORDON,  C.  H.:  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Pro/.  Paper  68,  pp.  77-78, 
1910. 


358      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


COPPER  AND   ASSOCIATED   METALS  PRODUCED  IN  BTJTTE   DISTRICT, 
MONTANA,  1913-1915° 


Year 

Copper 
pounds 

Yield,  copper, 
per  cent. 

Gold 
ounces, 
per  ton 

Silver 
ounces, 
per   ton 

1913 

283,600,000 

2.70 

0  0058 

1.92 

1914  
1915  

235,700,000 
267,000,000 

2.66 
2.65 

0.0059 
0.0066 

1.83 
1.82 

0  BUTLER,  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  700,  1916. 

The  Butte  district1  is  an  area  of  quartz  monzonite  (frequently 
called  granite),  which  is  intruded  by  a  later  aplite,  and  by  rhyo- 
lite  porphyry.  Dikes  of  late  Tertiary  rhyolite  cut  the  granite, 
and  effusive  rhyolite  rests  upon  it.  In  the  western  part  of  the 
region  are  Tertiary  lake  beds  more  recent  than  the  granitic  rocks. 
These  are  composed  of  sand,  gravel,  and  water-laid  tuff. 

The  quartz  monzonite,  aplite,  and  porphyry,  which  contain 
all  the  ores,  are  phases  of  the  great  Boulder  batholith,  which 
extends  some  64  miles  southward  from  a  point  near  Helena  and 
is  12  to  16  miles  wide.  This  batholith  intrudes  Paleozoic  and 
Cretaceous  sedimentary  rocks  and  along  its  borders  has  induced 

1  WEED,  W.  H.,  EMMONS,  S.  F.,  and  TOWER,  G.  W.,  JR.  :  U.  S.  Geol.  Sur- 
vey Geol.  Atlas,  Butte  folio  (No.  38),  1897). 

WEED,  W.  H.:  Geology  and  Ore  Deposits  of  the  Butte  District,  Mon- 
tana. U.  S.  Geol.  Survey  Prof.  Paper  74,  1912. 

WINCHELL,  H.  V. :  Synthesis  of  Chalcocite  and  Its  Genesis  at  Butte,  Mon- 
tana. Eng.  and  Min.  Jour.,  vol.  75,  pp.  782-784,  1903. 

SALES,  R.  H. :  Ore  Shoots  at  Butte,  Montana.  Econ.  Geol.,  vol.  3,  pp. 
326-331,  1908;  Superficial  Alteration  of  the  Butte  Veins.  Idem,  vol.  5,  pp. 
15-21,  1910;  Ore  Deposits  of  Butte,  Montana.  Am.  Inst.  Min.  Eng.  Trans., 
vol.  40,  pp.  3-106,  1914. 

SIMPSON,  J.  F. :  The  Relation  of  Copper  to  Pyrite  in  the  Lean  Copper 
Ores  of  Butte,  Montana.  Econ.  Geol,  vol.  3,  pp.  628-636,  1908. 

KIRK,  C.  T. :  Conditions  of  Mineralization  in  the  Copper  Veins  at  Butte, 
Montana.  Econ.  Geol.,  vol.  7,  pp.  35-82,  1912. 

RAT,  J.  C. :  Paragenesis  of  the  Ore  Minerals  in  the  Butte  District,  Mon- 
tana. Econ.  Geol,  vol.  9,  pp.  463-481,  1914. 

LINPORTH,  F.  A.:  Applied  Geology  in  the  Butte  Mines.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  46,  pp.  110-127,  1914. 

BARD,  D.  C.,  and  GIDEL,  M.  H.:  Mineral  Associations  at  Butte,  Montana. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  46,  pp.  123-127,  1914. 

ATWOOD,  W.  W. :  The  Physiographic  Conditions  at  Butte,  Montana,  and 
Bingham  Canyon,  Utah,  when  the  Copper  Ores  in  These  Districts  were 
Enriched.  Econ.  Geol,  vol.  11,  pp.  697-741,  1916. 


COPPER  359 

contact  metamorphism  by  which  the  typical  garnet  zones  have 
been  developed  in  the  calcareous  sediments.  There  are, 
however,  no  metamorphosed  sediments  in  the  Butte  mining 
district. 

Although  the  rocks  of  the  batholith  are  in  general  of  com- 
paratively uniform  composition,  the  Butte  quartz  monzonite 
is  a  somewhat  more  basic  phase.  The  aplite  represents  a  dif- 
ferentiation product  that  was  forced  into  cracks  in  the  quartz 
monzonite  after  that  rock  had  cooled. 

There  seems  to  be  a  genetic  relation  between  the  copper  ores 
and  the  porphyry  intrusives.  The  porphyry  is  found  mainly 
in  the  eastern  portion  of  the  copper  area,  where  it  is  younger 
than  the  Butte  quartz  monzonite  and  older  than  the  veins,  for 
even,  the  oldest  veins  cut  through  it.  The  veins  in  the  por- 
phyry as  in  the  aplite,  are  narrower  and  poorer  than  in  the  quartz 
monzonite.  The  porphyry  is,  however,  the  youngest  igneous 
rock  exposed  that  is  older  than  the  oldest  veins. 

The  copper  ores  are  included  in  an  area  about  1%  miles  long 
and  a  mile  wide,  and  this  area  is  almost  surrounded  by  a 
much  larger  area  containing  closely  spaced  silver-bearing  veins. 
Pronounced  parallelism  is  noticeable  in  veins  of  both  groups. 

The  fissuring  in  the  district  is  exceedingly  complex  (Figs. 
156,  157).  The  systems  as  outlined  by  Sales  are  (1)  Anaconda 
system;  (2)  Blue  system  of  fault  fissures;  (3)  Mountain  View 
breccia  faults;  (4)  Steward  system;  (5)  Rarus  fault;  (6)  Middle 
faults;  (7)  Continental  fault. 

1.  The  Anaconda  system  is  composed  of  easterly  fissures  which 
are  generally  heavily  mineralized  and  along  which  there  has 
been  but  little  displacement.  In  general  they  dip  south  at  high 
angles.  •  In  the  copper-producing  area  there  are  two  notable 
groups  of  veins  belonging  to  the  Anaconda  system.  On  the  south 
or  Anaconda  group  are  the  deposits  of  the  Gagnon,  Original, 
Parrot,  Never  Sweat,  Anaconda,  St.  Lawrence,  Mountain  View, 
Leonard,  West  Colusa,  and  other  mines.  North  of  this  group 
and  separated  from  it  by  a  comparatively  barren  area  is  the 
second  group,  which  includes  the  Syndicate,  Bell,  Speculator,  and 
associated  fissures.  Some  of  the  easterly  fractures  are  joined 
by  many  closely  spaced  smaller  fractures,  doubtless  of  the  same 
age.  They  strike  about  N.  20°  W.  Many  of  them  play  out 
toward  the  southeast  or  join  other  fractures.  They  form  alto- 
gether a  network  having  what  Sales  has  designated  "horse- 


360         THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


COPPER 


361 


362      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tail"  structure.  In  some  mines  the  most  valuable  ore  bodies  are 
along  these  northwest  conjugated  fractures.  They  are  well  de- 
veloped in  the  Tramway,  Leonard,  and  West  Colusa  mines,  where 
large  ore  bodies  are  stoped  from  them.  North  of  the  copper- 
bearing  area  are  the  great  easterly  silver  and  zinc  lodes.  These 
also  are  believed  to  belong  to  the  Anaconda  system  of  fissures. 

2.  The  Blue  system  is  composed  of  several  fissures  that  strike 
northwest.     These  cross  and  fault  the  veins  of  the  Anaconda 
system.     Members  of  the  Blue  system  include  the  Clear  Grit, 
Blue,  Diamond,  High  Ore,  South  Bell,  Skyrme,  Edith  May, 
and  other  veins.     These  veins   are   spaced  with  considerable 
regularity  and  show  regular  strike  lines,  although  they  differ 
greatly  in  dip.     The  prevailing  dip,  however,  is  to  the  south- 
west.    The  fissures  are  faults,  many  of  which  have  displacements 
of  150  to  300  feet.     The  grooves  in  the  fault  planes  are  generally 
rather  flat-lying,  and  the  displacements  are  nearly  horizontal 
shifts  to  the  northwest  on  the  northeast  sides  of  the  fault  fissures. 
Although  the  movements  of  the  hanging  walls  have  downward 
components  as  well  as  horizontal  components,  owing  to  the  larger 
amount  of  horizontal  movement  the  faults  appear  on  sections 
as  "reverse"  faults.     The  lodes  of  the  Blue  vein  system  carry 
large  deposits,  although  they  are  much  less  valuable  and  less 
uniformly  mineralized  than  the  easterly  veins,  on  which  the 
deposits   are   almost    continuous   except   where    displaced    by 
faults. 

3.  The   Mountain   View   breccia  faults,   which   are   notably 
developed  in  the  Mountain  View,  Leonard,  and  Gagnon  mines, 
are  persistent  fissures  filled  with  angular  or  rounded  fragments 
of  country  rock  and  of  earlier  veins.     They  strike  about  N.  75° 
E.     Near  veins  they  contain  locally  enough  brecciated  ore  to  be 
worked.     They  are  later  than  the  deposits  of  the  Blue  vein 
system. 

4.  The  Steward  system  includes  fault  fissures  that  strike  about 
N.  65°  E.,  dip  about  65°  S.,  and  extend  across  the  Butte  district. 
They  include  the  Rob  Roy,  Mollie  Murphy,  Steward,  Modoc, 
and  Poser  fissures.     The  Steward  fissures  are  in  general  planes 
of  movement.     Displacements  range  from  50  feet  or  less  to  150 
feet.     As  a  rule  the  hanging  wall  moved  downward  in  these  faults 
at  an  angle  of  70°  with  the  strike.     Few  of  them  carry  much  ore. 

5.  The  Rams  fault  is  a  complex  fissure  that  is  later  than  the 
Anaconda,  Blue,  and  Mountain  View  systems.     It  is  known 


COPPER  363 

also  to  be  later  than  some  of  the  veins  of  the  Steward  system. 
It  strikes  northeast  and  dips  45°  N W.  It  is  a  broad  crushed  zone 
from  20  to  250  feet  wide  and  is  limited  by  tabular  masses  of 
fault  gouge  from  1  to  8  inches  wide.  The  displacement  is  not 
uniform.  In  the  northeastern  part  of  the  district  it  is  small;  in 
the  southwestern  part  it  is  as  much  as  350  feet.  The  hanging 
wall  at  some  places  moved  downward  on  the  dip,  but  in  the 
Leonard  mine  the  movement  was  at  angles  of  60°  with  the 
strike  or  less.  The  ore  in  the  Rarus  fault  is  drag  ore  from 
older  veins. 

6.  The  Middle  faults,  as  developed  in  the  Mountain  View 
mines,  represent  a  period  of  movement  later  than  the  Rarus. 
They  are  not  mineralized. 

7.  The  Continental  fault,  which  is  on  the  east  edge  of  the 
mineralized  area,  is  likewise  later  than  the  metallization  of  the 
district. 

The  relations-  of  the  fracture  systems  to  one  another  are 
stated  above.  The  earliest  or  easterly  fracturing  followed  the 
consolidation  of  aplite  and  quartz  porphyry.  The  rhyolitic 
intrusion  was  subsequent  to  the  earlier  vein  fissures;  the  silver 
veins  are  cut  off,  in  places  cut  in  two,  by  intrusive  dikes  of 
rhyolite.  Hydrothermal  alteration  of  the  quartz  monzonite  is 
extensive.  Where  large  veins  are  closely  spaced  the  entire  area 
of  quartz  monzonite  is  hydrothermally  altered;  where  the  veins 
are  less  closely  spaced  fresh  rock  is  found  between  them.  Kirk 
recognizes  two  phases  of  alteration — an  earlier  chloritic  phase 
and  a  later  sericitic  phase. 

The  veins  are  replacement  deposits,  and,  according  to  Sales, 
60  to  80  per  cent,  of  the  ore  is  altered  quartz  monzonite  with 
disseminated  sulphides.  The  ores  are  of  three  classes — copper, 
siliceous  silver,  and  zinc.  The  copper  ores  contain  a  little  silver; 
the  silver  ores  rarely  contain  much  copper;  both  copper  and 
silver  ores  contain  a  little  gold,  and  the  high-grade  silver  ores 
contain  it  in  notable  amounts. 

Chalcocite,  enargite,  and  bornite  are  the  most  common  copper 
minerals.  Covellite  occurs  in  large  amounts  in  the  Leonard  mine 
and  in  small  amounts  in  others.  Chalcopyrite  is  present  in 
workable  quantities  in  a  few  properties  but  is  an  insignificant 
part  of  the  total  copper-ore  output.  Tetrahedrite  is  found  in 
the  deep  workings  of  a  few  mines.  Chalcanthite  is  common  in 
the  old  workings.  Pyrite  is  the  most  common  sulphide.  It  is 


364      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

estimated  by  Weed  that  since  the  beginning  of  mining  about 
75  per  cent,  of  the  copper  produced  has  come  from  glance,  20 
per  cent,  from  enargite,  4  per  cent,  from  bornite,  0.5  per  cent, 
from  covellite,  and  0.5  per  cent,  from  chalcopyrite. 

Native  silver  occurs  in  the  copper  ores,  especially  in  those  from 
the  upper  levels.  Ruby  silver  and  indeterminable  black  sulph- 
antimonites  and  sulpharsenides  occur  in  the  siliceous  silver  ores. 
Free  gold  is  rare  but  occurs  in  some  silver  ores.  The  gangue 
minerals  include  quartz,  sericite,  and  several  residual  minerals 
of  the  altered  country  rock.  Much  rhodonite  and  rhodochrosite 
and  some  fluorite  occur  in  the  silver  and  zinc  ores. 

Sales  states  that  there  is  a  central  zone  of  copper  ore,  mainly 
chalcocite  and  enargite,  which  grades  into  an  intermediate  zone 
that  contains  ores  with  the  same  minerals  and  also  sphalerite, 
rhodochrosite,  and  rhodonite,  with  a  slight  increase  of  silver 
content.  In  an  outer  or  peripheral  zone  the  ores  carry  sphalerite, 
rhodonite,  rhodochrosite,  tetrahedrite,  tennantite,  and  chalco- 
pyrite, but  rarely  chalcocite  or  bornite.  Their  chief  metals  are 
silver,  gold,  zinc,  and  some  lead. 

The  oldest  lodes,  including  the  Parrot,  Anaconda,  and  Syndi- 
cate, occupy  openings  along  which  there  was  but  slight  tangential, 
movement.  They  have  been  fractured,  however,  since  the  ore 
was  deposited.  The  ore  minerals  in  these  lodes  consist  chiefly 
of  pyrite,  chalcopyrite,  chalcocite,  and  covellite.  According  to 
Sales  they  contain  some  enargite  also. 

The  veins  of  the  Blue  system  contain  the  minerals  named  as 
constituents  of  the  earlier  veins  and  large  quantities  of  enargite. 
Along  these  veins  evidence  of  movement  parallel  to  the  planes 
of  the  deposits  is  pronounced.  Although  the  mineral  composi- 
tion of  the  veins  is  comparatively  uniform,  the  later  fissures 
are  characterized  by  barren  patches  separating  rich  ore  shoots. 
These  ore  shoots,  as  shown  by  Sales,  are  of  primary  origin,  the 
course  of  the  mineralizing  solutions  having  been  determined  by 
fault  gouge,  which  effectively  dammed  back  the  waters  from  the 
portions  of  the  fissures  that  are  barren. 

Some  silver  lodes  crop  out  conspicuously,  but  the  outcrops  of 
copper  lodes  are  not  prominent.  The  leached  zone  extends 
downward  in  places  300  or  400  feet  below  the  surface.  It  con- 
tains silver,  locally  30  ounces  or  more  to  the  ton,  but  little  copper. 
Below  the  oxidized  zones  of  copper  lodes,  grading  into  them  locally 
within  2  or  3  feet,  are  enormous  masses  of  chalcocite,  with  some 


COPPER  365 

bornite  and  covellite.  This  ore  carries  in  general  2  or  3  ounces 
of  silver  to  the  ton. 

In  the  great  ore  bodies  of  the  upper  levels  of  the  Anaconda 
veins  chalcocite  occurred  in  nearly  pure  masses  20  feet  or  more 
wide.  In  depth  the  mineral  shows  a  more  crystalline  structure, 
and  it  is  found  in  all  the  mines  in  greater  or  less  abundance  and 
purity,  but  as  a  rule  it  forms  small  grains  scattered  through  the 
ores.  The  chalcocite  ores  are  present  in  large  quantities  also 
between  the  2,000-  and  3,000-foot  levels. 

Emmons,  Weed,  and  many  others  who  studied  the  copper 
lodes  in  the  earlier  stages  of  their  development  regarded  the 
chalcocite  ores  as  secondary  deposits  formed  by  descending 
waters.  More  recent  investigations,  including  those  of  R.  H. 
Sales  and  his  associates,  have  shown  that  the  deeper  chalcocite 
ores  are  primary. 

Bingham,  Utah. — The  Bingham  district,  Utah,  is  in  the  Oquirrh 
Range  about  20  miles  southwest  of  Salt  Lake  City.  In  1915 
the  district  produced  165,000,000  pounds  of  copper.  Its 
measured  reserves  of  copper  ore  are  probably  the  largest  in  the 
United  States.  It  has  produced  also  large  amounts  of  silver, 
lead,  and  gold. 

The  Bingham  district1  is  an  area  of  Carboniferous  quartzites 
and  limestones  intruded  by  monzonite  and  monzonitic  porphyry 
and  covered  in  part  by  andesites,  andesitic  porphyries,  and 
breccias.  The  quartzite  series  ("Bingham  quartzite")  is  several 
thousand  feet  thick.  It  contains  at  least  seven  limestone  lenses, 
some  of  them  300  feet  thick. 

The  region  is  crossed  by  many  faults  and  fissures  which  trend 
in  all  directions.  The  faults  are  both  normal  and  reverse,  and 
some  carry  ore.  Extensive  fissuring  and  some  faulting  has  taken 
place  also  after  the  deposition  of  the  ores. 

The  ore  deposits  are  in  or  near  the  intrusive  monzonite  or 
monzonitic  porphyry.  They  include  fissure  veins  in  several 
formations,  bedding-plane  replacement  deposits  in  limestone, 
and  disseminated  deposits  in  shattered  porphyry.  Boutwell 

1  BOUTWELL,  J.  M.:  Economic  Geology  of  the  Bingham  Mining  District, 
Utah.  U.  S.  Geol.  Survey  Prof.  Paper  38,  1905. 

BEESON,  J.  J.:  The  Disseminated  Copper  Ores  of  Bingham  Canyon, 
Utah.  Am.  Inst.  Min.  Eng.  Bull.  107,  pp.  2191-2236,  1916. 

ATWOOD,  W.  W. :  The  Physiographic  Conditions  at  Butte,  Montana, 
and  at  Bingham  Canyon,  Utah,  when  the  Copper  Ores  in  These  Districts 
were  Enriched.  Econ.  Geol.,  vol.  11,  pp.  697-741,  1916. 


366      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

recognized  two  periods  of  mineralization — one  during  the  in- 
trusion of  the  monzonite  porphyry,  and  a  second  closely  follow- 
ing the  solidification  and  fracturing  of  the  porphyry.  The  ore 
bodies  that  were  most  productive  in  the  earlier  history  of  the 
district  are  large  replacement  deposits  of  sulphide  ore  in  lime- 
stone. These  ores  consist  chiefly  of  pyrite,  chalcopyrite,  sphal- 
erite, and  chalcocite,  with  a  little  bornite  and  enargite  and 
oxidation  products.  In  many  of  the  deposits  a  little  pyrrhotite 
is  present,  and  in  some  ore  from  the  Highland  Boy  mine  pyrrhotite 
is  abundant. 

On  the  borders  of  the  great  ore  body  of  the  Highland  Boy, 
which  partly  replaces  a  limestone  lens,  garnet  and  specularite 
are  intergrown  with  calcite,  chalcopyrite,  and  sphalerite.  This 
ore  body  was  formed  under  contact-metamorphic  condi- 
tions; the  heavy  silicates,  however,  are  not  conspicuously  de- 
veloped. 

The  ores  of  the  Highland  Boy  mine  near  the  surface  were 
extensively  oxidized  and  carried  concentrated  gold.  The  mine 
was  first  exploited  for  gold,  but  deeper  developments  disclosed 
great  bodies  of  copper  ore  containing  gold  and  silver.  The 
copper  ores  carry  little  chalcocite  and  are  in  the  main 
primary. 

The  argentiferous  lead  ores  are  deposits  of  galena  that  generally 
carry  a  high  content  of  silver.  They  occupy  veins  in  igneous  and 
sedimentary  rocks  and  replace  limestone. 

Another  type  of  deposit  which  in  recent  years  has  become 
highly  productive  contains  copper  ore  disseminated  in  monzonite 
porphyry.  The  principal  deposit  of  this  type  is  in  the  intrusive 
body  at  Upper  Bingham.  The  mineralized  tract  contains  a 
multitude  of  thin  unsystematized  parting  planes.  The  rock  is 
bleached,  silicified,  and  sericitized  in  and  near  the  areas  of  great 
shattering.  The  copper  content  is  lowest  in  the  oxidized  zone 
at  the  surface.  Farther  down,  in  the  unoxidized  rock,  the 
secondary  sulphide  ore  consists  of  chalcocite,  covellite,  and  chalco- 
pyrite. It  lies  in  scales  and  films  in  silicified  walls  of  cracks,  and 
in  areas  of  great  shattering  it  occurs  abundantly  on  quartz- 
coated  cracks  and  is  disseminated  through  the  silicified  bleached 
walls.  Quartz,  sericite,  brown  mica,  and  some  orthoclase1  in 

1  BUTLER,  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1912,  part  1, 
p.  325,  1913. 


COPPER 


367 


this  ore  are  later  than  the  original  rock-making  minerals.  The 
deposit  lies  like  a  thick  blanket.  Below  it  the  porphyry  carries 
pyrite  and  chalcopyrite  but  is  of  too  low  grade  to  work. 

The  Utah  Copper  Co.,  which  is  exploiting  the  disseminated 
ores  of  Bingham,  has  developed  about  338,000,000  tons,  the 
average  content  of  which  is  about  1.5  per  cent,  of  copper  and 
from  20  to  30  cents  a  ton  in  gold  and  silver.  The  surface  of  this 
ore  body  covers  211  acres,  and  the  average  thickness  of  the 
workable  ore  is  probably  about  414  feet. 


METALS  PRODUCED  BY  UTAH  COPPER  Co.,  1913-1915 


Year 

Ore 

milled, 
short  tons 

Copper 
content, 
per   cent. 

.   Mill 

recovery, 
per   cent. 

Refined 
copper, 
pounds 

Gold  and 
silver  per 
pound  of 
copper, 
cents 

Cost  per 
pound  of 
copper, 
cents 

1913 

7,519,392 

1.25 

63.95 

113,942,834 

0.64 

9.498 

1914 

6,470,166 

1.42 

66.04 

115,690,445 

0.75 

8.131 

1915 

.  8,494,300 

1.43 

64.13 

148,397,006 

0.62 

7.560 

The  leached  capping,  which  is  stripped  by  steam  shovels,  has 
an  average  thickness  for  the  entire  area  of  110  feet  (see  Fig. 
67,  page  141).  The  average  depth  of  the  bottom  of  the  de- 
posit now  workable  is  about  530  feet  below  the  surface,  although 
in  places  it  extends  downward  more  than  900  feet.  In  much  of 
the  ore,  especially  in  that  of  lower  levels,  chalcopyrite  is  abundant, 
and  considerable  masses  of  the  ore  carry  very  little  chalcocite, 
chalcopyrite  and  covellite  being  the  principal  minerals.  The 
Utah  Copper  Co.  produces  annually  from  the  disseminated  copper 
ore  more  than  a  million  dollars  in  precious  metals,  about  four- 
fifths  of  which  is  gold  and  about  one-fifth  silver.  This  is  equi- 
valent to  about  22^  cents  per  ton  of  ore  and  1.07  cents  per  pound 
of  copper. 

The  Ohio  Copper  Mining  Co.  mines  ores  disseminated  in 
fractured  quartzite  near  the  Upper  Bingham  intrusive  mass. 

Bisbee,  Ariz. — The  Bisbee  (Warren)  district,  Arizona,  is  situated 
in  the  Mule  Mountains,  a  low  range  in  the  southeast  corner  of 
the  State. 

Production  in  this  district  began  in  1880,  and  to  the  end  of 
1915  it  had  yielded  2,025,860,000  pounds  of  copper.  The  oldest 


368      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 
COPPER  PRODUCED  IN  BISBEE   DISTRICT,   ARIZONA,    1913-1915° 


Year 

Quantity, 
pounds 

Average  yield 
per  ton  in  copper, 
per  cent. 

Value  in  gold 
and  silver  per  ton 

1913 

155  100  000 

5  6 

$1  16 

1914  

151,600,000 

5.4 

1.35 

1915  

164,600,000 

5.0 

1.66 

BDTLEK,  B.  S.:  U.  S.  Geol.  Survey,  Mineral  Resources,  part  1,  p.  674,  1916. 


rocks1  of  the  district  belong  to  the  Final  schist,  of  pre-Cambrian 
age.  The  granite  mass  of  Jumper  Flat,  north  of  Bisbee,  which 
is  intruded  into  the  schist,  is  also  pre-Cambrian.  Resting  un- 
conformably  on  the  schist  is  the  Cambrian  Bolsa  quartzite  with 
a  basal  conglomerate  and  above  the  quartzite,  are  several  lime- 
stone beds  with  an  aggregate  thickness  of  over  4,000  feet.  These 
rocks  include  the  Abrigo,  Martin  (?),  Escabrosa,  and  Naco 
limestones.  They  were  intruded  in  early  Mesozoic  time  by 
stocks,  dikes,  and  sills  of  granite  porphyry  and,  after  deep 
erosion,  were  covered  by  a  thick  series  of  Cretaceous  (Comanche) 
beds.  These  have  in  greater  part  been  eroded  from  the  productive 
area. 

The  district  is  cut  by  numerous  faults,  some  older  and  some 
younger  than  the  Cretaceous  beds.  In  some  of  the  tilted  fault 
blocks  the  strata  are  gently  folded  (Fig.  158). 

The  primary  ores  were  deposited  during  or  after  the  intrusion 
of  the  granite  porphyry  and  before  the  deposition  of  the  Cre- 
taceous beds.  Their  age  is  therefore  early  Mesozoic.  Their 


1  RANSOME,  F.  L. :  Geology  and  Ore  Deposits  of  the  Bisbee  Quadrangle, 
Arizona.  U.  S.  Geol.  Survey  Prof.  Paper  21,  1904. 

RICE,  C.  T.:  Camp  of  Bisbee,  Ariz.  Mines  and  Methods,  vol.  1,  p.  241, 
Salt  Lake  City,  Utah,  1909. 

DOUGLAS,  JAMES,  NOTMAN,  ARTHUR,  LEGRAND,  CHARLES,  LEE,  G.  B. : 
The  Copper  Queen  Mines  and  Works.  Inst.  Min.  and  Met.  Trans.,  vol. 
21,  pp.  532-590,  1913;  Eng.  and  Min.  Jour.,  Mar.  8,  1913. 

RANSOME,  F.  L.:  Notes  on  the  Bisbee  District,  Arizona.  U.  S.  Geol. 
Survey  Bull.  529,  pp.  179-182,  1913. 

JENNET,  J.  B. :  Bisbee  Porphyry  Deposits.  Eng.  and  Min.  Jour.,  vol. 
97,  p.  467,  1914. 

BONILLAS,  Y.  S.,  JENNET,  J.  B.,  and  FEUCHERE,  LEON:  Geology  of 
the  Warren  District.  Am.  Inst.  Min.  Eng.  Bull.  117,  pp.  1397-1466,  1916. 


COPPER 


369 


Final  Schist  Sedimentary  Granite  Porphyry 

Pie-Cambrian  Paleozoic  Mesozoic 


Shaft 


1  Ore  Replaces  Limestone  and  I't        a000  *«*«  aboy« 
is  Concentrated  in  this  Syncline  8ea  LeTe' 


5000  leet  above 


FIG.  158. — Sketch  and  sections  of  part  of  Bisbee  district,  Arizona. 
on  map  by  Ransome,  U.  S.  Geol.  Survey.) 


370      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

deposition  was  connected  with  contact  metamorphism  of  the  lime- 
stones, shown  by  the  development  of  tremolite,  diopside,  garnet, 
and  other  silicates,  generally  in  crystals  of  microscopic  size.  The 
ore  bodies  first  worked  are  those  in  the  Carboniferous  and  De- 
vonian limestones,  but  recent  developments  have  discovered  lentic- 
ular masses  of  ore  in  the  Cambrian  limestone,  and  disseminated 
ore  has  been  found  in  the  porphyry. 

According  to  Ransome  the  larger  structural  features  to  which 
the  occurrence  of  the  ores  is  related  are  (1)  the  Dividend  fault, 
which  has  brought  Paleozoic  beds  on  the  southwest  side  of  the 
fissure  against  Final  schist  on  the  northeast  side;  (2)  a  small 
stock  of  granite  porphyry  intruded  on  the  line  of  the  fault  and 
invading  the  contiguous  schist  and  Paleozoic  beds;  (3)  an  open 
syncline  in  the  down-faulted  Paleozoic  beds,  which  dip  in  part 
toward  the  porphyry  stock  and  in  conjunction  with  the  fault 
plane  form  a  trough,  pitching  to  the  southeast;  and  (4)  a  gentle 
tilt  to  the  southeast,  as  shown  by  the  present  slope  of  the  pre- 
Comanche  erosion  surface.  The  ore  bodies  occur  in  the  down- 
faulted  fragment  of  a  syncline,  are  disposed  in  roughly  semi- 
circular fashion  around  the  porphyry  stock,  and  have  radial 
prolongations  along  certain  zones  of  fissuring. 

Though  very  irregular  in  form,  the  ore  masses  as  a  rule  are 
roughly  lenticular  and  tend  to  conform  with  the  bedding  of  the 
limestone.  Their  average  thickness  is  about  33  feet.  The  shape 
and  position  of  many  of  them,  however,  are  determined  by  zones 
of  fissuring  and  by  the  form  of  intruded  porphyry  masses,  many 
of  which  do  not  extend  to  the  surface.  The  principal  ore  bodies 
were  originally  deposited  by  replacement  of  the  limestones.  In 
many  of  the  ore  bodies  in  the  Abrigo  limestone  the  positions  of 
former  bedding  planes  are  clearly  shown  by  a  banding  in  the 
ore. 

Up  to  1904  nearly  all  the  copper  obtained  at  Bisbee  came  from 
oxidized  or  enriched  ore.  Of  late  years,  however,  primary  ores 
have  been  worked.  The  most  abundant  sulphides  in  the  primary 
ore  are  pyrite  and  chalcopyrite.  With  these  may  be  associated 
considerable  bornite  and,  in  certain  ore  bodies,  magnetite.  Both 
sphalerite  and  galena  have  been  found  in  considerable  quantity 
near  the  porphyry  of  Sacramento  Hill,  but  these  minerals  are  not 
widely  distributed  in  the  copper  ores. 

In  general  in  the  northern  part  of  the  productive  area  much 
enriched  sulphide  ore  lies  above  the  original  water  level,  and 


COPPER  371 

in  the  southern  part  there  is  considerable  oxidized  material 
below  it. 

The  lower  limit  of  enrichment  is  irregular  and  ill  defined  but, 
like  the  lower  limit  of  oxidation,  is  deeper  in  the  southern  part 
of  the  productive  area  than  in  the  northern  part.  In  some  places 
sulphide  enrichment  has  worked  down  to  the  bottom  of  a  pyrite 
ore  body;  in  others  it  has  worked  around  and  under  residual 
masses  of  unenriched  pyritic  material;  and  in  parts  of  the  Briggs 
mine  large  masses  of  leached  and  oxidized  material  rest  directly 
on  unenriched,  low-grade  pyrite.  The  enriching  mineral  is 
generally  chalcocite.  In  some  loose,  friable  ore  the  chalcocite 
occurs  as  thin  shells  around  grains  of  pyrite  and  as  a  sooty 
interstitial  powder. 

The  great  depths  to  which  oxidation  and  enrichment  have 
penetrated  at  Bisbee  and  the  inclined  position  of  these  zones 
of  alteration  with  reference  to  the  present  underground  water 
level  and  their  approximate  parallelism  with  the  old  pre-Co- 
manche  erosion  surface  indicate  that  much  of  the  oxidation  and 
enrichment  were  effected  before  the  final  tilting  and  before  the 
deposition  of  the  Cretaceous  formations. 

Globe  and  Miami,  Ariz. — The  Globe  district,  in  Gila 
County,  Arizona,  ranks  fifth  in  the  United  States  (1915)  in  the 
production  of  copper.  At  Miami,  6  miles  west  of  Globe,  there  are 
enormous  quantities  of  low-grade  chalcocite  ores  disseminated 
in  schist.  The  Miami  Copper  Co.  and  Inspiration  Copper  Co., 
mine  a  great  tonnage  of  these  ores,  concentrating  them  in  huge 
mills. 

The  region  contains  pre-Cambrian  crystalline  rocks,  including 
the  Final  schist  and  various  granitic  intrusives,  overlain  uncon- 
formably  by  Paleozoic  beds.1  The  section,  after  Ransome,  is 
as  follows: 


1  RANSOME,  F.  L. :  Geology  of  the  Globe  Copper  District,  Arizona.  U.  S. 
Geol.  Survey  Prof.  Paper  12,  1903;  Geology  at  Globe,  Arizona,  Min.  and 
Sci.  Press,  vol.  100,  p.  256,  1910;  Geology  of  the  Globe  District,  Arizona, 
Idem,  vol.  102,  p.  747,  1911. 

TOLMAN,  C.  F.,  JR.:  Min.  and  Sci.  Press,  vol.  99,  p.  646,  1909. 

RANSOME,  F.  L.:  U.  S.  Geol.  Survey  Bull.  529,  pp.  183-186,  1913. 

TOVOTE,  W.  L. :  Globe  Mining  District,  Arizona.  Min.  and  Sci.  Press, 
vol.  108,  pp.  442-449,  487-492,  1914. 

BECKETT,  P.  G.:  Water  Condition  in  the  Old  Dominion  Mine.  Am. 
Inst.  Min.  Eng.  Bull.  112,  pp.  679-710,  1916. 


372      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

PRE-MESOZOIC  STRATIGRAPHIC  COLUMN  IN  THE  GLOBE  REGION, 
ARIZONA 

Erosion  surface  Feet 

8.  Thick-bedded  gray  limestone  (Carboniferous) 1,000 

7.  Thin-bedded  limestone  (Devonian) 325 

6.  Quartzite  (Cambrian) 400 

5.  Cherty,  dolomitic  limestone  (Cambrian) 250 


4.  Dripping  Spring  quartzite 

3.  Barnes  conglomerate 

2.  Pioneer  shale 

1.  Scanlan  conglomerate 


Cambrian  or  older 


450 
10  to  55 

200 
Ito  6 


These  rocks  were  extensively  intruded,  probably  during  the 
Mesozoic  era,  by  diabase,  largely  as  great  irregular  sills,  and 
also  by  certain  masses  of  granite  and  quartz  monzonite,  includ- 
ing probably  the  Schultze  granite.  All  the  rocks  mentioned 
above  were  covered  wholly  or  in  part  by  a  thick  flow  of  dacite, 
probably  in  early  Tertiary  time.  The  rocks  are  cut  by  numerous 
faults,  some  older  and  some  younger  than  the  dacite.  Normal 
faults  greatly  predominate. 

The  copper  deposits  occur  (1)  as  lodes  in  schist,  quartzite, 
limestone,  and  diabase,  which,  where  they  pass  through  or  along- 
side of  limestone,  as  in  the  Old  Dominion  mine,  may  be  connected 
with  large  replacement  bodies  in  that  rock;  (2)  as  disseminated 
deposits  of  chalcocitic  ore  in  the  Final  schist  near  the  Schultze 
granite  (Miami  and  Inspiration  mines) ;  (3)  as  secondary  deposits 
of  chrysocolla  in  dacite  tuff  or  in  fissures. 

The  primary  ore  of  the  lodes  consists  essentially  of  pyrite  and 
chalcopyrite,  with  some  bornite  and  specularite.  Galena  and 
sphalerite  are  rather  rare.  In  the  disseminated  deposits  in 
schist  the  primary  metallic  minerals  are  pyrite  and  chalcopyrite 
with  a  little  molybdenite. 

The  deposition  of  the  disseminated  protore  in  schists  was 
connected  with  the  intrusion  of  the  Schultze  granite,  the  constit- 
uents of  the  sulphides  probably  emanating  from  the  magma 
reservoir  that  supplied  that  rock.  The  lode  ores  were  also  de- 
posited at  high  temperature  and  may  likewise  be  genetically  con- 
nected with  the  granitic  magma.  They  are,  however,  more 
closely  associated  with  the  diabase,  and  possibly  this  rock  had 
some  share  in  their  genesis. 

The  ore  bodies  in  limestone  of  the  Old  Dominion  (Fig.  159) 
and  neighboring  mines  were  large  irregular  masses  of  oxides  and 


COPPER 


373 


374      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

carbonates  associated  with  much  limonite  and  hematite.  In  the 
lodes  connected  with  these  masses  oxidized  ore  prevails  generally 
to  a  depth  of  700  to  800  feet  but  extends  much  deeper  in  the 
much  faulted  ground  under  the  Gila  conglomerate,  in  the  western 
part  of  the  mine,  where  it  is  found  on  the  1,600-foot  or  bottom 
level  about  1,200  feet  below  the  surface.  It  is  highly  probable 
that  much  of  the  oxidation  and  enrichment  at  the  Old  Dominion 
lode  was  effected  before  the  deposition  of  the  Gila  conglomerate. 

Under  the  oxidized  ore  in  the  Old  Dominion  and  adjacent  lodes, 
mainly  in  diabase,  are  large  bodies  of  ore  enriched  by  chalcocite, 
which  grade  irregularly  downward  into  pyrite  or  into  pyritic 
ore  containing  chalcopyrite,  bornite,  and  specularite.  The  Old 
Dominion  ore1  (1911)  averaged  5.84  per  cent,  copper.  Silver 
and  gold,  though  present,  are  not  as  abundant  as  in  the  ores 
replacing  limestone  at  Bingham,  Utah. 

The  disseminated  deposits  in  schists  near  Miami  form  a  chain 
of  large  ore  bodies  extending  in  a  gentle  curve  from  the  Miami 
mine  on  the  east  through  the  Inspiration,  Keystone,  and  Live 
Oak  mines  toward  the  west.  The  western  limit  of  this  ore  belt 
is  undetermined.  The  chain  as  at  present  developed  is  2  miles 
long  and  a  quarter  of  a  mile  in  maximum  width,  and  the  greatest 
thickness  of  ore  along  any  one  vertical  line  is  about  300  feet. 
Estimates  by  the  engineers  of  the  mines  give  a  total  of  80,000,000 
to  90,000,000  tons  of  ore  averaging  between  2  and  2.5  per  cent, 
of  copper.  The  ore  of  workable  grade  will  probably  amount  to 
150,000,000  tons. 

The  secondary  ore  consists  of  pyrite  and  chalcocite  in  a  siliceous 
gangue.  Some  gold  and  silver  are  present.  In  milling  this  ore 
is  concentrated  about  20  to  1,  with  a  high  extraction  of  copper. 
The  concentrates  carry  40  per  cent,  of  copper.  Apparently  the 
chalcocitization  of  iron  sulphides  has  gone  more  nearly  to  com- 
pletion at  Miami  than  in  disseminated  ores  elsewhere  in  the 
Southwest.  At  Ray  and  Morenci,  Ariz.;  Bingham,  Utah;  and 
Ely,  Nev.,  the  concentrates  carry  much  more  iron  sulphide. 

The  country  rock  of  the  Miami-Inspiration  ore  zone  is  in 
general  Final  schist,  but  good  ore  occurs  also  in  dikes  and  small 
offshoots  that  extend  into  metallized  schist  frpm  the  main  granite 
mass,  which  grades  into  porphyry  on  its  margins.  In  general  the 
unmetallized  Final  schist  is  a  bright-gray  fissile  rock,  splitting 

SUTLER,  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911.  part  1,  p. 
284,  1912. 


COPPER  375 

with  a  satiny  sheen  and  showing  considerable  variations  in  color, 
texture,  and  degree  of  metamorphism.  It  is  composed  of  quartz, 
sericite,  a  little  microcline  and  plagioclase,  magnetite,  zircon, 
tourmaline,  hornblende,  biotite,  and  chlorite.  As  a  whole,  the 
schist  was  derived  by  metamorphism  from  arkose  sedimentary 
beds,  although  it  includes  a  little  material  of  probable  igneous 
origin. 

During  the  primary  period  of  metallization  pyrite,  chalco- 
pyrite,  and  quartz  were  deposited  in  the  fractured  schists,  partly 
in  fissures  an  inch  or  so  wide  but  chiefly  in  much  smaller  cracks 
or  along  cleavage  planes.  During  the  period  of  enrichment  the 
downward-moving  cupriferous  solutions  replaced  the  chalco- 
pyrite  and  pyrite  wholly  or  in  part  by  chalcocite.  Enrichment 
probably  antedated  the  deposition  of  the  Gila  conglomerate, 
according  to  Ransome,  and  possibly  preceded  the  eruption  of 
the  dacite,  and  it  has  probably  continued  to  the  present  day. 
Where  erosion  has  overtaken  the  chalcocite  zone  enrichment  has 
apparently  been  checked  because  little  pyrite  is  available  in  the 
thoroughly  chalcocitized  schist  to  form  strongly  acid  solutions, 
and  the  copper,  instead  of  migrating  downward,  remains  near 
the  surface  as  chrysocolla  and  carbonates. 

As  a  whole,  the  disseminated  ore  bodies  form  an  irregularly 
undulating  ribbon  of  very  uneven  thickness.  The  distance  from 
the  surface  of  the  ground  to  the  top  of  the  ore  varies  widely 
from  place  to  place  and  is  not  definitely  related  to  the  present 
topography,  which  apparently  is  of  later  development  than  the 
main  period  of  enrichment.  At  the  Miami  mine  the  depth  of 
ore  is  in  general  between  200  and  400  feet,  on  the  Inspiration 
ground  it  ranges  from  50  to  600  feet,  and  on  the  Live  Oak  ground 
it  reaches  1,000  feet. 

Ray,  Ariz. — The  Ray  (Mineral  Creek)  district,1  is  near 
Kelvin,  Ariz.,  and  about  20  miles  southwest  of  Globe.  It  occupies 

1  RANSOME,  F.  L.:  Geology  of  the  Globe  District,  Arizona.  Min.  and 
Sri.  Press.,  vol.  102,  p.  747,  1911. 

WEED,  W.  H. :  The  Ray  Copper  Mining  District,  Arizona.  Min.  World, 
vol.  34,  p.  53,  1911. 

TOLMAN,  C.  F.,  JK.:  Disseminated  Chalcocite  Deposits  at  Ray,  Arizona. 
Min.  and  Sci.  Press,  vol.  99,  p.  622,  1909. 

CLIFFORD,  J.  O. :  Ray  Consolidated  Properties.  Mines  and  Methods, 
vol.  4,  p.  83,  Salt  Lake  City,  Utah. 

RANSOME,  F.  L.:  U.  S.  Geol.  Survey  Bull.  529,  p.  186,  1913,  Bvtt.  625, 
pp.  216-217,  1917. 


376      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

a  hilly  depression  that  is  drained  by  Mineral  Creek,  -between  the 
Dripping  Spring  and  Tortilla  ranges.  In  the  productive  area 
the  altitude  ranges  from  1,900  to  2,900  feet  above  sea  level,  and 
the  low  hills  are  steep  and  rugged.  The  production  of  the  dis- 
trict is  large,  and  it  is  stated  that  83,000,000  tons  of  ore,  with  an 
average  content  of  2.17  per  cent,  of  copper,  are  developed.  The 
ore  is  concentrated  to  a  product  containing  22  per  cent,  of  copper. 
The  geologic  formations  are  in  general  similar  to  those  in  the 
Globe  region,  the  Final  schist  at  Ray  being  intruded  by  two  varie- 
ties of  granite  porphyry,  one  of  which  very  closely  resembles  the 
porphyritic  facies  of  the  Schultze  granite. 


FIG.  160. — Section  of  part  of  an  ore  body  at  Ray,  Arizona.     \AfterTolman.} 

The  ore  bodies  are  mainly  in  the  schist,  although  masses  of 
granite  porphyry  within  the  generally  metallized  area  have  also 
been  converted  to  ore.  The  deposits  are  of  the  disseminated 
type;  the  siliceous,  sericitized  schist  is  sheeted,  fractured,  and 
filled  with  innumerable  closely  spaced  veinlets  of  copper  sul- 
phides, which  occur  also  in  the  schist  between  the  veinlets. 
The  protore  is  probably  connected  genetically  with  the  granite 
porphyry. 

The  ore  bodies  underlie  a  group  of  hills  stained  here  and  there 
with  copper  minerals.  The  principal  area  showing  this  altera- 
tion is  of  elongated  oval  shape  and  extends  west-northwestward 
from  Mineral  Creek  for  about  2^  miles.  Within  this  area  there 
is  a  continuous  ore  body  about  8,000  feet  long  and  2,500  feet  in 
greatest  width.  As  at  Miami,  the  layer  of  ore  has  many  irregular 
undulations  (Fig.  160)  that  apparently  have  no  relation  to  the 
present  topography.  The  average  thickness  of  the  ore  body  is 
101  feet;  of  the  overburden  250  feet.  The  depth  to  ore  ranges 
from  10  to  300  feet  and  the  thickness  of  the  ore  from  a  thin  film 
to  400  feet. 

Morenci,  Ariz. — The  Morenci  district,  in  eastern  Arizona, 
to  the  end  of  1915,  has  produced  1,221,296,000  pounds  of  copper, 


COPPER  377 

ranking  fourth  among  the  copper  districts  in  the  United  States. 
Some  of  the  ore,  yielding  about  4  per  cent,  of  copper,  is  smelted 
directly;  a  larger  amount,  yielding  about  2  per  cent.,  is  con- 
centrated. Only  a  little  gold  and  silver  is  present  in  the  ores. 
The  Morenci  district1  is  an  area  of  pre-Cambrian  granite  and 
quartzitic  schists,  above  which  rest  unconformably  about  1,500 
feet  of  Paleozoic  sandstones,  limestones,  and  shales  that  are 
locally  overlain  unconformably  by  Cretaceous  shales  and  sand- 
stones (Fig.  161) .  These  rocks  are  intruded  by  masses  of  granitic 


Tb  "  Basalt  Flows  Ol  =  Longfellow  Limestoue  Formation 

gp  =  Intrusive  Granite  Porphyry  Co  =  Corona, lo  Quartzite 

Kv=  Pinkard  Sandstone  Formation  Or  —  Pre  Cambrian  Granite 

FIG.  161. — Section   through    Morenci,    Arizona.     (After  Lindgren,    U.   S. 
Geol.  Survey.) 

and  dioritic  porphyries,  which  form  stocks,  dikes,  laccoliths,  and 
sheets.  All  these  rocks  have  been  subjected  to  uplift  and  warping 
or  doming,  succeeded  by  much  faulting  during  latest  Cretaceous 
or  earliest  Tertiary  time.  The  domed  area  of  older  rocks  is 
framed  by  great  masses  of  Tertiary  basalt  and  rhyolite,  with 
some  andesite.  Active  erosion  has  taken  place  since  early  Ter- 
tiary time,  removing  the  covering  of  lavas.  Some  of  the  material 
eroded  was  deposited  at  the  foot  of  the  mountains,  forming  the 
Quaternary  Gila  conglomerate.  Still  later  deep  canyons  were  cut 
in  the  conglomerate. 

The  ore  bodies,  as  stated  by  Lindgren,  are  veins  and  dis- 
seminated deposits  in  the  granitic  and  quartz  monzonite  por- 
phyry and  contact-metamorphic  deposits  in  the  limestone  and 
shale  near  the  porphyry.  The  veins  and  disseminated  deposits 
are  most  productive. 

The  ore  deposits  are  in  or  near  the  intruding  porphyry  and 
were  probably  formed  by  solutions  emanating  from  igneous 
bodies.  The  contact-metamorphic  deposits  have  formed  in 
Paleozoic  limestone  and  shale  near  the  porphyry;  where  the 

1LiNDGEEN,  WALDEMAR:  The  Copper  Deposits  of  the  Clifton-Morenci 
District,  Arizona.  U.  S.  Geol.  Survey  Prof.  Paper  43,  1905. 

REBER,  L.  E. :  The  mineralization  at  Clifton-Morenci.  Econ.  Geol.,  vol. 
11,  pp.  528-573,  1916. 


378      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

porphyry  cuts  granite  or  quartzite  little  change  is  noted.  By 
contact  metamorphism  pure  limestones  and  calcareous  shales 
were  changed  to  ore  consisting  of  pyrite,  chalcopyrite,  zinc  blende, 
magnetite,  garnet,  epidote,  diopside,  tremolite,  and  quartz. 
At  many  places  the  ores  were  formed  along  certain  beds  that  were 
evidently  more  favorable  for  their  development  than  others. 
In  form  these  deposits  are  irregular,  but  some  are  rudely  tabular, 
the  mineralization  having  followed  certain  beds  or  the  walls  of 
dikes.  Lindgren  considers  these  deposits  contemporaneous  with 
the  cooling  and  solidification  of  the  porphyry. 

Oxidation  of  the  contact-metamorphic  deposits  is  not  so  deep 
as  in  the  veins  in  porphyry.  Cuprite  and  copper  carbonates 
constitute  extensive  and  rich  ore  bodies  in  the  shales  and  lime- 
stones. Magnetite  and  garnet  decompose  to  limonite  and  quartz; 
sphalerite  is  removed  almost  completely.  By  oxidation  copper 
is  concentrated;  some  also  is  scattered  in  the  country  rock. 

The  veins  cut  granitic  porphyry,  and  sedimentary  rocks. 
They  are  composed  of  pyrite,  chalcopyrite,  sphalerite,  molyb- 
denite, sericite,  and  quartz.  A  few,  generally  of  small  value, 
carry  also  magnetite,  tremolite,  diopside,  and  epidote. 

The  disseminated  ores  in  porphyry  are  formed  by  filling  small 
but  closely  spaced  cracks  in  the  porphyry  and  replacing  the 
rock  nearby.  Some  of  these  deposits  are  large,  and  they 
constitute  the  mainstay  of  the  camp. 

COPPEK  PRODUCED  IN  CLIFTON-MORENCI  DISTRICT,  ARIZONA,  1913-1915° 


Year 

Quantity, 
pounds 

Average  yield  of  copper 
from  ore,  per  cent. 

1913 
1914 
1915 

70,100,000 
66,000,000 
51,096,000 

2.00 
1.95 

2.01 

"BTJTLEB,  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  682,  1916. 

The  veins  and  disseminated  deposits  owe  much  of  their  worka- 
ble ore  to  processes  of  enrichment.  Near  the  surface  they  are 
oxidized  and  generally  leached  of  copper.  Not  all  are  marked 
by  heavy  gossans,  and  in  the  outcrops  of  some  there  is  but  little 
iron.  Below  the  leached  zone  is  a  zone  of  chalcocite  ore  in  which 
the  copper  sulphide  replaces  pyrite  and  zinc  blende,  below  the 
chalcocite  ore  the  primary  sulphides  are  found  including  pyrite, 
chalcopyrite  and  sphalerite. 


COPPER 


379 


Ely,  Nev.— The  Ely  (Robinson)  district,  in  White  Pine 
County,  eastern  Nevada,  has  long  been  known  as  a  metallized 
area  but  did  not  begin  active  production  of  copper  until  1908. 
The  barren  capping  is  removed  and  the  ore  is  mined  by  steam 
shovels  and  concentrated  about  11  to  1.  About  10,000  tons  of 
ore  are  treated  daily.  The  Nevada  Consolidated  Copper  Co.  is 
the  principal  operator  in  the  district. 


METALS  PRODUCED  BY  NEVADA  CONSOLIDATED  COPPER  Co.,  ELY,  NEV., 
1912-1915- 


Year 

Ore  treated, 
short   tons 

Copper, 
per  cent. 

Extraction 
of  copper, 
per  cent. 

Gold   and 
silver  recov- 
ered per  ton, 
cents 

Copper 
produced, 
pounds 

Cost    of 
copper  per 
pound, 
cents 

1913 

3,139,137 

1.599 

68.52 

12.32 

64,972,829 

9.99 

1914 

2,640,294 

1.483 

68.48 

13.45 

49,244,056 

10.16 

1915 

3,081,520 

1.540 

70.18 

18.56 

62,726,651 

8.67 

0  BUTLER.  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  706,  1916. 

The  Ely  district1  is  an  area  of  folded  and  faulted  Paleozoic 
limestones  and  shales  ranging  in  age  from  Silurian  to  Carbon- 
iferous. These  formations  are  intruded  by  monzonite  porphyry 
along  an  east-west  zone  about  9  miles  long  and  from  J^  to  1 
mile  wide.  These  rocks  are  locally  overlain  by  rhyolite  flows 
of  Tertiary  age.  The  sedimentary  rocks  near  the  porphyry 
intrusions  are  locally  garnetized  or  changed  to  jasperoid  and 
commonly  charged  with  great  quantities  of  pyrite.  In  places 
near  the  igneous  masses  considerable  amounts  of  chalcopyrite 
occur  with  the  pyrite,  and  some  sphalerite  is  present.  Galena 
and  its  oxidation  products  occur  in  irregular  lodes  within  the 
metamorphic  area,  principally  at  some  distance  from  the 
porphyry  masses.  Gold  ores  with  lead  carbonate  occur  mainly 
as  blanket  lodes. 

Of  many  superficial  showings  of  copper  carbonates  none  have 
been  developed  profitably,  but  oxidized  ores  of  relatively  high 
grade  have  been  discovered  in  the  Alpha  mine,  at  considerable 

1  LAWSON,  A.  C. :  The  Copper  Deposits  of  the  Robinson  Mining  District, 
Nevada.  Cal.  Univ.,  Dept.  Geology  Bull,  vol.  4,  No.  14,  pp.  287-357, 1906. 

SPENCER,  A.  C.:  Preliminary  Geologic  Map  of  the  Vicinity  of  Ely, 
Nev  U.  S.  Geol.  Survey,  1912.  U.  S.  Geol.  Survey  Bull.  529,  pp.  189- 
191,  1913;  Bull  625,  pp.  219-221,  1917. 


380      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


depth.  This  ore  body  is  inclosed  by  metamorphosed  and  thor- 
oughly oxidized  sedimentary  rocks  that  lie  several  hundred  feet 
from  the  nearest  porphyry. 

The  principal  deposits  are  low-grade  disseminated  ores  in  the 
porphyry  (Fig.  162).  As  stated  by  Spencer,  the  porphyry  was 
locally  fractured  after  its  intrusion,  and  great  masses  of  it  were 
filled  with  veinlets  of  quartz,  carrying  pyrite  and  chalcopyrite. 


Limestone 


^/^^ 

,->.!"V.'  v"  /; 'Primary  Sulphidej-'s  ," 


Fia.  162. — Map  and  cross-section  of  disseminated  deposit  in  porphyry   at 
Ely,  Nevada.     (Based  on  maps  by  Nevada  Consolidated  Copper  Co.) 

Near  the  fractures  the  porphyry  was  charged  with  sulphides  by 
replacement,  and  the  rock  was  greatly  altered,  lime,  magnesia, 
soda,  and  iron  being  removed  and  considerable  potash  being 
added.  These  hydrothermal  changes  involved  the  destruc- 
tion of  hornblende  and  lime-soda  feldspar  and  the  formation 
of  brown  mica  and  sericite.  Some  fluorite  is  present.  The  out- 
crops of  the  ore  masses  are  said  to  carry  not  over  0.5  per  cent, 
of  copper.  There  is  an  abrupt  change  from  this  capping  to  soft 
blue-white  porphyry  ore,  which  carries  disseminated  sulphide 
minerals,  including  copper  glance,  as  films  coating  grains  of 
pyrite  and  chalcopyrite,  or,  less  commonly,  completely  replacing 
such  grains. 


COPPER  381 

The  Nevada  Consolidated  Co.'s  ore  bodies  have  a  capping  that 
averages  102.5  feet  in  thickness,  below  which  lies  the  chalcocite 
zone,  which  has  an  average  thickness  of  218  feet.  This  company 
has  developed  more  than  50,000,000  tons  of  the  disseminated  ore 
averaging  1.7  per  cent,  of  copper.  The  profitable  ore  is  known 
to  extend  locally  to  depths  of  about  600  feet.  A  hole  was  put 
down  nearly  400  feet  below  the  ore  body,  in  material  which  carried 
less  than  0.4  per  cent,  of  copper.1 

A  composite  analysis  of  1,000  samples  of  ore  from  the  Ruth 
mine,  stated  by  A.  C.  Lawson,  may  be  calculated  as  equivalent 
to  pyrite,  10  per  cent.;  chalcopyrite,  1.8  per  cent.;  and  chalcocite, 
2  per  cent.  According  to  Spencer  any  porphyry  in  this  district 
carrying  more  than  1  per  cent,  of  copper  probably  owes  its  grade 
to  the  presence  of  chalcocite,  the  enrichment  having  resulted 
from  precipitation  of  this  mineral  out  of  solutions  derived  from 
overlying  material. 

Santa  Rita,  N.  Mex.— The  Santa  Rita  district,2  in  Grant 
County,  New  Mexico,  was  known  to  the  Spaniards  in  the  later 
part  of  the  eighteenth  century,  when  copper  was  mined  and 
carried  to  Mexico  City.  The  yield  was  small,  however,  until 
1911,  when  the  Chino  Co.  began  production  on  a  large  scale. 
The  deposits  are  of  the  disseminated  type,  but  they  differ  from 
the  disseminated  ores  of  Morenci,  Ely,  and  Bingham,  for  a  large 
proportion  of  the  copper  is  native  metal  or  in  cuprite.  The  ore 
is  mined  by  steam  shovels,  concentrated  near  the  mines,  and 
smelted  at  El  Paso,  Texas.  In  the  disseminated  ores  on  the 
Chino  ground  90,000,000  tons  of  ore  averaging  1.75  per  cent,  of 
copper  was  developed  in  1913.  Most  of  it  is  near  the  surface. 
The  average  thickness  of  the  capping  is  82  feet,  and  the  average 
thickness  of  the  ore  below  the  capping  is  107  feet. 

The  country  (Fig.  163)  comprises  an  area  of  Paleozoic  quartz- 
ites  and  limestones,  unconformably  above  which  lie  Cretaceous 
quartzites  and  schists.  Above  the  Cretaceous  rocks,  also  un- 
conformably, are  Tertiary  lavas  and  tuffs.  The  Cretaceous 

1  Nevada  Consolidated  Copper  Co.,  Fifth  Ann.  Rept.,  for  15  months  ended 
Dec.  31,  1911,  p.  8. 

2  PAIGE,  SIDNEY:  The  Geologic  and  Structural  Relations  of  Santa  Rita 
(Chino),  N.  Mex.     Econ.  Geol,   vol.  7,  p.  547,  1913;   U.  S.  Geol.  Survey 
Geol.  Atlas,  Silver  City  folio  (No.  199),  1916. 

LINDGBEN,  WALDEMAR,  GRATON,  L.  C.,  and  GORDON,  C.  H.:  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Prof.  Paper  68,  p.  305,  1910. 


382      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


and  older  rocks  are  intruded  by  diorite  and  associated  igneous 
rocks,  by  quartz  diorite  porphyry,  and  by  quartz  monzonite 
and  quartz  monzonite  porphyry.  Subsequently,  in  Tertiary 
time,  the  rhyolite  flows  and  intrusions  and  basalt  flows  were 
formed.  Folding  attended  the  intrusions  of  diorite,  and  faulting 


Legend 

1  Silurian  Limestone 

2  Devonian  Shale 

3  Carboniferous  Limestone 

4  Cretaceous  Quartzite 

5  Cretaceous  Shale 
G  Rhyolite  Lara 

7  Tuft  and  Sand 

8  Andesltic  and  Basaltic 
Lava 

0  Quartz  Diorite  Porphory 

10  Grano-Diorite  (Quartz 
onI0nite  Porphory) 

U  Undiffercntiated  Grano- 
Diorite  ami  Qnartz  Diorite 
Porphory  at  Santa  Etta 

12  Metamorphosed 
Limestone 

13  Diorite-Andesite-Breccia 
Complex 

F  Fault 


FIG.  163.  —  Map  of  Santa  Rita  district,  New  Mexico.  1,  Silurian  lime- 
stone^, Devonian  shale;  3,  Carboniferous  limestone  ;  4,  Cretaceous  quartzite 
5,  cretaceous  shale;  6,  rhyolite  lava;  7,  tuff  and  sand;  8,  andesitic  and 
basaltic  lava;  9,  quartz  diorite  porphyry;  10,  granodiorite  (quartz  monzonite 
porphyry;  11,  undifferentiated  granodiorite  and  quartz  diorite  porphyry  at 
Santa  Rita;  12,  metamorphosed  limestone;  13,  diorite  andesite  breccia  com- 
plex; F,  fault.  (After  Paige.) 


followed  the  extravasation  of  Tertiary  lavas.  Finally,  Quater- 
nary sediments  with  interbedded  basalts  were  deposited.  Zones 
of  garnet  are  extensively  developed  in  limestone  and,  according 
to  Paige,  the  quartz  monzonite  porphyry  has  greatly  metamor- 
phosed the  quartz  diorite  porphyry  intrusives  by  the  develop- 
ment of  sericite,  epidote,  and  quartz  and  the  recrystallization  of 
the  iron  in  the  dark  silicates  to  form  sulphides.  The  quartz 
monzonite  porphyry  carries  most  of  the  disseminated  ore.  It  is 
highly  sericitized  and  contains  sulphides  and  secondary  silica. 
By  oxidation  iron  oxides  and  kaolin  have  been  formed.  The 
primary  mineralization  followed  the  deposition  of  the  Cretaceous 
quartzite  and  preceded  the  extrusion  of  the  Tertiary  lavas. 
According  to  Paige  the  country  was  peneplained  after  the  ore 


COPPER 


383 


was  deposited,  before  Tertiary  time,  and  probably  enrichment 
was  then  well  advanced. 

At  Santa  Rita  most  of  the  copper  occurs  as  native  metal,  oxide, 
or  sulphide  in  altered  porphyry.  Chalcocite  with  kernels  of 
pyrite  is  disseminated  both  in  the  quartzite  and  in  the  porphyry, 
and  much  of  the  chalcocite  has  doubtless  replaced  pyrite.  Out- 
side the  bodies  of  copper  ore  pyrite  is  distributed  similarly  to  the 
chalcocite  within  the  ore  bodies.  Primary  sulphides  have  altered 
to  chalcocite  and  chalcocite  to  oxide  and  native  copper. 

METALS  PRODUCED  BY  CHINO  COPPER  Co.,  1913-1915° 


Year 

Ore    milled, 
short    tons 

Copper 
content, 
per  cent. 

Mill  recov- 
ery, 
per  cent. 

Production 
of  refined 
copper, 
pounds 

Cost  of  cop- 
per per  pound, 
cents 

1913 

1,942,700 

2.033 

67.310 

50,511,661 

8.77 

1914 

1,907,300 

2.115 

67.858 

53,999,928 

7.60 

1915 

2,379,800 

2.155 

66,588 

64,887,788 

7.12 

BUTLER,  B.  S.:    U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  103,  1916. 


Burro  Mountain  District,  N.  Mex.  —  The  Burro  Mountain 
region,1  in  Grant  County,  New  Mexico,  is  arid  and  stands  5,000 
to  8,000  feet  above  sea  level.  The  main  mass  of  the  mountains 
consists  of  pre-Cambrian  granite  and  pegmatite  and  dioritic 
rock,  which  is  locally  gneissic .  These  rocks  are  intruded  by  quartz 
porphyry  and  quartz  monzonite  porphyry.  Remnants  of 
volcanic  breccia  cap  the  older  intrusive  rocks. 

The  monzonite  porphyry  and  quartz  porphyry  are  locally 
fractured  and  are  traversed  by  innumerable  small  joints  and 
fissures.  The  fracture  zone  in  the  principal  mineralized  area  is 
believed  by  Somers  to  have  been  formed  by  stresses  that  attended 
the  extrusion  of  lavas;  that  it  is  not  caused  by  contraction  of  the 

1  LINDGREN,  WALDEMAR,  GRATON,  L.  C.,  and  GORDON,  C.  H. :  The  Ore 
Deposits  of  New  Mexico.  U.  S.  Geol.  Survey  Prof.  Paper  68,  pp.  322-323, 
1910. 

LANG,  S.  S. :  The  Burro  Mountain  Copper  District.  Eng.  and  Min.  Jour., 
vol.  82,  p.  395,  1906. 

PAIGE,  SIDNEY:  Metalliferous  Ore  Deposits  near  the  Burro  Mountains, 
Grant  County,  New  Mexico.  U.  S.  Geol.  Survey  Bull.  470,  p.  131,  1910. 

SOMERS,  R.  E.:  The  Burro  Mountain  Copper  District.  Am.  Inst.  Min, 
Eng.  Butt.  101,  pp.  957-996,  1915. 


384      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

monzonite  on  cooling  is  evident,  for  only  a  small  part  of  the 
monzonite  mass  is  fractured.  In  these  fractures  and  in  the  wall 
rock  between  them  much  quartz,  pyrite,  chalcopyrite,  and 
sphalerite  have  been  deposited.  Sericite  and  quartz  have  been 
developed  in  the  wall  rock  by  hydrothermal  metamorphism, 
particularly  near  the  veins.  The  ore  near  the  surface  is  leached 
of  the  sulphides;  limonite,  hematite,  chalcedony,  and  kaolin 
mark  the  outcrops.  With  these  minerals  in  the  oxidized  zone 
are  associated  some  malachite,  azurite,  and  chrysocolla  and  small 
quantities  of  cuprite  and  native  copper.  Below  the  surface  a 
zone  of  chalcocite  having  a  maximum  thickness  of  200  to  300  feet 
is  developed  over  most  of  the  mineralized  area.  The  rock  in  this 
zone  carries  about  2  or  3  per  cent,  of  copper.  In  it  chalcocite 
has  replaced  pyrite,  chalcopyrite,  and  sphalerite.  Below  the 
chalcocite  zone  is  low-grade  material  carrying  pyrite  and  finely 
divided  chalcopyrite  and  sphalerite. 


—    ***Vffi?:?jEt>          0~'    200    '    40)  '    000 !.«» 

FIG.  164. — Vertical  section  through  New  Cornelia  ore  body,  Ajo,  Arizona. 
(After  Joralemon.) 

Ajo,  Ariz. — The  Ajo  district,1  in  south-central  Arizona,  con- 
tains disseminated  copper  ores  in  porphyry,  in  which  oxida- 
tion appears  to  have  been  attended  by  little  leaching  and  chalco- 
citization.  The  most  notable  feature  of  this  region  is  an  intrusive 
mass  of  monzonite  porphyry,  which  has  domed  up  the  older 
rhyolite  beds  (Fig.  164). 

Some  rich  copper  veins  occur  in  the  porphyry  and  in  the  rhyo- 
lite, but  the  most  valuable  deposits  developed  are  in  a  mass  of 
shattered  porphyry  that  occupies  about  55  acres  and  has  a  maxi- 
mum depth  of  600  feet,  carrying  about  12,000,000  tons  of  car- 
bonate ore,  below  which  lies  about  28,000,000  tons  of  sulphide  ore. 
Unlike  the  disseminated  deposits  at  Bingham  and  Ely,  in  which 
the  copper  ore  is  largely  chalcocite,  the  disseminated  ores  in  the 
Ajo  district  are  chalcopyrite  and  bornite.  The  porphyry  is 

JORALEMON,  I.  B.:  The  Ajo  Copper-Mining  District.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  49,  p.  593,  1915. 


COPPER  385 

highly  shattered.  Along  some  of  the  larger  fractures  the  seams 
of  bornite  and  chalcopyrite  widen  to  veins  an  inch  or  more 
across.  Where  several  of  these  veins  are  parallel  and  closely 
spaced  there  are  bands  of  ore,  10  to  100  feet  wide,  assaying 
from  3  to  5  per  cent,  of  copper.  The  grade  of  the  ore  in 
general  is  variable,  changing  abruptly  from  porphyry  containing 
less  than  0.5  per  cent,  of  copper  to  ore  assaying  over  3  per  cent. 
There  are  excellent  examples  of  the  replacement  of  bornite  by 
chalcocite.1 

The  outcrop  and  oxidized  zone  consist  of  silicified  monzonite 
porphyry,  with  seams  and  stains  of  malachite,  limonite,  hematite, 
and  a  little  chrysocolla.  The  feldspar  is  partly  kaolinized. 
About  85  per  cent,  of  the  copper  in  the  oxidized  zone  is  said  to 
be  malachite.  Here  and  there  disseminated  chalcopyrite  and 
bornite  remain  unaltered  in  hard  ore  between  fractures,  but  such 
remnants  of  sulphides  are  insignificant  compared  with  the  great 
mass  of  carbonate  ore.  The  oxidized  ore  extends  downward 
to  an  almost  horizontal  plane  about  20  feet  below  the  deepest 
arroyos  and  150  feet  below  the  highest  hills.  This  plane  is 
approximately  the  present  ground-water  level,  and  the  transition 
from  carbonate  to  sulphide  ore  is  very  abrupt. 

Jerome,  Ariz. — The  Jerome  district2  is  in  Yavapai  County, 
east-central  Arizona,  in  a  group  of  moderately  low  and  arid  hills. 
Although  several  mines  are  being  explored,  only  two,  the  United 
Verde  and  United  Verde  Extension,  have  produced  much  ore. 
The  United  Verde  mine  was  once  worked  for  gold  but  since  1888 
has  been  a  steady  producer  of  copper,  giving  the  district  the  fifth 
rank  in  the  United  States.  In  1915  the  Jerome  district  produced 
53,260,000  pounds  of  copper.  Recent  developments  promise  an 
increased  production. 

The  United  Verde  mine  is  in  an  area  of  pre-Cambrian  schists, 
probably  faulted  upward,  and  on  the  hillside  west  of  the  mine 
the  schist  is  overlain  unconformably  by  Cambrian,  Devonian, 

1  GRATON,  L.  C.,  and  MURDOCH,  JOSEPH:  The  Sulphide  Ores  of  Copper. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  45,  p.  78,  1914. 

2  GRATON,  L.  C. :  U.  S.  Geol.  Survey  Mineral  Resources,  1907,  part  1,  p. 
597,  1908. 

RANSOME,  F.L.:  U.  S.  Geol.  Survey  Bull.  529,  p.  192,  1913;  Butt.  625,  pp. 
232-233,  1917. 

TUPPER,  C.  A.:  Mine  and  Smelter  of  the  United  Verde  Copper  Co.  Min. 
World,  vol.  42,  pp.  717-727,  1915. 

25 


386      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

and  Carboniferous  beds.  In  the  vicinity  of  the  mine,  as  de- 
scribed by  Ransome,  the  schist  stands  nearly  vertical,  with  rough 
irregular  cleavage,  and  strikes  a  little  west  of  north.  There  are 
three  varieties — (1)  a  green  rock,  schistose  on  its  margins  but 
grading  into  massive  material,  which  is  evidently  an  altered  dio- 
ritic  intrusive;  (2)  a  rough  gray  schist  containing  abundant 
phenocrysts  of  quartz,  apparently  an  altered  rhyolite;  and  (3) 
a  satiny  greenish-gray,  very  fissile  sericitic  schist 'that  may  be  a 
metamorphosed  sediment.  The  ore  occurs  in  the  gray  schist 
and  in  the  sericitic  schist,  the  main  belt  of  dioritic  rock  lying  just 
west  of  the  ore  bodies.  The  ore  is  said  to  follow  as  a  rule  the 
layers  of  fine  sericitic  schist. 

The  ore  shoot  is  oval  in  plan,  about  1,300  feet  long  horizontally 
and  700  feet  wide.  It  trends  north-northwest  and  pitches  in 
that  direction  45°.  It  has  been  worked  to  a  vertical  depth  of 
1,200  feet.  The  great  shoot  is  in  reality  a  complex  of  smaller  but 
nevertheless  large  irregular  or  lenticular  ore  bodies,  apparently 
related  in  part  to  cross  fissuring  but  showing  a  tendency  toward 
parallelism  with  the  schistosity. 

The  ore  was  deposited  in  pre-Cambrian  time.  The  chief 
mineral  is  chalcopyrite,  associated  with  which  are  pyrite  and 
sphalerite.  The  sulphides  occur  partly  in  small  irregular  frac- 
tures and  along  planes  of  schistosity,  but  to  a  large  extent  they 
have  metasomatically  replaced  the  schist,  particularly  the  fine- 
grained variety.  The  ore  contains  very  little  vein  quartz  or  other 
gangue  mineral. 

Oxidized  ore  containing  malachite,  azurite,  and  cuprite  ex- 
tended to  a  depth  of  about  400  feet  and  in  its  upper  part  was 
comparatively  rich  in  gold.  Below  the  level  of  complete  oxida- 
tion there  was  chalcocite  ore  with  a  relatively  high  proportion 
of  silver.  Recently  large  bodies  of  rich  chalcocite  ore  inclosed 
in  schist  have  been  discovered  below  the  flat-lying  Cambrian 
sedimentary  rocks  in  the  United  Verde  Extension  mine. 

Shasta  County,  Calif. — The  copper-bearing  region  of  Shasta 
County,  California,1  is  in  the  Klamath  Mountains,  near  the 

1  DILLER,  J.  S.:  U.  S.  Geol.  Survey  GeoZ.  Atlas,  Redding  folio  (No.  138), 
1906. 

GRATON,  L.  C. :  The  Occurrence  of  Copper  in  Shasta  County,  California. 
U.  S.  Geol.  Survey  Bull.  430,  pp.  71-111,  1910. 

BOYLE,  A.  C.,  JR.:  The  Geology  and  Ore  Deposits  of  the  Bully  Hill 
Mining  District,  California.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  48,  pp.  67- 
115,  1915. 

HERSHEY,  0.  H.:  The  Geology  of  Iron  Mountain.  M in.  and  Sci.  Press, 
Dot,  23,  1915,  pp.  633-638. 


COPPER  387 

head  of  Sacramento  River,  a  few  miles  north  of  Redding. 
The  ores  yield  about  3  per  cent,  of  copper  and  $1.50  to  $2  a  ton 
in  precious  metals.  In  1915  Shasta  County  produced  30,500,000 
pounds  of  copper. 

It  is  an  area  of  sedimentary  rocks  complexly  intruded  by 
several  varieties  of  consanguineous  igneous  rocks.  The  oldest 
sedimentary  series  comprises  the  Kennett  formation  (Middle 
Devonian),  which  consists  chiefly  of  black  fissile  shale,  with 
scattered  lenses  of  light-gray  limestone  and  numerous  gray  or 
yellowish  beds  of  tuffaceous  material.  Overlying  the  Kennett 
formation  unconformably  is  the  Bragdon  formation  (Mississip- 
pian),  which  consists  chiefly  of  black  and  gray  shales,  with  thin 
interbedded  layers  of  tuff  and  sandstone  and  bands  of  conglom- 
erate. Above  the  Bragdon  is  the  Pit  shale,  of  Middle  and  Upper 
Triassic  age,  consisting  chiefly  of  shales  with  interbedded  layers 
of  volcanic  tuff. 

The  oldest  igneous  rock  is  an  altered  andesite  called  by  Diller 
the  meta-andesite,  which  underlies  and  is  older  than  the  Kennett 
and  Bragdon  formations.  A  massive,  less  altered  andesite  (Dek- 
kas  andesite)  overlies  the  Bragdon  and  underlies  and  grades  into 
the  Pit.  Younger  than  all  these  rocks  and  cutting  the  latest  of 
them,  the  Pit  shale,  are  intrusives  of  soda-rich  alaskite  porphyry, 
or  soda  granite.  The  alaskite  porphyry  is  cut  by  quartz  diorite, 
which  in  turn  is  cut  by  acidic  and  basic  dikes  that  are  genetically 
very  closely  related  to  the  alaskite  porphyry  and  to  the  pegma- 
tites of  the  region.  These  pegmatites,  according  to  Graton,  in 
places  pass  over  into  siliceous  masses  that  are  virtually  quartz 
veins  and  carry  sulphides.  There  are  two  centers  of  alaskite 
porphyry,  and  each  is  a  center  of  ore  deposition. 

The  valuable  copper  deposits  consist  of  large  masses  of  pyritic 
ore,  surrounded  in  most  places  by  alaskite  porphyry  but  here  and 
there  extending  into  shale.  The  ores  are  in  part  replacement 
deposits  in  crushed  and  shattered  zones  of  alaskite  porphyry, 
which  is  highly  altered  by  sericitization  (soda  sericite).  The  ore 
bodies  are  rudely  tabular.  The  Bully  Hill  deposits,  in  the  eastern 
district,  are  steeply  pitching.  The  Shasta  King  and  Balaklala, 
in  the  western  district,  are  large  flat-lying  "lenses."  Some  of 
the  deposits  are  over  1,200  feet  long  and  300  feet  wide  (Fig.  165). 

The  deposits  are  mineralogically  simple.  Pyrite  is  the  most 
abundant  mineral,  and  chalcopyrite  is  the  chief  copper  mineral. 
Sphalerite  is  present  in  varying  amount.  Some  galena  is  associ- 


388      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

ated  with  the  zinc  sulphide,  especially  in  the  eastern  districts. 
The  gangue  minerals  are  gypsum,  calcite,  and  barite.  The  min- 
erals that  have  resulted  from  the  alteration  of  the  primary  ore 
minerals  include  limonite,  magnetite,  wad;  secondary  chalco- 
pyrite,  bornite,  and  chalcocite;  cuprite;  native  copper;  small 
amounts  of  malachite  and  azurite ;  and  several  sulphates.  The 
ores  are  believed  to  have  been  deposited  by  hot  waters  originating 
in  alaskite  porphyry.  Graton1  estimates  the  depth  at  which  the 
ores  were  deposited  at  5,000  to  6,000  feet  and  classes  them  with 
veins  of  the  middle  zone. 


Metamorphic 
Slate 

FIG.  165.— Cross-section  of  ore  bodies  at  Balaklala,  Shasta  County,  Cali- 
fornia.    (After  Weed.) 


The  secondary  zone  was  highly  productive  of  copper  and  silver 
at  the  Iron  Mountain  and  Bully  Hill  mines,  but  at  other  mines 
it  is  of  little  economic  importance.  The  secondary  ores  appear 
to  have  extended  to  no  great  depth. 

Foothill  Copper  Belt,  Calif.— The  "foothill  belt"  in  Cali- 
fornia lies  along  the  foothills  of  the  Sierra  Nevada,  extending 
northwestward  from  Copperopolis  to  a  point  near  Marysville,  a 
distance  of  more  than  100  miles.  It  includes  the  deposits  of 
Copperopolis,  Campo  Seco,  and  Dairy  Farm.  The  deposits  were 
worked  actively  in  the  sixties  and  again  in  recent  years.  The 
production  to  and  including  1915  is  about  129,400,000  pounds 
of  copper.2  The  principal  ores  are  heavy  sulphides,  pyrite  and 
.chalcopyrite.  Those  treated  in  1915  averaged  5.4  per  cent,  of 
copper,  with  about  $1.88  in  precious  metals  to  the  ton.  Sulphuric 
acid  is  made  from  the  sulphur  gases. 

1  GRATON,  L.  C.:  Op.,  cit.  p.  110. 

3  BUTLER,  B.  S. :  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p. 
692,  1916. 


COPPER  389 

The  rocks  of  this  area1  include  the  "Bedrock  series,"  mainly 
meta-andesites,  greenstones,  slates,  tuffs  and  intrusives,  meta- 
morphosed in  varying  degrees.  These  rocks  are  cut  by  horn- 
blendite,  gabbro,  granodiorite,  serpentine,  etc.  The  deposits  are 
related  genetically  to  basic  igneous  rocks  that  are  older  than 
granodiorite,  dikes  of  which  cut  the  copper  ores.  At  Copper- 
opolis,  according  to  Reid,  the  ore  occurs  in  overlapping  lenses, 
each  of  which  is  composed  of  pyrite  and  chalcopyrite,  or  as  a 
complex  series  of  veinlets  of  these  minerals,  or  as  bands  of  them 
parallel  to  the  schists.  Only  a  little  quartz  is  present  and  a  little 
ilmenite  and  sphene,  which  are  probably  alteration  products  of 
a  titaniferous  pyroxene.  Reid  states  that  the  ores  are  related 
genetically  to  the  small  dikes  of  hornblendite,  which  are  numerous 
and  which  were  found  to  carry  original  chalcopyrite.  Some 
others,  however,  maintain  that  they  have  been  formed  through 
concentration  of  copper  from  surrounding  rocks  during  the  dy- 
namic metamorphism  that  made  amphibolites  of  basic  rocks. 

At  Copperopolis  the  ore  is  capped  by  an  extensive  gossan, 
locally  rich,  30  feet  or  less  thick.  There  is  no  zone  of  sulphide 
enrichment,  a  fact  which  Reid  attributed  to  impermeability  of 
the  ores.  In  depth  the  heavy  sulphides  come  in,  either  nearly 
pure  or  with  a  varying  proportion  of  schist.  The  hypothesis 
that  these  ores  were  deposited  before  the  period  of  dynamic  meta- 
morphism has  not  been  directly  stated,  although  Reid  attributes 
their  impermeability  to  intense  lateral  pressure. 

Encampment,  Wyo. — The  Encampment  district,2  in  southern 
Wyoming  near  the  Colorado  line,  has  produced  over  20,000,000 
pounds  of  copper,  with  some  gold  and  silver.  The  country 
is  an  area  of  pre-Cambrian  metamorphosed  igneous  and  sedi- 
mentary rocks,  which  are  cut  by  pre-Cambrian  gabbro,  granite, 
and  quartz  diorite.  The  principal  mines  are  the  Doane  and  the 

1  REID,  J.   A. :  The  Ore  Deposits  of  Copperopolis,  Calaveras  County, 
California.     Econ.  Geol,  vol.  2,  pp.  380-417,  1907. 

KNOPF,  ADOLPH:  Notes  on  the  Foothill  Copper  Belt  of  the  Sierra  Nevada. 
Cal.  Univ.  Dept.  Geol.  BuU.,  vol.  4,  No.  17,  pp.  411-421,  1906. 

LONG,  HERBERT:  The  Copper  Belt  of  California.  Eng.  and  Min.  Jour., 
vol.  84,  p.  909,  1907. 

TURNER,  H.  W.:  U.  S.  Geol.  Survey  Geol.  Atlas,  Jackson  folio  (No.  11), 
1894. 

2  SPENCER,  A.  C.:  The  Copper  Deposits  of  the  Encampment  District, 
Wyoming.     U.  S.  Geol.  Survey,  Prof.  Paper,  25,  1904. 


390      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Ferris-Haggarty  (Fig.  166),  both  of  them  in  fractured  quartzite. 
The  primary  copper  ores  are  chalcopyrite  and  pyrrhotite.  The 
secondary  ores  include  azurite,  malachite,  chrysocolla,  bornite, 
chalcocite,  and  covellite. 

The  country  is  about  10,000  feet  above  the  sea;  the  climate  is 
moist,  and  ground  water  is  near  the  surface.  The  chalcocite  zone 
is  shallow,  well  denned,  and  carries  a  high  content  of  precious 
metals. 


FIG.  166. — Horizontal  section  of  Ferris-Haggarty  mine,  Encampment  dis- 
trict, Wyoming.     (After  Spencer,  U.  S.  Geol,  Survey.) 

Ducktown,  Tenn. — The  mineral  deposits  of  Ducktown  are  in 
the  southeast  corner  of  Tennessee,  near  the  North  Carolina 
line,  and  extend  southward  into  Georgia.  They  were  first 
worked  in  the  late  forties.  Besides  large  amounts  of  copper  they 
have  produced  1,500,000  tons  of  iron  ore  and  a  relatively  small 

METALS  PRODUCED  IN  DUCKTOWN  DISTRICT,  TENN.,  1912-1915" 


Year 

Ore  treated, 
short  tons 

Copper 
produced, 
pounds 

Average  yield 
of  copper, 
per  cent. 

Gold  and  silver 
per  ton  of  ore, 
cents 

1912 

.  603,229 

18,395,256 

1.530 

10.5 

1913 

652,253 

19,489,654 

1.476 

11.2 

1914 

653,621 

18,661,112 

1.435 

9.0 

1915 

623,534 

18,205,308 

1.450 

"BUTLER,  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  708,  1916. 

amount  of  silver  and  gold.  They  yield  at  present,  in  addition 
to  the  metals,  over  1,000  tons  of  sulphuric  acid  daily.  The  ore 
now  worked  carries  less  than  2  per  cent,  of  copper.  It  is  heavy 
iron  sulphide  ore,  principally  pyrrhotite,  pyrite,  and  chalcopyrite, 


COPPER  391 

with  heavy  silicates.  It  is  smelted  without  mechanical  concen- 
tration. The  first  smelting  yields  a  12  per  cent,  copper  matte, 
which  is  smelted  again  to  a  high-grade  matte  carrying  about  50 
per  cent,  of  copper.  This  is  blown  to  blister  copper  in  Bessemer 
converters,  or  shipped  without  converting.  The  furnace  gases 
are  led  to  two  great  acid  plants,  one  of  them  the  largest  in  the 
world. 

The  prevailing  rocks  of  the  district1  are  sandy  schists  and  gray- 
wackes,  with  which  are  interbedded  mica  schists.  The  dominant 
series  is  the  metamorphosed  product  of  Cambrian  sedimentary 
beds,  including  conglomerate,  grits,  sandstones,  and  shales.  The 
beds  grade  into  one  another  along  the  strike  and  across  the  bed- 
ding. They  contain  small  bodies  of  pegmatites  and  peculiar 
masses  and  stringers  of  an  actinolite-feldspar  rock  which  has  a 
composition  near  that  of  quartz  diorite.  These  are  supposed  to 
have  been  developed  by  metamorphism  from  material  of  sedi- 
mentary origin.  The  schists  are  cut  by  dikes  of  gabbro,  which 
are  not  so  highly  metamorphosed  by  pressure  as  the  sedimentary 
beds.  The  schistosity  and  the- bedding  of  the  sedimentary  rocks 
strike  nearly  everywhere  northeast,  and  the  prevailing  dip  is 
southeast.  These  rocks  have  been  folded  into  sharp  folds,  many 
of  them  isoclines.  Many  of  the  folds  were  broken  along  the 
crests  of  anticlines  and  pass  into  strike  faults  that  nearly  every- 
where dip  southeast. 

The  ore  bodies2  (Fig.  167)  are  replacements  of  limestone  lenses 
which  without  much  doubt  were  originally  deposited  at  a  single 
stratigraphic  horizon.  Anticlines  and  faulted  anticlines,  which 

1  KEITH,  ARTHUR:  U.  S.  Geol.  Survey,  Geol.  Atlas,  Nantahala  folio  (No. 
143),  1907. 

'SAFFORD,  J.  M.:  "Geology  of  Tennessee,"  pp.  469-482,  Nashville,  1869. 

WHITNEY,  J.  D. :  Remarks  on  *  *  *  the  East  Tennessee  Copper  Mines. 
Am.  Jour.  Sci.,  2d  ser.,  vol.  20,  pp.  53-57,.  1855. 

ANSTED,  D.  T. :  On  the  Copper  Lodes  of  Ducktown,  in  East  Tennessee. 
Geol.  Soc.  Quart.  Jour.,  vol.  13,  pp.  245-254,  1857. 

HEINRICH,  CARL:  The  Ducktown  Ore  Deposits.  Am.  Inst.  Miii.  Eng. 
Trans.,  vol.  25,  p.  173,  1896. 

KEMP,  J.  F.:  The  Deposits  of  Copper  Ores  at  Ducktown,  Tennessee. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  31,  pp.  244-265,  1902. 

WEED,  W.  H. :  Types  of  Copper  Deposits  in  the  Southern  United  States. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  30,  p.  480,  1901. 

EMMONS,  W.  H.,  and  LANEY,  F.  B. :  Preliminary  Report  on  the  Mineral 
Deposits  of  Ducktown,  Tenn.  U.  S.  Geol.  Survey  Bull.  470,  pp.  151- 
172,  1911. 


392      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


are  characteristic  of  this  region,  are  shown  also  in  the  ore  zone. 
The  ores  themselves  are  somewhat  metamorphosed  by  dynamic 
processes,  and  the  gangue  minerals  are  bent,  but  at  most  places 
they  do  not  exhibit  a  well-defined  schistosity.  They  were  de- 


•7*; 

/     /CULCHOTt 


C23 

Staurolitic  beds 


FIG.  167. — Sketch  showing  location  of  principal  ore  deposits  in  Ducktown 
district.     Tennessee.     Arrows  indicate  dips  of  lodes. 

posited  after  the  rocks  that  now  inclose  them  and  after  the  lime- 
stone they  replace  had  been  subjected  to  considerable  dynamic 
metamorphism.  The  outcrops  are  composed  of  iron  oxides  and 
quartz  and  contrast  strongly  with  the  country  rock. 

The  primary  ore  consists  of  pyrrhotite,  pyrite,  chalcopyrite, 


COPPER 


393 


zinc  blende,  bornite,  specularite,  magnetite,  actinolite,  calcite, 
tremolite,  quartz,  pyroxene,  garnet,  zoisite,  chlorite,  micas,  graph- 
ite, titanite,  and  feldspars.  Essentially  the  same  minerals  are 
found  in  all  the  deposits,  but  they  appear  in  varying  proportions 
at  different  places  in  the  lodes.  Where  its  content  of  copper  is 
above  1.5  per  cent.,  or  where  its  sulphur  content  is  high,  the 
material  is  ore,  but  where  the  proportion  of  actinolite  and  other 
lime  silicates  is  greater  and  the  sulphides  are  less  abundant  the 
material,  though  containing  copper  and  other  sulphides,  is  not 
workable. 

The  association  of  primary  minerals  is  one  that  is  commonly 
developed  by  contact-metamorphic  processes  or  in  the  deep-vein 


FIG.  168. — Plan   of   20-fathom   level,    East   Tennessee   mine,    Ducktown, 
Tennessee. 

zone.  The  deposits  are  not  in  contact  with  igneous  rocks  but 
are  near  intruding  gabbro,  and  a  few  miles  away  granites  are 
intruded  in  rocks  as  late  as  the  ore-bearing  series.  The  heavy 
silicates  of  the  gangue  are  in  part  contemporaneous  with  the  sul- 
phides, but  in  much  of  the  ore  pyrrhotite  and  chalcopyrite  fill 
cracks  in  the  silicates. 

Characteristic  structure  is  shown  by  Fig.  168.  It  has  resulted 
from  the  folding  of  a  limestone  bed  that  was  later  replaced  by  ore. 
The  replacing  solutions,  probably  originating  in  a  deep-seated 
magma,  deposited  sulphides  only  sparingly  in  the  inclosing  schists 
but  replaced  the  limestones  almost  completely. 

The  gossan  extends  from  the  surface  to  a  maximum  depth  of 
100  feet.  It  carries  40  to  50  per  cent,  of  iron,  generally  less  than 
12  per  cent,  of  silica  and  alumina,  and  0.3  to  0.7  per  cent,  of  cop- 


394      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY  - 

per.  The  minerals  are  hydrous  iron  oxides,  quartz,  jasper,  and 
kaolin,  with  a  little  cuprite,  native  copper,  and  sulphur.  Below 
the  gossan  iron  ores  is  a  zone  of  rich  copper  ores,  consisting  of 
chalcocite  and  other  copper  minerals  in  a  gangue  of  sulphates, 
quartz,  and  decomposed  silicates.  Under  the  higher  portions  of 
the  outcrops  of  the  lodes  the  top  of  this  zone  is  about  100  feet 
below  the  surface,  but  the  depth  decreases  down  the  slopes,  and 
where  the  lodes  are  crossed  by  running  streams  the  secondary  cop- 
per ores  are  exposed  at  the  surface.  The  upper  limit  of  the  chalco- 
cite zone  follows  the  level  of  ground  water,  which  in  turn  follows  the 
contour  of  the  country  but  is  less  accentuated  (Fig.  73,  page  164). 

The  secondary  minerals  include  chalcanthite,  chalcocite,  chal- 
copyrite,  cuprite,  gypsum,  and  iron  sulphate,  The  copper  con- 
tent of  much  of  this  ore  ranges  from  20  to  30  per  cent.  An 
analysis  of  the  gossan  and  primary  ore  is  given  on  page  165. 

Chitina  Copper  Belt,  Alaska.— The  Chitina  copper  belt,  Alaska,1 
is  an  area  of  greenstone,  mainly  diabase,  which  is  overlain  by 
Triassic  limestones.  Above  the  limestones  are  later  sedimentary 
rocks;  all  these  rocks  are  cut  by  quartz  diorite  porphyry.  Andes- 
ite  and  other  volcanic  rocks,  probably  of  Tertiary  age,  are  also 
present.  The  country  is  rugged,  and  the  region  of  the  deposits 
has  been  deeply  eroded.  The  principal  deposit  is  a  rudely  tabular 
mass  of  nearly  pure  chalcocite,  which  occurs  in  a  fractured  or  fis- 
sured zone  in  the  limestone  just  above  the  contact  with  greenstone. 
The  very  rich  ore  can  be  traced  on  the  surface  for  about  250  feet. 
The  deposit  carries  more  than  60  percent,  of  copper  and  22  ounces 
of  silver  to  the  ton  and  in  1908  was  estimated  to  contain 
100,000,000  pounds  of  copper.2  More  recently  other  very  large 
reserves  have  been  discovered  in  this  region  by  the  Kennicott 
Mining  Co. 

In  the  Bonanza  mine  the  very  rich  ore,  with  its  included  lime- 
stone, as  seen  at  the  surface,  has  a  width  of  approximately  25  feet, 
although  the  thickness  of  ore  sufficiently  rich  to  be  mined  may  be 
greater.  Below  the  deposit  a  little  chalcocite  and  bornite  are 
found  in  some  of  the  shearing  planes  in  the  greenstone. 

1  MOFFIT,  F.  H.,  and  MADDREN,  A.  G. :  Mineral  Resources  of  the  Kotsina- 
Chitina  Region,  Alaska.     U.  S.  Geol.  Survey  Bull.  374,  p.  80,  1909. 

MOFFIT,  F.  H.,  and  CAPPS,  S.  R.:  Geology  and  Mineral  Resources  of  the 
Nizina  District,  Alaska.  U.  S.  Geol.  Survey  Bull.  448,  1911. 

2  GRATON,  L.  C. :  U.  S.  Geol.  Survey  Mineral  Resources,  1907,  part  1,  p. 
592,  1908. 


COPPER  395 

Though  it  extends  to  the  very  surface  and  accumulates  in  talus 
from  the  cliff,  the  chalcocite  ore  has  no  great  vertical  range.  Ow- 
ing to  the  rapid  mechanical  disintegration  and  the  cold  climate 
little  or  no  gossan  is  developed.  Open  cavities  in  the  fractured 
limestone  have  been  filled  with  ice,  and  both  the  country  rock  and 
the  talus  are  frozen  all  summer  except  for  a  few  feet  at  the  surface. 
Tolman1  has  shown  that  in  the  Bonanza  mine  chalcocite  replaces 
bornite. 

Lake  Superior  Copper  Deposits. — The  Lake  Superior  copper 
deposits  are  in  the  Keweenawan,  the  upper  series  of  the  Algon- 
kian  of  the  Lake  Superior  region.  The  Keweenawan  series  con- 
sists of  interbedded  lava  flows,  sandstones,  and  conglomerates, 
intruded  by  acidic  and  basic  porphyries.  The  bedded  series  is 
perhaps  25,000  feet  or  more  thick.  It  borders  the  larger  part  of 
the  west  half  of  Lake  Superior  and  extends  southwestward 
through  Wisconsin  to  Taylors  Falls  and  Pine  City,  Minn.,  and 
beyond.  On  Keweenaw  Point,  Mich.  (Fig.  169),  the  series 
dips  northwest;  on  Isle  Royale  and  on  the  north  shore  of 
Lake  Superior  it  dips  southeast.  Thus  the  Keweenawan  rocks 
form  a  broad  geosyncline,  the  axis  of  which  is  in  Lake  Superior 
(see  Fig.  132,  page  298).  The  section  for  Keweenaw  Point,2  as 
described  by  Irving,  is  stated  below. 

12.  Eastern  sandstone. 
Keweenawan  series: 

11.  Red  sandstone. 

10.  Black  shale  and  gray  sandstone  ("Nonesuch  belt"). 
9.  Red  sandstone  and  conglomerate  ("Outer  conglomerate"). 
8.  Diabase  and  diabase  amygdaloid,  including  at  least  one  con- 
glomerate belt  ("Lake  Shore  trap"). 

7.  Red  sandstone  and  conglomerate  ("Great  conglomerate"). 
6.  Diabase  and  diabase  amygdaloid,  including  several  sandstone 
belts. 

1  TOLMAN,  C.  F. :  "Observations  on  Certain  Types  of  Chalcocite  and  Their 
Characteristic  Etch  Patterns.     Am.  Inst.  Min.  Eng.  Bull.  110,  pp.  401-408, 
1916. 

2  IRVING,  R.  D. :  The  Copper-Bearing  Rocks  of  Lake  Superior.     U.  S. 
Geol.  Survey  Mon.  5,  pi.  17,  1883. 

WRIGHT,  F.  E.:  The  Intrusive  Rocks  of  Mount  Bohemia,  Michigan. 
Mich.  Geol.  Survey  Ann.  Rept.  for  1908,  pp.  361-402,  1909. 

LANE,  A.  C.:  Geology  of  Keweenaw  Point.  Lake  Superior  Min.  Inst., 
Proc.,  vol.  12,  p.  93,  1907. 

RICKARD,  T.  A.:  "The  Copper  Mines  of  the  Lake  Superior  Region," 
New  York,  1905. 


396      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


COPPER 


397 


5.  Diabase  and  diabase  amygdaloid,  including  conglomerates. 

4.  Luster-mottled  melaphyres  and  coarse-grained  gabbros  and 
diabases  ("Greenstone  group"). 

3.  Diabase,  diabase  amygdaloid,  and  luster-mottled  melaphyre, 
including  a  number  of  conglomerate  beds. 

2.  Quartz  porphyry  and  felsite. 

1.  Diabase,  diabase  amygdaloid,  melaphyre,  diabase  porphyry, 
and  orthoclase  gabbro,  including  also  conglomerate  beds  and 
beds  or  areas  of  quartz  porphyry  and  granite  porphyry  ("Bo- 
hemian Range  group"). 


These  rocks  have  a  general  dip  to  the  northwest  and  are 
intruded  by  gabbro,  orthoclase  gabbro,  and  acidic  rocks.  A 
fault  strikes  northeast  along  Keweenaw  Point  near  its  center. 


COPPER  ORE  PRODUCED  IN  LAKE  SUPERIOR  DISTRICT,  MICHIGAN, 
1912-1915° 


Year 

Quantity, 
short  tons 

Yield  of  copper, 
per  cent. 

1912  

11,411,941 

0.96 

1913  

7,016,370 

0.97 

1914  

9,269,413 

0.89 

1915 

12,334,700 

1.07 

"  BUTLEB,  B.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1  p.  695,  1916. 

Southeast  of  the  fault  the  copper-bearing  beds  are  probably 
covered  by  the  nearly  horizontal  Eastern  (Cambrian)  sandstone 
(see  Fig.  169).  Northwest  of  it  the  Keweenawan  beds  dip  north- 
west at  angles  of  40°  to  20°  or  less. 

The  copper  deposits  are  in  the  main  broad  tabular  bodies 
that  strike  and  dip  with  the  beds.  The  principal  productive 
belt,  which  is  in  the  Middle  Keweenawan,  extends  from  a  point 
near  Rockland,  in  Ontonagon  County,  northeastward  to  a  point 
near  Eagle  River,  a  distance  of  about  70  miles.  Numerous 
veins  cut  across  the  lodes,  but  these  are  at  present  of  no  great 
commercial  importance.  The  copper  now  mined  occurs  as 
native  metal,  although  formerly  some  copper  was  obtained  as 
sulphide  from  the  crosscutting  veins.  The  value  of  the  ore  is 
low,  ranging  at  present  from  about  0.6  to  2  per  cent.  A  little 
native  silver  occurs  with  the  copper;  in  the  ore  now  mined  it  is 
almost  negligible. 


398      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Owing  to  the  occurrence  of  the  copper  as  the  native  metal  the 
problem  of  beneficiation  is  simple.  The  ore  is  stamped  in  huge 
steam  stamps  and  jigged  or  otherwise  concentrated  by  gravity. 
"Lake  copper"  is  of  high  grade  and  commands  a  premium  in  the 
markets. 

The  principal  gangue  minerals  are  calcite,  quartz,  chlorite 
(delessite),  prehnite,  and  laumontite,  but  considerable  quantities 
of  analcite,  orthoclase,  thomsonite,  apophyllite,  natrolite,  and 
other  zeolites  are  present,  with  many  other  minerals. 

The  copper  of  the  bedded  deposits  replaces  the  conglomerates 
and  amygdaloids,  fills  amygdules  and  other  openings  in  them, 
and  cements  small  fissures  in  the  trap  series.  At  present  the 
amygdaloidal  deposits  supply  about  twice  as  much  ore  as  the 
conglomerates.  The  latter  are  tnined  extensively  in  the  vicinity 
of  Calumet. 

The  amygdaloidal  and  conglomerate  deposits  extend  great 
distances  along  the  strike;1  the  Kearsarge  lode  is  mined  almost 
without  break  for  14  miles,  and  other  lodes  are  mined  for  2 
miles  or  more  along  their  strike.  They  have  been  followed  down 
the  dip  more  than  1^  miles,  or  about  1  mile  below  the  surface. 

Copper  is  found  at  many  stratigraphic  horizons  in  the  amyg- 
daloids. Some  of  the  lava  flows  are  100  feet  thick  or  even  thicker. 
At  some  places  the  amygdaloidal  ore  is  at  the  top,  at  others  at  the 
bottom  of  a  flow.  In  general  the  productive  amygdaloids  are 
mined  for  a  width  of  30  or  40  feet.  At  some  places  the  amygda- 
loids carry  copper  for  a  width  of  only  3  or  4  feet;  at  such  places 
they  are  generally  unprofitable. 

Although  a  number  of  conglomerate  beds  contain  small 
amounts  of  copper,  only  two  conglomerates,  the  Allouez  and  the 
Calumet,  are  workable.  One  of  these,  the  Calumet,  is  the  main- 
stay of  the  Calumet  &  Hecla  Co.,  the  greatest  copper  producer  in 
the  region.  This  conglomerate  dips  39°  NW.  at  the  surface 
and  is  followed  down  the  dip  8,100  feet,  to  a  vertical  depth  of 
4,748  feet.  A  vertical  shaft  about  a  mile  from  the  outcrop 
passes  through  the  lode  at  a  depth  of  3,287  feet.  One  of  the 
Tamarack  shafts  reaches  a  depth  of  5,229  feet. 

The  conglomerate  lode  is  13  feet  wide  at  the  surface  and  20 
feet  wide  in  the  deep  workings.  The  upper  half  of  the  bed  is 
richer  than  the  lower  half.  The  copper  content  of  this  conglom- 

1  VAN  HISE,  C.  R.,  and  LEITH,  C.  K. :  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  25,  p.  575,  1911. 


COPPER  399 

erate  was  about  4  per  cent,  near  the  surface,  but  a  mile  verti- 
cally below  the  surface  it  is  only  1  to  1.5  per  cent. 

The  genesis  of  these  deposits  is  a  vexed  problem.  They  doubt- 
less were  formed  by  hot  waters.  Lane1  has  shown  that  the  waters 
of  deep  levels  are  strong  chloride  solutions,  chiefly  calcium  and 
sodium  chlorides,  much  like  sea  water.  He  considers  them 
"connate"  or  fossil  waters  of  great  antiquity.  They  are  un- 
like the  surface  waters  of  the  mines,  a  fact  which  shows  that 
there  is  at  present  no  active  deep  circulation.  Many  zeolites 
like  those  with  which  the  ores  are  associated  have  been  formed 
synthetically  in  hot  solutions,2  but  generally  below  200°  C.  F.  E. 
Wright3  has  shown  that  copper  and  silver  will  be  alloyed  when 
formed  above  500°  C.,  and  as  these  metals  are  not  alloyed  in  the 
Lake  Superior  deposits  it  may  be  inferred  that  the  temperatures 
of  deposition  there  were  lower. 

It  is  certain  that  some  of  the  copper  was  deposited  in  Middle 
Keweenawan  time,  for  boulders  in  some  of  the  conglomerates 
show  mineralization  that  must  have  taken  place  before  the 
fragments,  now  forming  the  conglomerate4  were  broken  from  the 
parent  ledges.5 

As  the  copper  ores  are  replacement  deposits  they  must  have 
formed  after  the  associated  rocks  had  solidified.6  This  places 
the  formation  of  some  of  the  deposits  at  a  time  after  the  effusion 
and  solidification  of  associated  diabase  amygdaloids,  and  before 
the  entire  series  was  deposited. 

The  associated  minerals,  many  of  which  are  formed,  so  far  as 
known,  only  near  or  above  100°  C.,  suggest  a  genesis  related  to 
magmatic  processes.  The  evidence  suggests  that  the  deposits  were 
formed  by  magmatic  solutions,  perhaps  mingled  with  meteoric 

1  LANE,   A.  C. :  Mine  Waters.     Lake  Superior  Min.  Inst.  Proc.,  vol.  13, 
pp.  74-126,  1908. 

2  DOELTEB,  C. :  Ueber  die  kuenstlichkeit  Darstellung  und  die  chemische 
constitution  einiger  Zeolithe.     Neues  Jahrb.,  1890,  Band.  1,  pp.  118-139. 

3  WRIGHT,  F.  E.:  The  Intrusive  Rocks  of  Mount  Bohemia,  Michigan. 
Mich.  Geol.  Survey  Ann.  Rept.  for  1908,  pp.  361-397,  1909. 

4  Compare  with  description  of  zeolitization  of  traps  of  White  River, 
Alaska,  by  ADOLPH  KNOPF  (Econ.  Geol,  vol.  5,  p.  247,  1910)  who  noted 
fragments  of  cupriferous  amygdaloid  in  a  pyroclastic  rock  that  overlies  a 
cupriferous  amygdaloidal  bed. 

6  VAN  HISE,  C.  R.,  and  LEITH,  C.  K:  Op  tit.,  p.  581. 

6  PUMPELLY,  RAPHAEL:  The  Paragenesis  and  Derivation  of  Copper  and 
its  Associates  on  Lake  Superior.  Am.  Jour.  Set.,  3d  ser.,  vol.  2,  pp.  188- 
189,  243-258,  347-355,  1871. 


400      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

waters,  reacting  on  the  solidified  lavas  and  conglomerates  that 
were  above  the  cooling  magmas.  The  mineralization  took  place 
before  the  beds  were  tilted,  perhaps  at  a  time  when  they  were 
nearly  flat-lying.  This  theory  is  not  out  of  keeping  with  the  ob- 
servations of  Lane  on  the  composition  of  mine  waters,1  or  those 
of  Biddle2  and  of  Fernekes3  on  the  chemistry  of  the  deposits, 
or  those  of  Pumpelly,4  Irving,5  and  Van  Hise  and  Leith6  on  the 
hydrothermal  alterations. 

Large  deposits  have  recently  been  developed  in  the  "None- 
such" beds.  These  consist  chiefly  of  chalcocite  disseminated  at 
certain  horizons  in  sandstone. 

1  LANE,  A.  C. :  Geological  Report  on  Isle  Royale,  Mich.     Geol.  Survey, 
vol.  6,  part  1,  pp.  99 — et  seq.,  1898.     The  Chemical  Evolution  of  the  Ocean. 
Jour.  Geol.,  vol.  14,  pp.  221-225,  1906. 

2  BIDDLE,  H.  C. :  The  Deposition  of  Copper  by  Solutions  of  Ferrous 
Salts.     Jour.  Geol,  vol.  9,  pp.  430-436,  1901. 

3  FERNEKES,   G. :  Precipitation  of   Copper  from   Chloride  Solution  by 
Means  of  Ferrous  Chloride.     Econ.  Geol,  vol.  2,  p.  580,  1907. 

4  PUMPELLY,  RAPHAEL:  Op  cit. 

6  IRVING,  R.  D.:  The  Copper-Bearing  Rocks  of  Lake  Superior.  U.  S. 
Geol.  Survey  Mon.  5,  pp.  419-426,  1883. 

6  VAN  HISE,  C.  R.,  and  LEITH,  C.  K.:  The  Geology  of  the  Lake  Superior 
Region.  U.  S.  Geol.  Survey  Mon.  52,  p.  583,  1911. 


CHAPTER  XXIV 
GOLD 


Minerals 

Percentage  of 
gold 

Composition 

Native  gold 

100  0 

Au 

Gold  amalgam  

Au  Hg 

Electrum  

Au,Ag 

Calaverite  

39.5 

(Au,Ag)Te» 

Krennerite  

39  5 

(Au  Ag)Te» 

Sylvanite  . 

24  5 

AuAgTe4 

Petzite  

25  4 

(Au  Ag)jTe 

MINERAL  ASSOCIATIONS 

Gold  is  one  of  the  rarer  metals  in  nature,  and  chemically  it  is 
one  of  the  most  inactive.  It  forms  stable  natural  compounds 
with  but  few  other  elements  and  only  with  metals.  Although 
rare,  gold  is  peculiarly  widespread;  traces  are  found  in  sea  water 
and  in  the  ashes  of  plants  and  animals.  Small  amounts  of  metal- 
lic gold  are  commonly  present  in  other  metallic  minerals,  espe- 
cially in  pyrite,  chalcopyrite,  galena,  sphalerite,  and  other  sul- 
phides. Some  gold  is  intimately  associated  also  with  quartz, 
calcite,  alunite,  and  other  gangue  minerals.  A  common  ore  is 
composed  of  quartz,  pyrite,  and  gold,  with  small  amounts  of 
other  minerals.  The  various  types  of  hydrothermal  alteration 
in  and  along  gold  veins  are  mentioned  on  pages  230  to  268.  The 
wall  rock  may  be  replaced  by  sericite,  calcite,  alunite,  gold, 
pyrite  or  tellurides,  generally  by  combinations  of  two  or  more  of 
these,  the  particular  minerals  depending  upon  the  character  of 
the  rock,  the  composition  of  the  depositing  solutions,  and  the 
physical  conditions  at  the  time  of  deposition. 

The  amount  of  gold  in  its  ores  varies  greatly.  In  1912  the 
"dry"  or  siliceous  ores  produced  in  Alaska  yielded  $2.85  a  ton  in 
precious  metals,  Nevada  ores  yielded  $14.74,  and  the  average 
value  of  gold  and  silver  recovered  from  10,584,777  tons  treated 

26  401 


402      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

in  the  United  States  was  $7.40.  With  improvements  in  mechan- 
ical concentration  and  in  the  cyanide  process,  it  becomes  possible 
to  treat  the  lower-grade  ores.  With  a  few  exceptions  copper 
ores  contain  gold,  some  of  them  large  amounts.  Many  silver  de- 
posits carry  much  gold,  also.  Some  lead  and  zinc  ores  carry 
gold  and  silver. 

GENESIS  OF  GOLD  DEPOSITS 

Gold  is  found  in  igneous  rocks  and  in  pegmatite  veins,  but  such 
occurrences  are  of  more  scientific  than  commercial  interest. 
Gold  occurs  in  some  contact-metamorphic  deposits,  and  in  a  few 
it  is  the  principal  metal.  As  a  syngenetic  mineral  in  sedimentary 
beds  gold  is  deposited  mainly,  if  not  exclusively,  by  mechanical 
processes. 

Except  placers,  nearly  all  workable  deposits  of  gold  are  veins 
and  related  deposits.  In  veins  gold  is  deposited  at  all  depths. 
In  the  Appalachian  region  gold  deposits  have  been  formed  some 
3  or  4  miles  below  the  surface,  and  at  many  places  the  gold  is 
associated  with  magnetite  and  pyrrhotite.  In  the  Dahlonega 
district,  Georgia,1  and  at  Pinetucky,  Ala.,2  the.  gold  is  associated 
with  veins  having  a  garnet  gangue.  In  the  Black  Hills,  South 
Dakota,  it  is  associated  with  garnet,  actinolite,  and  tremolite. 
Many  of  the  great  gold  deposits  of  the  American  Cordillera  have 
been  formed  at  moderate  depths,  as  in  the  Mother  Lode,  Grass 
Valley,  and  Ophir,  Cal.  Other  gold  deposits  have  been  formed 
relatively  near  the  surface,  as  at  Goldfield,  Nev.,  and  Cripple 
Creek,  Colo.  Gold  is  persistent,  then,  from  the  deep  vein  zone 
to  the  surface.  It  has  been  deposited  in  the  sinters  of  some 
hot  springs. 

On  weathering  gold  behaves  more  like  iron  than  like  copper. 
It  is  concentrated  mechanically  in  placers,  and  the  outcrops  of  its 
deposits  are  enriched  by  the  removal  of  other  materials  more 
rapidly  than  gold.  In  a  suitable  environment,  where  the  waters 
carry  chlorides  and  the  ores  contain  manganese,  the  gold  will 
be  dissolved  and  may  be  transported  and  reprecipitated  at  con- 
siderable depths. 

1LiNDGREN,  WALDEMAR:  Notes  on  the  Dahlonega  Mines.  U.  S.  Geol. 
Survey  Bull.  293,  p.  126,  1906. 

2  McCASKEY,  H.  D. :  Notes  on  Some  Gold  Deposits  of  Alabama.  U.  S. 
Geol.  Survey  Bull.  340,  p.  36,  1908. 


GOLD  403 

AGE  AND  ASSOCIATIONS  OF  GOLD  DEPOSITS 

Practically  all  the  gold  output  of  North  America  has  been 
derived  from  veins  formed  by  ascending  hot  solutions,  from 
nearly  related  deposits,  and  from  placers.  The  lode  deposits 
are  found  in  rocks  of  all  kinds,  both  igneous  and  sedimentary, 
and  of  various  ages,  from  pre-Cambrian  to  Pliocene.  Many 
deposits  are  associated  with  granite,  granodiorite,  monzonite, 
and  their  porphyries,  or  with  andesite  and  dacite.  Some,  how- 
ever, are  associated  with  alkali-rich  rocks  such  as  phonolite,  and 
a  few  with  gabbro  and  diabase.  Gold  veins  are  found  also  in 
rhyolite,  basalt,  and  other  surface  lavas,  but  generally  such  rocks 
are  found  to  be  cut  by  intrusives  near  the  veins. 

Gold  deposits,  like  those  of  iron,  copper,  and  other  metals, 
have  been  formed  more  abundantly  in  certain  geologic  periods 
than  in  others.1 

Pre-Cambrian  Deposits. — In  the  pre-Cambrian  and  probably 
in  early  Paleozoic  time  many  gold  deposits  were  formed  in  the 
southern  Appalachian  Mountains.  Gold  deposits  also  were 
formed  in  pre-Cambrian  time  in  the  Black  Hills,  where  placer 
gold  is  found  in  Cambrian  conglomerates.  Many  of  the  pre- 
Cambrian  deposits  formed  in  the  deep  vein  zone  carry  garnet, 
actinolite,  magnetite,  pyrrhotite,  and  other  minerals  deposited 
at  high  temperatures.  Not  all  the  deposits  are  of  this  class, 
however.  In  the  pre-Cambrian  time,  as  later,  gold  deposits  were 
doubtless  formed  at  various  depths,  but  because  much  erosion 
has  since  taken  place  in  the  Appalachian  region  most  of  the 
deposits  remaining  are  but  the  "roots"  of  deposits  that  were 
once  much  more  extensive  vertically. 

Cretaceous  Veins  of  the  Pacific  Coast. — The  most  productive 
gold  belt  in  the  United  States  is  along  the  Pacific  coast,  where 
there  were  great  intrusions  of  granitic  rocks  in  Cretaceous  time. 
The  gold  deposits  are  found  in  California,  Oregon,  Wash- 
ington, British  Columbia,  and  Alaska.  The  veins  are  large  and 
numerous  but  are  mainly  of  low  grade,  the  average  value  being 
about  $5  a  ton.  Much  of  the  production  has  been  derived  from 
placers.  The  lodes  contain  quartz,  alkali  earth  carbonates,  and 
some  sericite,  with  subordinate  chlorite.  Alteration  generally 
extends  only  a  few  feet  from  the  lodes.  These  deposits  are  near 

1  LINDGREN,  WALDEMAR:  The  Geological  Features  of  the  Gold  Production 
of  North  America.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  33,  p.  790,  1902 


404      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  roots  of  veins,  but  in  general  they  were  formed  above  the 
zone  corresponding  to  that  which  now  remains  of  many  pre- 
Cambrian  deposits.  Tabular  forms  are  more  prevalent  than 
stock  works  and  reticulated  masses,  though  in  California  there 
are  many  sheeted  zones  and  "stringer  lodes."  In  the  main, 
however,  the  veins  have  remarkable  continuity  and  seem  to  have 
been  formed  under  conditions  which  favored  development  of 
stronger  and  more  persistent  fractures  than  those  of  the  Appa- 
lachian pre-Cambrian  veins.  The  deposits  everywhere  contain 
sulphides,  though  they  are  not  abundant;  silver  is  insignificant 
in  quantity,  and  bonanzas  are  rare.  The  minerals  are  quartz, 
gold,  a  little  pyrite,  arsenopyrite,  chalcopyrite,  here  and  there 
galena  and  zinc  blende,  rarely  tetrahedrite,  telluride,  and  molyb- 
denite. Gangue  minerals  include  calcite,  dolomite,  siderite,  and 
rarely  barite  and  fluorite.  Some  of  the  veins  carry  pyrrhotite 
and  albite,  suggesting  affiliation  with  deposits  of  the  deeper 
zone. 

Late  Cretaceous  or  Early  Tertiary  Deposits. — Volcanic  ac- 
tivities moved  eastward  about  the  end  of  the  Cretaceous  period, 
and  in  early  Tertiary  time  there  were  extensive  igneous  intrusions 
in  Montana,  Idaho,  Utah,  Arizona,  and  Colorado.  Many  gold 
deposits  are  associated  with  the  intrusives  in  this  area,  which 
Lindgren  has  designated  the  "central  belt."  The  deposits  of 
late  Cretaceous  or  early  Tertiary  age  occur  in  Montana,  where 
they  have  yielded  great  quantities  of  placer  gold  in  Alder,  Con- 
federate, and  Prickly  Pear  gulches  and  on  Gold  Creek.  The 
lodes  from  which  these  deposits  were  derived  were  disappointing. 
In  this  region  deposits  of  similar  age  but  carrying  more  silver  than 
gold  are  found  at  Philipsburg,  Butte,  and  other  places.  The 
deposits  of  the  central  belt  are  mainly  sericitic  and  calcitic 
(page  231).  They  were  formed  at  intermediate  depths.  Many 
of  them  carry  the  complex  antimony  or  arsenic-silver  minerals. 
Bonanzas  are  not  uncommon. 

Middle  and  Late  Tertiary  Deposits. — The  Tertiary  gold 
deposits,  which  form  the  latest  group,  are  abundant  in  the 
Great  Basin — in  Nevada  and  portions  of  adjoining  States. 
They  are  also  developed  at  many  places  in  Colorado  and  to  some 
extent  over  other  portions  of  the  western  Cordillera,  where  there 
was  extensive  igneous  intrusion  in  middle  and  late  Tertiary  time. 
Bonanzas  are  characteristic,  and  placers  are  less  common  than 
in  deposits  of  the  Pacific  coast  type.  Many  of  these  deposits 


GOLD  405 

were  formed  near  the  surface,  probably  most  of  them  at  depths  of 
less  than  a  mile.  Hydrothermal  metamorphism  extends  to  great 
distances  from  the  veins,  especially  in  andesite,  which  is  a  com- 
mon country  rock.  Propylitic  alteration  is  characteristic.  It 
is  attended  by  the  development  of  chlorite,  calcite,  sericite,  and 
pyrite,  with  the  formation  of  adularia  and  devitrification  of  any 
glass  present.  The  ores  are  largely  silver-gold  ores  that  contain 
too  little  lead  or  copper  to  smelt  without  mixing  with  other  ores. 
Typical  districts  of  the  Basin  province  are  Tonopah,  Washoe, 
Goldfield,  Tuscarora,  and  Rhyolite,  Nev.;  and  Bodie,  Cal. 
In  Colorado  the  deposits  of  Cripple  Creek  and  some  at  George- 
town and  Idaho  Springs  belong  to  this  group.  Notable  examples 
are  found  also  in  Mexico  and  Central  America. 

SUPERFICIAL  ENRICHMENT 

General  Features. — Unlike  copper  and  silver,  gold  is  insoluble 
in  sulphuric  acid.  Wurtz1  stated,  in  1858,  that  ferric  sulphate 
dissolves  gold,  and  his  statement  has  frequently  been  quoted  in 
discussions  of  the  processes  of  enrichment  of  gold  deposits. 
Stokes2  showed,  however,  that  ferric  sulphate  will  not  dissolve 
gold,  even  at  200°C.,  except  in  the  presence  of  a  chloride,  and 
its  insolubility  in  ferric  chloride  and  in  ferric  sulphate  at  ordinary 
temperatures  has  been  verified  by  Brokaw.3  Gold  is  dissolved 
in  the  presence  of  a  chloride  and  manganese  dioxide.  Hydro- 
chloric acid  forms  in  the  presence  of  sodium  chloride  and  sulphuric 
acid,  and  in  the  presence  of  an  oxidizing  agent  the  hydrogen  ion 
is  removed  to  form  water,  leaving  the  chlorine  in  the  so-called 
"nascent  state."  In  this  state  its  attack  is  vigorous.  It  is 
known  that  several  oxides  will  release  "nascent  chlorine"  at  low 
temperatures  if  the  solutions  are  sufficiently  concentrated,  but  in 
moderately  dilute  solutions  manganese  oxides  are  the  only  com- 
mon ones  that  are  appreciably  effective.4 

1  WTJRTZ,  HENRY:  Contributions  to  Analytical  Chemistry.     Am.  Jour. 
Set.,  2d  ser.,  vol.  26,  p.  51,  1858. 

2  STOKES,  H.  N. :  Experiments  on  the  Solution,  Transportation,  and  Depo- 
sition of  Copper,  Silver,  and  Gold.     Econ.  Geol,  vol.  1,  p.  650,  1906. 

3  BROKAW,  A.  D. :  The  Solution  of  Gold  in  the  Surface  Alterations  of  Ore 
Bodies.     Jour.  Geol.,  vol.  18,  p.  322,  1910. 

4  EMMONS,  W.  H. :  The  Agency  of  Manganese  in  the  Superficial  Alteration 
and  Secondary  Enrichment  of  Gold  Deposits  in  the  United  States.     Am, 
Inst.  Min.  Eng.  Bull.  46,  pp.  789-791,  1910, 


405      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Experiments  on  the  solubility  of  gold  in  cold  dilute  solutions 
were  made  by  Brokaw.1  The  nature  of  these  experiments  is 
shown  by  the  following  statements,  in  which  a  and  b  represent 
duplicate  tests: 

(1)  Fe2(SO4)3  +  H2SO4  +  Au. 

(a)  No  weighable  loss  (34  days). 
(6)  No  weighable  loss. 

(2)  Fe2(SO4)3  +  H2S04  +  MnO2  +  Au. 

(a)  No  weighable  loss  (34  days). 
(6)  0.00017  gram  loss.2 

(3)  FeCl3  +  HC1  +  Au. 

(a)  No  weighable  loss  (34  days). 
(6)  No  weighable  loss. 

(4)  FeCl3  +  HC1  +  MnO2  +  Au. 

(a)  0.01640  gram  loss;  area  of  plate,  383  square  milli- 
meters (34  days). 

(6)  0.01502  gram  loss;  area  of  plate,  348  square  milli- 
meters. 

To  approximate  natural  waters  more  closely,  a  solution  was 
made  one-tenth  normal  as  to  ferric  sulphate  and  sulphuric  acid 
and  one  twenty-fifth  normal  as  to  sodium  chloride.  Then  1  gram 
of  powdered  manganese  dioxide  was  added  to  50  cubic  centi- 
meters of  the  solution,  and  the  experiment  was  repeated.  The 
time  was  14  days. 

(5)  Fe2(S04)3  +  H2S04  +  NaCl  +  Au. 

No  weighable  loss. 

(6)  Fe2(SO4)3  +  H2S04  +  NaOl  +  Mn02  +  Au. 

Loss  of  gold,  0.00505  gram. 

Although  the  concentration  of  chlorine  in  most  of  these  experi- 
ments is  greater  than  that  which  is  found  in  many  mineral  waters, 
it  is  noteworthy  that  solution  of  gold  will  take  place  with  even  a 
trace  of  chlorine  (see  experiment  26),  and  without  much  doubt 
these  reactions  will  go  on  also  in  the  presence  of  only  minute 
quantities  of  manganese  oxides. 

The  solution  of  gold  is  most  extensive  in  the  upper  parts  of 

1  BBOKAW,  A.  D.:  Op.  cU.,  pp.  321-326. 

2  This  duplicate  was  found  to  contain  a  trace  of  Cl,  which  accounts  for 
the  loss. 


GOLD  407 

the  oxidized  zones,  where  most  of  the  pyrite  has  been  removed, 
for  pyrite  precipitates  gold,  and  on  oxidation  pyrite  yields  fer- 
rous sulphate,  which  tends  to  inhibit  solution. 

Where  it  is  held  in  solution  as  chloride,  gold  is  readily  precipi- 
tated by  ferrous  sulphate.  If  much  manganese  oxide  is  present, 
however,  the  ferrous  sulphate  is  immediately  oxidized  to  ferric 
sulphate,  which  does  not  precipitate  gold  from  solutions  in  which 
it  is  held  as  chloride.  In  the  presence  of  manganese  oxides, 
therefore,  not  only  is  gold  dissolved  in  acid  solution,  but  the  con- 
ditions under  which  it  is  precipitated 
may  be  delayed.  Gold  may  be  carried 
in  acid  solution  so  long  as  the  higher 
oxides  of  manganese  are  present.  In 
many  gold  deposits  manganese  oxides 
and  gold  are  intimately  associated 
and  without  doubt  have  been  pre- 
cipitated together. 

If  a  crystal  of  calcite  is  placed  in 
an  acid  solution  in  which  gold  is  dis- 
solved  in  the  presence  of  manganese, 
gold  is  precipitated  with  manganese  manganiferous  acid  gold  solu- 
oxide  on  the  surface  and  in  the  cleav-  fed.^^^KS 
age  cracks  of  the  calcite  crystal1  (Fig.  together  in  cleavage  cracks. 
170).  The  reactions  may  be  stated: 

2AuCl3  +  3MnCl2  +  6H2O^2Au  +  3Mn02  +  12HC1 
It  follows  also  from  this  experiment  that  gold  will  not  be  dissolved 
readily  when  calcite  is  present  in  excess.2 

These  reactions  indicate  processes  by  which  gold  held  in  acid 
solution  in  the  presence  of  manganese  salts  may  be  precipitated  in 
the  deeper  zones,  together  with  manganese  oxides,  when  the  solu- 
tions reacting  on  alkaline  minerals  lose  acidity. 

In  some  deposits  there  is  evidence  that  gold  has  been  dissolved 
and  reprecipitated  in  the  surficial  zone,  yet  the  secondary  gold  ore 
carries  no  manganese,  or  at  least  not  more  than  a  trace  of  manga- 
nese compounds.  Ferrous  sulphate  will  precipitate  gold  even 
from  strongly  acid  solutions  in  which  manganese  would  still  re- 

^ROKAW,  A.  D.:  The  Secondary  Precipitation  of  Gold  in  Ore  Bodies. 
Jour.  Geol.  vol.  21,  p.  251,  1913. 

2  EMMONS,  W.  H. :  The  Enrichment  of  Ore  Deposits.  U.  S.  Geol.  Sur- 
vey Bull  625,  p.  314,  1917. 


408      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

main  in  solution.  It  follows  that  manganese  might  not  be  pre- 
cipitated with  gold  from  acid  solutions  even  where  manganese 
dioxide  has  supplied  conditions  for  its  solution  in  chloride  waters ; 
but  from  neutralized  solutions  the  gold  and  manganese  would  go 
down  simultaneously.  Gold  is  precipitated  from  chloride  solu- 
tion also  by  native  metals,  sulphides,  organic  matter,  and  many 
other  materials. 

Placers  and  Outcrops. — Those  deposits  in  which  enrichment  in 
gold  is  believed  to  have  taken  place  are,  perhaps  without  excep- 
tion, manganiferous.  Inasmuch  as  enrichment  is  produced  by 
the  downward  migration  of  the  gold  instead  of  by  its  superficial 
removal  and  accumulation,  it  follows  that  both  gold  placers  and 
outcrops  rich  in  gold  are  generally  found  in  connection  with  non- 
manganiferous  deposits.1  Placer  deposits  in  the  main  are  asso- 
ciated with  nonmanganiferous  lodes,  and  such  lodes  are  generally 
richer  at  the  outcrops  and  in  the  oxidized  zones  than  at  greater 
depths,  the  enrichment  being  due  to  a  removal  of  the  material 
associated  with  gold.  Even  under  favorable  conditions,  however, 
gold  is  generally  dissolved  less  readily  than  copper  or  silver  and 
precipitated  more  readily  than  either.  Consequently  its  enriched 
ores  are  likely  to  be  found  nearer  the  surface.  In  a  manganifer- 
ous calcite  gangue  gold  may  accumulate  at  the  very  outcrop, 
for  the  solutions  do  not  long  remain  acid  if  passing  through 
strongly  alkaline  rocks.  Some  placer  deposits,  indeed,  are  asso- 
ciated with  gold  lodes  having  a  manganiferous  calcite  gangue. 

Concentration  in  the  Oxidized  Zone. — The  concentration  of 
gold  in  the  oxidized  zone  near  the  surface,  where  the  waters  re- 
move the  valueless  elements  more  rapidly  than  gold,2  is  an  effect- 
ive process  in  lodes  that  do  not  contain  manganese  or  in  man- 
ganiferous lodes  in  areas  where  the  waters  do  not  contain  appre- 
ciable chloride  (see  Fig.  171, a).  In  the  oxidized  zone  in  some 
mines  it  is  difficult  to  distinguish  the  ore  which  has  been  enriched 
by  this  process  from  ore  which  has  been  enriched  lower  down  by 
the  solution  and  precipitation  of  gold  and  which,  as  a  result  of 
erosion,  is  now  nearer  the  surface.  Rich  bunches  of  ore  are  much 

1  EMMONS,  W.  H. :  The  Agency  of  Manganese  in-  the  Superficial  Alteration 
and  Secondary  Enrichment  of  Gold  Deposits  in  the  United  States.     Am. 
Inst.  Min.  Eng.  Bull.  46,  pp.  813,  814-816,  1910.     Jour.  Geol,  vol.  19,  pp. 
15-46,  1911. 

2  RICKARD,  T.  A. :  The  Formation  of  Bonanzas  in  the  Upper  Portions  of 
Gold  Veins.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  31,  pp.  198-220,  1902. 


GOLD 


409 


more  common  in  the  oxidized  zone  than  in  the  primary  sul- 
phides of  such  lodes.  They  are  present  in  some  lodes  which  carry 
little  or  no  manganese  in  the  gangue  and  which  below  the  water 
level  show  no  deposition  of  gold  by  descending  solutions.  Some 
of  them  are  doubtless  residual  pockets  of  rich  ore  that  were  richer 
than  the  main  ore  body  when  deposited  as  sulphides,  but  others 
are  very  probably  ores  to  which  gold  has  been  added  in  the  proc- 
ess of  oxidation  near  the  water  table  by  the  solution  and  precipi- 


Fio.  171. — Ideal  sections  showing  influence  of  manganese  on  secondary 
concentration  of  gold  in  deposits  of  sulphide  ores,  a,  Longitudinal  projection 
of  gold  vein  without  manganese  minerals.  The  gold  is  concentrated  at  the 
surface  by  removal  of  valueless  material  and  accumulates  as  placers,  b, 
Longitudinal  projection  of  gold  vein  with  manganese  minerals  and  with 
calcite  or  other  minerals  that  readily  reduce  acidity.  Some  gold  is  dissolved 
but  is  not  carried  to  great  depth,  c,  Longitudinal  projection  of  gold  vein 
with  considerable  manganese  and  without  minerals  that  rapidly  reduce 
acidity.  Gold  is  dissolved  and  reprecipitated,  and  some  is  deposited  at 
considerable  depths. 

tation  of  gold  in  the  presence  of  the  small  amount  of  manganese 
contributed  by  the  country  rock.  In  view  of  the  relations  shown 
by  experiments  it  is  probable  that  a  very  little  manganese  will 
accomplish  the  solution  of  gold,  but  it  requires  considerably  more 
manganese  to  form  appreciable  amounts  of  the  higher  manganese 
compounds  that  delay  the  deposition  of  gold  by  suppressing  its 
precipitation.  In  the  absence  of  larger  amounts  of  the  higher 
manganese  compounds  the  gold  would  probably  be  precipitated 
almost  as  soon  as  the  solutions  encountered  the  zone  where  any 
considerable  amounts  of  pyrite  or  other  sulphides  were  exposed  in 
the  partly  oxidized  ore;  for  oxygenated  solutions  dissolve  pyrite 
and  even  traces  of  the  ferrous  sulphate  formed  in  the  oxidizing 
reactions  precipitate  gold  almost  immediately.  Many  other 
sulphides  precipitate  gold  very  readily.  Deposits  showing  only 


410      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

traces  of  manganese,  presumably  supplied  from  the  country  rock, 
are  not  much  enriched  below  the  zone  of  oxidation. 

Relation  of  Gold  Enrichment  to  Chalcocitization. — In  several 
of  the  great  copper  districts  of  the  West  gold  is  a  valuable  by- 
product. In  other  deposits  silver  and  gold  are  the  principal 
metals,  and  copper,  where  present,  is  only  a  by-product.  But  in 
some  of  these  precious-metal  ores  chalcocite  is  nevertheless  the 
most  abundant  metallic  mineral  and  constitutes  several  per  cent, 
of  the  vein  matter.  In  many  ores  it  forms  a  coating  over  pyrite 
or  other  minerals.  Some  of  this  ore,  appearing  in  general  not 
far  below  the  water  table,  is  fractured,  spongy  quartz  coated  with 
pulverulent  chalcocite.  A  part  of  it  contains  silver  and  more 
gold  than  the  oxidized  ore  or  the  deeper-seated  sulphide  ore. 
Clearly  the  conditions  that  favor  chalcocitization  are  favorable 
also  to  the  precipitation  of  silver  and  gold. 

Chalcocitization  sets  free  ferrous  sulphate  (page  356) ,  and  even  a 
trace  of  ferrous  sulphate  precipitates  gold  from  acid  solutions  in 
which  it  is  dissolved  as  chloride.  Furthermore,  chalcocite  itself 
readily  precipitates  gold.  It  follows  that  gold  is  not  precipitated 
by  downward  enrichment  below  the  zone  where  chalcocite  is 
being  deposited. 

Gold  and  silver  are  commonly  associated  in  their  deposits. 
Although  gold  is  dissolved  in  chloride  solutions,  silver  chloride 
is  but  slightly  soluble,  and  high  concentration  of  the  two  metals 
could  not  exist  in  the  same  solution.  Silver  chloride  is  slightly 
soluble  in  water,  and  silver  may  be  held  in  small  concentration 
in  solutions  in  which  gold  also  is  dissolved.  A  mine  water  from 
the  Comstock  lode1  carried  188  milligrams  of  silver  and  4.15 
milligrams  of  gold  in  a  ton  of  solution.  As  ferrous  sulphate  and 
certain  sulphides  precipitate  both  gold  and  silver  from  acid  solu- 
tion, alloys  of  these  metals  might  form  as  secondary  minerals. 
As  ferrous  sulphate  is  released  in  the  chalcocitization  of  pyrite, 
the  secondary  ores  of  gold  and  silver  can  be  deposited  simultane- 
ously with  chalcocite. 

Tellurides. — Petzite,  sylvanite,  krennerite,  and  calaverite  are 
tellurides  of  gold  and  silver,  the  precious  metals  being  present 
in  varying  proportions.  All  are  primary. 

Summary. — The  materials  whose  presence  is  favorable,  if  not 
essential,  for  gold  enrichment  are  (1)  chloride  solutions,  (2)  iron 

1  REID,  J.  A. :  The  Structure  and  Genesis  of  the  Comstock  Lode.  Cal. 
Univ.  Dept.  Geol.  Bull,  vol.  4,  No.  10,  p.  193,  1905. 


GOLD  411 

sulphides,  (3)  manganese  compounds.  Where  these  materials 
exist  and  where  no  very  effective  precipitant  is  at  hand  and  ero- 
sion is  not  too  rapid,  gold  placers  are  rarely  formed,  and  outcrops 
of  gold  ores  are  likely  to  be  less  rich  than  the  ores  that  lie  deeper 
(Fig.  171,  c).  Where  these  materials  exist  and  where  the  lodes 
are  fractured,  gold  will  migrate  downward  in  solution.  Many 
minerals  will  precipitate  gold,  but  in  the  presence  of  some, 
including  calcite,  siderite,  rhodochrosite,  pyrrhotite,  chalcocite, 
nepheline,  olivine,  and  leucite  its  precipitation  is  especially  rapid. 
In  deposits  that  carry  appreciable  quantities  of  these  minerals 
in  ore  or  wall  rock  the  downward  migration  of  gold  is  delayed, 
and  gold  bonanzas  are  likely  to  be  formed  at  or  very  near  the 
surface  (Fig.  171,  6).  Under  these  conditions,  also,  placers  may 
be  produced,  even  from  manganiferous  lodes.  In  a  gangue  of 
adularia,  sericite,  and  quartz,  with  pyrite,  chalcopyrite,  galena, 
and  sphalerite,  gold  may  be  carried  downward  several  hundred 
or  even  a  thousand  feet,  but  this  depth  should  be  regarded  as 
nearly  the  maximum  and  exceptional.  Gold  is  readily  precipi- 
tated by  many  common  minerals;  it  will  generally  migrate  slowly 
in  ground  water. 

GOLD  PLACERS 

General  Features. — A  large  part  of  the  world's  gold  has  come 
from  placers,  which  now  yield  about  20  per  cent,  of  the  annual 
production.  Placer  deposits  as  a  rule  are  the  most  easily  dis- 
covered of  all  metalliferous  deposits,  and  because  their  product 
is  so  easily  transported  and  marketed  the  placers  are  commonly 
the  first  resources  of  a  region  to  be  exploited.  The  lure  of  gold 
has  resulted  in  the  exploration  and  development  of  many  a  new 
country. 

As  a  land  surface  is  worn  away  in  regions  where  conditions  are 
unfavorable  for  solution,  gold  contained  in  veins  and  veinlets  or 
disseminated  in  the  rock  tends  gradually  to  become  concentrated 
at  the  surface.  Some  of  it  may  remain  practically  in  place. 
The  rotten  outcrops  (saprolites)  of  many  veins  in  the  southern 
Appalachians  were  washed  for  gold.1  As  erosion  goes  on  the 
gold-bearing  mantle  rock  of  a  deposit  on  a  slope  will  gradually 
settle  downhill,  constituting  an  eluvial  deposit.  As  erosion  is 
continued  further,  however,  the  gold  ultimately  finds  lodgment 

1  BECKER,  G.  F.:  Gold  Fields  of  the  Southern  Appalachians.  U.  S. 
Geol.  Survey  Sixteenth  Ann.  Rept.,  part  3,  p.  289,  1895. 


412      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


in  streams  together  with  sand  and  gravel.     Such  accumulations 
constitute  the  principal  placer  deposits. 

In  placer  mining  commonly  a  box-like  trough,  called  a  sluice, 
is  fitted  on  the  bottom  with  movable  cross  pieces  or  riffles.  Al- 
though tons  of  rock  may  be  washed  through  the  sluice,  nearly 
all  the  gold  collects  in  the  pockets  behind  the  riffles.  The  bed  of 
a  stream  acts  much  like  the  placer  miner's  sluice.  The  gold  sinks 
to  the  bottom  and  remains  on  bedrock,  especially  in  joints  and 
seams  or  in  low  places  in  the  bed.  As  gold  (specific  gravity  15.6 
to  19.3)  is  about  six  to  seven  times  as  heavy  as  rock  (specific 
gravity  2.5  to  3),  it  will  sink,  except  the  very  finest  dust,  which 
may  be  carried  away.  The  coarser  gold  generally  remains  in 
gulches  and  creeks  near  its  source,  forming  gulch  and  creek 


FIG.  172. — Sketch  showing  placer  deposits  on  terraces  of  a  stream.  These 
were  formed  when  the  terraces  were  parts  of  the  flood  plain  of  the  stream. 
(After  Tyrrell.) 

gravels;  the  finer  gold  may  be  carried  to  rivers  and  supply  gold 
for  river  gravels;  some  gold  may  be  carried  even  to  the  sea. 

Where  a  shore  line  is  receding  by  wave  action  the  gold  may  be 
concentrated  along  the  beaches  as  beach  placers  or  marine  placers. 
At  Nome,  Alaska,1  placer  deposits  occur  both  along  streams  and 
along  the  beach. 

Placers  formed  in  gulches,  in  beds  of  rivers,  or  on  the  sea  shore 
may  be  elevated  by  general  uplift  of  the  region  to  levels  above 
the  present  drainage  lines  or  along  the  coast  far  above  sea  level. 
As  streams  approach  grade  they  meander  in  their  flood  plains, 
and  gold  contained  in  flood-plain  deposits  may  be  distributed 
over  wide  areas.  In  the  normal  history  of  erosion  a  stream  will 
cut  below  its  flood  plain,  and  the  abandoned  flood  plain  will  later 
appear  high  above  the  stream  as  a  terrace.2  Some  deposits 

1  COLLIER,  A.  J.,  HESS,  F.  L.,  SMITH,  P.  S.,  and  BROOKS,  A.  H. :  The  Gold 
Placers  of  Parts  of  Seward  Peninsula,  Alaska,  U.  S.  Geol.  Survey  Bull.  328, 
1908. 

2CHAMBERLiN,  T.  C.,  and  SALISBURY,  R.  D.:  "College  Geology," 
pp.  191-194,  1909. 


GOLD  413 

worked  for  placer  gold  are  far  above  the  present  streams  (Fig. 
172). 

Placer  deposits  may  be  buried  under  later  deposits.  Lava  flows 
that  are  extravasated  upon  a  rugged  surface  containing  placers 
generally  cover  the  lower  areas — the  bottoms  of  valleys.  The 
streams  that  subsequently  flow  over  the  lavas  will  take  new 
courses,  and  after  they  have  sunk  their  beds  below  the  ancient 
beds  that  were  filled  by  the  lavas  the  gold  deposits  will  crop 
out  on  hillsides  where  the  ancient  channels  are  crossed  by  the 
later  ones.  Such  ancient  gravels  in  California  have  yielded 
much  gold.  As  the  land  surface  is  eroded  the  gold  in  the  old 
channels  is  carried  down  into  present  stream  valleys.  Thus  the 
gold-bearing  gravels  in  present  channels  may  represent  not  only 
the  waste  of  lodes  that  are  now  being  eroded,  but  reconcentra- 
tions  from  ancient  placers  that  were  recently  exhumed. 

Scour  and  Fill. — There  is  a  strong  tendency  for  gold  to  work 
downward  in  gravels.  It  may  halt  at  a  stiff  clay  seam  in  the 
gravel  bed,  or  it  may  descend  to  the  bedrock  and  be  concentrated 
in  joints  and  fractures  of  'the  rock.  Cross-currents,  eddies,  and 
whirlpools  are  common  features  of  streams,  and  normal  stream 
erosion  is  attended  by  extensive  reassortment  of  stream  deposits. 

The  development  of  potholes  is  common  in  swift  streams,  and 
even  sluggish  streams  scour  the  loose  unconsolidated  material  of 
their  beds.  Soundings  in  rivers  show  that  deep  holes  are  sunk 
in  fluvial  deposits  by  running  water.  Even  the  more  sluggish 
streams  that  are  filling  their  channels  will  continually  reassert 
the  material  deposited  in  them  by  dropping  the  coarse  and  pick- 
ing up  the  fine.  In  flood  time  material  is  deposited  on  the  flood 
plain,  but  at  the  same  time  material  in  the  main  or  central 
channel  may  be  picked  up  and  carried  downstream  because 
the  velocity  and  volume  of  the  stream  are  greater  than  during 
low  water.  At  Nebraska  City,  Nebr.,1  the  scouring  action  is 
known  to  extend  to  depths  of  70  to  90  feet.  Holes  scoured  out 
at  one  time  will  be  filled  later.  Thus  material  is  continually 
agitated  and  shifted  downstream,  and  any  gold  present  tends 
to  settle  to  bedrock.  In  gold-bearing  gravels  the  gold  is  generally 
greatly  concentrated  on  bedrock.  Dredging  or  sluicing  opera- 
tions yield  the  greatest  profit  from  the  material  lying  within  a 
few  inches  of  the  bottom  of  the  gravel  bed.  It  is  common  prac- 

^HAMBERLIN,  T.  C.,  and  SALISBURY,  R.  D.:  "College  Geology,"  pp. 
184-185,  1909. 


414      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tice,  indeed,  to  dredge  out  the  bedrock  in  order  to  obtain  the 
gold  deposited  in  its  crevices. 

Where  the  grade  of  a  swift  stream  becomes  lower  gravels  with 
contained  gold  will  accumulate.  Alluvial  fans,  gravel  plains,  or 
any  other  depositional  features  may  be  the  sites  of  placer  gold. 

Minerals  Associated  with  Gold  in  Placers. — The  common  asso- 
ciates of  the  gold  may  be  hard  or  soft,  heavy  or  light,  brittle  or 
malleable.  They  have  one  feature  in  common — they  are  not 
readily  dissolved  in  ground  water  nor  in  stream  waters.  Stream 
gravels  are  made  up  of  various  materials.  Minerals  that  are  com- 
monly found  with  placer  gold  are  the  heavier  minerals  that  are 
stable — magnetite,  ilmenite,  hematite,  pyrite,  garnet,  etc.  In 
regions  containing  deposits  of  cassiterite,  monazite,  zircon, 
platinum,  or  diamonds,  these  minerals  also  will  accumulate  in 
placers.  Silver,  mercury,  copper,  and  lead  are  alloyed  with  gold, 
and  considerable  silver  may  be  present  with  gold  in  placers  that 
have  not  moved  far  from  their  sources.  These  metals,  however, 
are  more  easily  dissolved  than  gold  and  rarely  accumulate  in 
large  quantities. 

Solution  of  Gold  in  Placer  Deposits. — Where  acid  waters  con- 
taining sodium  chloride  attack  gold  in  the  presence  of  an  oxidiz- 
ing agent,  such  as  manganese  oxide,  gold  will  be  dissolved.  But 
waters  of  streams 'normally  contain  very  little  sodium  chloride, 
and  by  the  processes  of  sedimentation  the  light  powdery  man- 
ganese oxides  are  generally  separated  from  the  heavier  particles 
of  gold.  Moreover,  acid  waters  are  neutralized  by  nearly  all 
minerals,  and  neutral  waters  do  not  dissolve  gold.  Organic  mat- 
ter also  is  generally  present  near  the  surface  and  in  streams,  and  it 
would  cause  any  dissolved  gold  to  be  precipitated  and  inhibit  its 
solution.  Thus  the  normal  conditions  are  adverse  to  the  exten- 
sive migration  of  gold  in  placer  deposits  by  solution  and  redeposi- 
tion,  although  under  exceptional  conditions  such  migration  prob- 
ably takes  place  to  a  minor  extent. 

Ground  waters  attack  other  metals  more  readily  than  gold. 
Silver,  lead,  and  copper  are  dissolved  from  minerals  in  which 
they  are  associated  with  gold.  Thus  the  gold  in  placer  deposits 
will  generally  be  purer  than  the  gold  of  the  lodes  from  which  it 
is  derived,  and  normally  the  farther  it  is  from  its  source  and  the 
finer  its  state  of  subdivision  the  purer  it  will  be.1 

1  BECKER,  G.  F. :  Gold  Fields  of  the  Southern  Appalachians.  U.  S. 
Geol.  Survey  Sixteenth  Ann.  Rept.,  part  3,  p.  292,  1895. 


GOLD  415 

Relation  of  Gold  Placers  to  Gold  Lodes.— Where  gold  lodes 
are  exposed  it  is  reasonable  to  expect  placers,  and  where  placers 
have  been  found  the  search  for  the  source  of  their  gold  suggests 
interesting  possibilities  for  prospecting.  An  experienced  prospec- 
tor will  pan  the  gravels  of  gulches  that  drain  a  region  which  con- 
tains gold  deposits  and  will  seek  gold  lodes  in  a  region  which  con- 
tains gold-bearing  gravels.  Either  the  placers  or  the  lodes  may  be 
found  first;  placer  gold  is  easily  discovered.  It  is  common  prac- 
tice to  follow  up  a  stream,  panning  it  at  intervals  and  to  scrutinize 
the  surrounding  country  carefully  where  "colors"  suddenly  cease 
to  appear.  This  method  has  proved  effective  also  in  prospecting 
hill  slopes  for  gold  lodes,  especially  in  regions  that  have  not  been 
glaciated.  The  mantle  rock  is  panned  at  intervals,  and  a  line 
may  be  found  below  which  it  contains  gold  and  above  which 
gold  is  absent.  Trenching  across  such  a  line  may  disclose  a  gold- 
bearing  lode. 

Not  all  gold  lode  deposits,  however,  have  associated  placers. 
In  some  regions  gold  is  dissolved  and  carried  downward  in  solu- 
tion, enriching  the  deposit  below.  Even  where  gold  is  not  dis- 
solved it  is  not  invariably  concentrated  in  gravels.  It  may  be 
so  finely  divided  that  it  is  carried  away  in  the  drainage.  In 
panning  some  deposits  a  film  of  gold  may  be  seen  floating  on  top 
of  the  water  in  the  pan.  Gold  as  fine  as  that  could  easily  be 
carried  away  by  streams  and  scattered.  Placers  are  not  devel- 
oped from  some  gold  deposits,  because  the  primary  ore  shoot  did 
not  reach  the  surface  and  no  auriferous  rock  has  yet  been  eroded. 
Glacial  erosion  may  remove  all  mantle  rock  and  stream  gravels 
from  lodes  in  mountainous  countries.  In  some  regions  fine  flaky 
gold  may  have  been  blown  away  by  winds. 

Conversely,  in  some  regions  that  contain  placer  deposits  no 
workable  gold  lodes  have  been  discovered.  Where  gold  is  not 
dissolved  from  its  deposits  weathering  and  erosion  are  generally 
very  efficient  processes  in  the  mechanical  concentration  of  gold. 
Hundreds  or  even  thousands  of  feet  of  material  may  be  eroded 
from  a  region,  the  rock  being  carried  away  in  streams,  whereas 
the  heavier  gold  remains  behind  to  enrich  gravels  near  the  de- 
posits. In  some  regions  the  gold  in  gravels  has  been  concen- 
trated from  many  veins  and  veinlets  that  are  too  small  and  too 
low  in  grade  to  be  worked  underground.  Many  quests  for  the 
"mother  lodes"  in  regions  containing  valuable  placers  have 
proved  disappointing. 


416      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Eolian  and  Glacial  Deposits  Containing  Gold. — Where  rocks 
are  deeply  decayed  in  arid  countries  where  strong  winds  blow, 
the  lighter  particles  may  be  blown  away  from  outcrops  of  depos- 
its, leaving  the  heavy  material  in  a  more  concentrated  state. 
The  deposit  becomes  enriched  in  the  coarse  gold  that  is  left  be- 
hind. Placers  have  formed  in  western  Australia  by  such  proc- 


When  glaciers  erode  a  gold-bearing  area  the  mantle  rock, 
gravel  and  loose  material  at  outcrops  of  lodes  will  be  carried  away 
in  the  ice  and  any  gold  it  contains  will  become  incorporated  in 
the  drift.  There  is  very  little  sorting,  however,  and  compara- 
tively few  glacial  deposits  are  valuable  except  where  the  material 
has  been  worked  over  by  running  water. 


FIG.  173. — Geologic  section  of  Homestake  vein,  Black  Hills,  South  Dakota. 
The  "cement"  mines  are  Cambrian  placers.     (After  Devereux.) 

Buried  Placers. — Not  only  are  placer  deposits  being  formed 
today,  but  they  have  been  formed  where  conditions  were  favor- 
able during  and  since  pre-Cambrian  time.  Becker2  mentions 
buried  placers  in  the  southern  Appalachian  region  and  in  the  Bald 
Mountain  region,  Wyoming,  and  states  that  the  Triassic  and 
Cretaceous  in  California  contain  placers.  In  the  Black  Hills, 
which  are  famous  for  their  buried  placers  (Fig.  173),  the  Cambrian 
basal  conglomerate  and  sandstone  rest  unconformably  above  the 
pre-Cambrian  schist  series  that  contains  the  Homestake  lode, 
and  in  the  conglomerate  are  found  rounded  grains  of  gold,  evi- 
dently derived  from  the  pre-Cambrian  deposits.  Associated  beds 

1  HOOVER,  H.  C. :  The  Superficial  Alteration  of  Western  Australian  Ore 
Deposits.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  28,  pp.  762-763,  1899. 

RICKARD,  T.  A.:  The  Alluvial  Deposits  of  Western  Australia.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  28,  pp.  490-537,  1899. 

2  BECKER,   G.   F.:  The   Witwatersrand   Banket,  with   Notes  on  Other 
Gold-Bearing  Pudding  Stones.     U.  S.  Geol.  Survey  Eighteenth  Ann.  Kept., 
part  5,  p.  181,  1897. 


GOLD  417 

contain  Cambrian  marine  fossils.  The  deposits  are  believed  to 
have  been  formed  along  an  ancient  shore.1 

When  they  were  worked  out  the  gold  was  found  to  be  concen- 
trated near  the  bottom  of  the  gravel  beds,  as  in  most  placers  now 
in  process  of  formation.  Becker  mentions  many  other  gold- 
bearing  conglomerates  in  America,  Australia,  and  South  Africa. 
The  presence  of  heavy  residual  minerals,  the  accumulation  of  the 
gold  in  the  lower  portions  of  the  gravel  beds,  and  the  rounding 
of  the  particles  of  gold  are  fairly  constant  characteristics  of 
buried  placers. 

Where  surfaces  sink  slowly  below  sea  level  and  are  covered 
by  marine  deposits  any  gold  present  on  the  sunken  land  surface 

<-VOLCAN/C  CAPPING 
<-KHYOUT/C  TUFF 

^-BEDROCK 

FIG.  174. — Section  of  Swift  Shore  mine,  Placer  County,  California,  show- 
ing Tertiary  deposits  (dotted  portions)  buried  below  lava  capping.  (After 
Browne.) 

will  be  washed  over  by  the  waves.  Thus  the  stream  gravels  are 
likely  to  be  spread  out  as  beach  gravels.  It  is  not  surprising 
that  marine  littoral  rather  than  fluviatile  deposits  predominate 
among  the  ancient  gravels  that  have  been  buried  in  the  sea. 

Where  land  sediments  or  lavas  cover  stream  beds,  the  stream 
placers  will  be  preserved.  The  most  productive  buried  placers 
of  North  America  are  the  Tertiary  gravels  of  California,  which, 
according  to  Lindgren2  have  yielded  about  $300,000,000.  This 
region  in  early  Tertiary  time  was  less  rugged  than  it  is  today 
(Fig.  174),  and  on  its  gentle  surface  gold  accumulated  in  streams 
from  the  weathered  lodes  near  by.  The  gravel  beds  were  covered 
with  rhyolite  tuffs,  andesitic  breccia,  and  basalt,  in  places  as 
much  as  1,500  feet  deep.  Later  the  country  was  elevated,  and 
deep  canyons  were  sunk  in  its  surface.  The  new  drainage  lines 

1  DEVEEEUX,  W.  B. :  The  Occurrence  of  Gold  in  the  Potsdam  Formation, 
Black  Hills,  Dakota.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  10,  pp.  465-475, 1882. 

IRVING,  J.  D.:  Economic  Resources  of  the  Northern  Black  Hills.  U.  S. 
Geol.  Survey  Prof.  Paper  26,  pp.  98-111,  1904. 

2  LINDGREN,  WALDEMAR:  The  Tertiary  Gravels  of  the  Sierra  Nevada  of 
California.     U.  S.  Geol.  Survey  Prof,  Paper  73,  p.  81,   1911— Mineral  de- 
posits, p.  206,  New  York,  1913. 

27 


418      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

did  not  follow  the  former  ones,  and  the  ancient  gravel  beds  where 
crossed  by  later  canyons  are  now  exposed,  some  of  them  hundreds 
of  feet  above  present  streams.  As  the  canyons  are  widened,  gold 
in  the  ancient  gravels  is  reconcentrated  in  the  present  gulches, 
where  it  mingles  with  material  that  is  accumulated  from  the 
weathered  outcrops  of  the  lodes. 

References 

GOLD  PLACERS 

BECKER,  G.  F. :  Gold  Fields  of  the  Southern  Appalachians.  TJ.  S.  Geol. 
Survey  Sixteenth  Ann.  Rept.,  part  3,  pp.  251-331,  1895. 

Reconnaissance  of  the  Gold  Fields  of  Southern  Alaska.     U.  S. 

Geol.  Survey  Eighteenth  Ann.  Rept.,  part  3,  pp.  1-86,  1897. 

BROOKS,  A.  H. :  Placer  Gold  Mining  in  Alaska  in  1902.  U.  S.  Geol.  Sur- 
vey Bull.  213,  pp.  41-48,  1903. 

Placer  Mining  in  Alaska  in  1903.      U.  S.  Geol.  Survey  Bull. 

225,  pp.  43-59,  1904. 

The  Geology  and  Geography  of  Alaska.  U.  S.  Geol.  Survey 

Prof.  Paper  45,  1906. 

BROWNE,  R.  E.:  The  Ancient  River  Beds  of  the  Forest  Hill  Divide. 
Cal.  State  Min.  Bur.  Tenth  Ann.  Rept.,  pp.  435-465,  1890. 

COLLIER,  A.  J.,  HESS,  F.  L.,  SMITH,  P.  S.,  and  BROOKS,  A.  H.:  The  Gold 
Placers  of  Parts  of  Seward  Peninsula,  Alaska.  U.  S.  Geol.  Survey  Bull.  328, 
1908. 

DERBY,  O.  A.:  Notes  on  Brazilian  Gold  Ores.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  33,  pp.  282-287,  1902. 

DEVEREUX,  W.  B.:  The  Occurrence  of  Gold  in  the  Potsdam  Formation, 
Black  Hills,  Dakota.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  10,  pp.  465-475, 1882. 

HAMMOND,  J.  H.:  The  Auriferous  Gravels  of  California.  Cal.  State 
Min.  Bur.  Ninth  Ann.  Rept.,  pp.  105-138,  1890. 

IRVING,  J.  D.,  EMMONS,  S.  F.,  and  JAGGAR,  T.  A.,  JR.:  Economic  Re- 
sources of  the  Northern  Black  Hills.  U.  S.  Geol.  Survey  Prof.  Paper  26, 
1904. 

LINDGREN,  WALDEMAR:  The  Tertiary  Gravels  of  the  Sierra  Nevada  of 
California.  U.  S.  Geol.  Survey  Prof.  Paper  73,  1911. 

Neocene  Rivers  of  the  Sierra  Nevada.      U.  S.  Geol.  Survey 

Bull.  213,  pp.  64-65,  1903. 

-     The  Gold  Belt  of  the  Blue  Mountains  of  Oregon.      U.  S.  Geol. 
Survey  Twenty-second  Ann.  Rept.,  part  2,  pp.  551-776,  1901. 

The  Mining  Districts  of  the  Idaho  Basin  and  the  Boise  Ridge, 

Idaho.     U.    S.  Geol.  Survey  Eighteenth  Ann.  Rept.,  part  3,  pp.  617-744, 
1898. 

An  Auriferous  Conglomerate  of  Jurassic  Age  from  the  Sierra 

Nevada.       Am.  Jour.  Sri.,  3d  ser.,  vol.  48,  pp.  275-280,  1894. 

McCoNNELL,  R.  G. :  Report  on  Gold  Values  in  the  Klondike  High-level 
•Gravels.  Canada  Geol.  Survey  Pub.  979,  1907. 


GOLD  419 

Klondike  District,  Yukon  Territory.      Canada  Geol.  Survey 

Ann.  Rept.,  vol   15  for  1903-1906;  Summ.  Rept.,  1904,  part  AA,  pp.  34-42.. 

Report  on  the  Klondike  Gold  Fields.      Canada  Geol.  Survey 

Ann.  Rept.,  vol.  14  for  1901,  part  B,  1905. 

:  The  Kluane  Mining  District.  Canada  Geol.  Survey  Ann. 

Rept.,  vol.  16,  for  1904-1906;  Summ.  Rept.,  part  A,  pp.  1-18,  1905. 

MACLAREN,  J.  M.:  "Gold — Its  Geological  Occurrence  and  Geographic 
Distribution,"  pp.  80-99,  London,  1908. 

MOFFIT,  F.  H.,  and  MADDREN,  A.  G.:  Mineral  Resources  of  the  Kotsina- 
Chitina  Region,  Alaska.  U.  S.  Geol.  Survey  Bull.  374,  1909. 

O'HARRA,  C.  C. :  The  Mineral  Wealth  of  the  Black  Hills.  S.  Dak.  Geol. 
Survey  Bull.  3,  pp.  32-37,  1902. 

PRINDLE,  L.  M.:  The  Gold  Placers  of  the  Fortymile,  Birch  Creek,  and 
Fairbanks  Region,  Alaska.  U.  S.  Geol.  Survey  Bull.  251,  1905. 

PRINDLE,  L.  M.,  and  HESS,  F.  L.:  The  Rampart  Gold-Placer  Region, 
Alaska.  U.  S.  Geol.  Survey  Bull.  280,  1906. 

The  Fairbanks  and  Rampart  Quadrangles,  Yukon-Tanana 

Region,  Alaska,  with  a  Section  on  the  Rampart  Placers.  U.  S.  Geol.  Survey 
Bull.  337,  1908. 

PURINGTON,  C.  W.:  The  Platinum  Deposits  of  the  Tura  River  System, 
Ural  Mountains,  Russia.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  29,  pp.  3-16, 1899. 

The  Occurrence  of  Platinum  in  the  Ural  Mountains.  Eng. 

and  Min.  Jour.,  vol.  77,  pp.  720-722,  1904. 

Methods  and  Costs  of  Gravel  and  Placer  Mining  in  Alaska. 

U.  S.  Geol.  Survey  Bull.  263,  1905. 

RICKARD,  T.  A.:  The  Gold  Fields  of  Otago.  Am.  Inst.  Min.  Eng.  Trans., 
vol.  21,  pp.  411-442,  1893. 

SMYTH,  H.  L.:  The  Origin  and  Classification  of  Placers.  Eng.  and  Min. 
Jour.,  vol.  79,  pp.  1045-1046,  1179-1180,  1228-1230,  1905. 

SPURR,  J.  E.:  "Geology  Applied  to  Mining,"  pp.  205-233,  1907. 

STORMS,  W.  H.:  Ancient  Gravel  Channels  of  Calaveras  County,  Cali- 
fornia. Min.  and  Sci.  Press,  vol.  91,  pp.  170-171,  192-193,  1905. 

TURNER,  H.  W. :  The  Cretaceous  Auriferous  Conglomerate  of  the  Cotton- 
wood  Mining  District,  Siskiyou  County,  California.  Eng.  and  Min.  Jour., 
vol.  76,  pp.  653-654,  1903. 

TYRRELL,  J.  B.:  The  Law  of  the  Pay  Streak  in  Placer  Deposits.  Inst. 
Min.  and  Met.  Trans.,  vol.  21,  pp.  593-612,  London,  1912. 

Concentration  of  Gold  in  the  Klondike.  Econ.  Geol.,  vol.  2, 

pp.  343-349,  1907. 

WITWATERSRAND  AURIFEROUS  CONGLOMERATES 

The  auriferous  conglomerates  of  Witwatersrand,  Transvaal, 
South  Africa,1  which  are  the  most  productive  gold  deposits  of 

1  HATCH,  F.  A.,  and  CORSTORPHINE,  G.  S.:  Petrography  of  the  Witwaters- 
rand Conglomerate.  South  Africa  Geol.  Soc.  Trans.,  vol.  7,  1904. 

HATCH,  F.  A. :  The  Conglomerate  of  the  Witwatersrand,  in  BAIN,  H.  F., 
and  others:  "Types  of  Ore  Deposits,"  pp.  202-218,  San  Francisco,  1911. 


420      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  world,  yield  about  $160,000,000  annually.  The  country  is  a 
hilly  area  of  crystalline  schists  (Swaziland  schists)  and  intruding 
granites  upon  which  rest  unconformably  slates,  quartzites,  and 
conglomerates  belonging  to  the  Witwatersrand  system,  which 
is  probably  also  pre-Cambrian  (Fig.  175).  Above  the  Wit- 
watersrand strata  are  conglomerates,  lavas,  and  breccias  of  the 
Ventersdorp  system;  above  the  Ventersdorp  unconformably  is 
the  Potchefstroom  system,  which  is  overlain  (also  uncon- 
formably) by  the  Table  Mountain  sandstones,  of  Devonian  age. 
Higher  still  in  the  series  are  the  coal-bearing  Karoo  beds,  which 


FIG.  175. — Plan  and  cross-section  of  part  of  Witwatersrand  region,  South 
Africa.     (Based  on  map  by  Hatch.) 


are  probably  of  Carboniferous  age.  The  Witwatersrand  system, 
which  contains  the  principal  ore  bodies,  forms  a  large  syncline 
in  which  the  beds  dip  at  high  angles. 

The  rocks  composing  the  Witwatersrand  system  consist  of 
conglomerate,  grit,  quartzite,  and  slate.  The  conglomerates 
contain  round  and  subangular  fragments  of  quartz  and  quartzite 
in  a  matrix  of  quartz  grains,  the  whole  cemented  by  secondary 
silica.  The  quartzites  are  made  up  of  quartz  sand,  thoroughly 
indurated  by  pressure  and  deposition  of  secondary  silica.  The 
slates  consist  largely  of  minute  quartz  fragments  and  sericite. 
The  Witwatersrand  beds  are  faulted  and  crossed  by  diabase 
dikes. 

The  principal  gold  deposits  are  in  the  upper  Witwatersrand 
series,  which  is  predominantly  quartzite  but  contains  four  well- 
defined  conglomerate  zones,  grouped  as  follows: 


GOLD  421 

4.  Kimberley  series. 
3.  Bird  Reef  series. 
2.  Livingstone  Reef  series. 
1.  Main  Reef  series. 

These  series  contain  numerous  beds  of  conglomerate  and 
sandstone  that  carry  gold.  The  Main  Reef  series  has  been 
worked  more  or  less  continuously  for  a  distance  of  46  miles. 

In  the  Central  Rand  (the  district  near  Johannesburg)  the  bulk 
of  the  gold  is  obtained  from  the  Main  Reef  leader  and  from  the 
South  Reef.  The  conglomerates  of  the  Main  Reef  consist 
mainly  of  rolled  fragments  of  quartz  pebbles,  with  fragments  of 
quartzite,  banded  chert,  and  slate.  The  pebbles,  which  are 
worn  smooth  and  round,  lie  in  a  matrix  which  originally  consisted 
of  quartz  sand  but  which  by  the  deposition  of  silica  has  been 
converted  into  a  compact  mass  of  quartz  on  freshly  fractured 
surfaces  of  which  even  the  boundaries  of  the  pebbles  are  difficult 
to  identify.  Besides  the  quartz  pebbles  and  the  quartz  sand, 
the  only  other  constituents  of  the  matrix  that  are  undoubtedly 
original  are  zircon  in  microscopic  crystals  and  chromite  and 
iridosmine  in  rounded  grains. 

The  minerals  later  than  the  quartz  pebbles  include  chloritoid, 
sericite,  calcite,  tourmaline,  rutile,  pyrite,  marcasite,  pyrrhotite, 
chalcopyrite,  zinc  blende,  galena,  stibnite,  cobalt  and  nickel 
arsenides,  graphite,  and  gold  telluride. 

The  origin  of  the  gold  in  the  Rand  conglomerates  is  still  in 
doubt.  Gregory,1  Becker,2  and  others  have  maintained  that  it 
is  of  placer  origin,  formed  by  concentration  from  the  Swaziland 
schists,  which  carry  stringers  of  quartz  and  gold.  They  state 
that  the  gold  is  concentrated  in  the  lower  parts  of.  the  beds  and 
attribute  its  angular  or  crystalline  condition  to  recrystallization, 
which  would  obliterate  rounded  surfaces  of  the  minute  particles 
of  gold  that  predominate  in  the  conglomerates.  Facts  that  are 
urged  as  opposed  to  the  theory  that  the  gold  was  carried  into  the 
conglomerates  by  hot  solutions  are:  an  absence  of  hydrothermal 
metamorphism,  a  fairly  regular  distribution  of  gold  in  the  con- 

1  GREGORY,  J.  W. :  The  Origin  of  the  Gold  in  the  Rand  Banket.     Econ. 
GeoL,  vol.  4,  pp.  118-129,  1909. 

2  BECKER,  G.  F. :  The  Witwatersrand  Banket,  with  Notes  on  other  Gold- 
Bearing  Pudding  Stones.     U.  S.  Geol.  Survey  Eighteenth  Ann.  Rept.,  part  5, 
p.  169,  1897. 


422      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

glomerate,  and  an  absence  of  gold  in  wall  rock  along  fractures  and 
of  cross-channels  of  .gold  ore. 

On  the  other  hand,  some  of  the  closest  students  of  the  area1 
believe  that  the  gold  was  brought  into  the  conglomerate  in 
solution,  together  with  other  metallic  minerals,  among  them 
pyrite,  sphalerite,  galena,  and  many  other  sulphides.  Some 
see  a  generic  relation  between  the  gold  deposits  and  diabase 
dikes  that  cut  the  ore-bearing  beds.  A  third  hypothesis  is  that 
there  has  been  some  infiltration  of  mineral-bearing  thermal 
waters  since  the  gold  was  deposited  as  placers. 

GOLD  LODES 

Porcupine,  Ontario. — The  Porcupine  gold  district,2  in  northern 
Ontario,  about  100  miles  northwest  of  Cobalt,  was  discovered  in 
1908.  It  lies  about  1,000  feet  above  the  sea  and  is  an  area  of  low 
relief  and  wooded  but  not  particularly  swampy.  The  surface  is 
covered  thinly  with  clay,  sand,  and  boulder  clay.  The  oldest 
formation  is  the  Keewatin,  consisting  of  volcanic  rocks  and  their 
metamorphosed  representatives.  This  formation  is  more  highly 
schistose  than  it  is  at  Cobalt.  The  prevailing  rock  at  Porcupine 
is  a  green  igneous  rock  (mainly  basalt)  which  is  cut  by  dikes  and 
other  masses  of  quartz  porphyry.  Associated  with  the  Keewatin 
also  are  sedimentary  rocks,  including  iron-bearing  jaspilites,  iron 
carbonate  rocks,  and  limestone.  Above  the  Keewatin  are  Huro- 
nian  quartzite  and  slate,  altogether  at  least  400  feet  thick.  This 
series  is  tilted  and  locally  rendered  schistose.  Granite  (Lauren- 
tian)  is  intruded  in  the  Keewatin;  possibly  some  of  the  granite  is 
intruded  in  the  Huronian  also,  but  as  granite  pebbles  are  found 
in  the  Huronian  sediments,  at  least  some  of  the  granite  is  older 

1  HATCH,  F.  A.,  and  CHALMERS,  J.  A.:  "The  Gold  Veins  of  the  Rand," 
London,  1895. 

HATCH,  F.  A.:  The  Conglomerates  of  the  Witwatersrand  Banket,  in 
BAIN,  H.  F.,  and  others:  "Types  of  Ore  Deposits,"  pp.  202-219,  San  Fran- 
cisco,  1911. 

HATCH,  F.  A.,. and  CCRSTORPHINE,  G.  S.:  Petrography  of  the  Witwaters- 
rand Conglomerate.  Geol.  Soc.  South  Africa  Trans.,  vol.  7,  part  3,  1904. 

8  BURROWS,  A.  G. :  The  Porcupine  Gold  Area,  Ontario  Bur.  Mines. 
Twenty-fourth  Ann.  Rept.,  part  3,  1915. 

HORE,  R.  E.:  The  Nature  of  Some  Porcupine  Gold  Quartz  Deposits. 
Canadian  Min.  Inst.  Jour.,  vol.  14,  p.  171,  1911. 

KNIGHT,  C.  W.:  Mineral  Associations  at  Porcupine.  Min.  and  Sci.  Press, 
April  15,  1911. 


GOLD 


423 


than  the  Huronian.     Both  Keewatin  and  Huronian  are  intruded 
by  diabase  dikes. 

The  ore  deposits  (Fig.  176)  are  veins  and  great  irregular  masses 
of  schist  seamed  and  impregnated  with  quartz  and  gold.  Some 
of  the  deposits  crop  out  conspicuously,  among  them  the  Dome 


FIG.  176. — Geologic  map  of  part  of  Porcupine  district,  Ontario.     (Based  on 
map  by  Burrows  and  Hopkins.) 


and  West  Dome,  so  called  from  the  shape  of  the  outcrops  at  the 
places  of  discovery.  The  domes  on  the  Dome  property  were 
about  100  by  125  feet.  The  deposits  occur  in  both  the  Keewatin 
and  Huronian  rocks,  and  generally  the  lodes  cut  across  the  schis- 
tosity.  The  lodes  range  in  attitude  from  horizontal  to  vertical 


424      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

and  show  a  strong  tendency  to  parallelism.  The  Hollinger 
group  strikes  northeast;  the  Dome  group  nearly  east.  Spectacu- 
lar showings  occur  on  many  properties,  but  these  -are  limited  to 
small  portions  of  the  veins.  Considering  the  irregular  character 
of  the  veins  and  the  large  quantity  of  country  rock  mined,  the 
ore  is  of  low  grade. 

Besides  quartz  and  pyrite  the  veins  carry  feldspar,  tourmaline, 
and  carbonates.  The  quartz  of  the  Hollinger  mine  contains 
liquid  and  gas  inclusions.  Copper  pyrite,  galena,  zinc  blende, 
and  pyrrhotite  are  found  in  some  veins.  Locally  the  ore  is 
fractured  and  cemented  with  quartz  and  pyrite.  These  veins 
were  probably  formed  at  great  depths  and  under  high  pressures, 
as  is  suggested  by  the  presence  of  tourmaline  and  gas  inclusions 
in  the  quartz.  Burrows1  believes  that  they  are  closely  related 
to  granitic  intrusions.  Near  Night  Hawk  Lake  are  aplite  dikes 
with  fine  veinlets  of  quartz  which  contain  a  little  gold. 

Southern  Appalachian  Region.— A  belt  of  ancient  rocks,  con- 
sisting of  crystalline  schists  and  granites  and  other  igneous  rocks, 
extends  from  Alabama  northeastward  to  Maine.  In  this  belt 
several  small  deposits  of  gold  have  been  found  in  Maine,  New 
Hampshire,  and  several  other  States,  but  none  north  of  Virginia 
has  produced  much  gold.  Valuable  deposits  are  located  in 
Alabama,  Georgia,  and  the  Carolinas.  Gold  has  been  mined  in 
the  southern  Appalachians  since  early  in  the  nineteenth  century. 
The  total  production  is  $50,000,000,  of  which  placers  have 
yielded  $30,000,000. 

The  southern  Appalachian  gold-bearing  region2  has  a  very  com- 
plex geologic  history.  The  rocks  are  of  Archean,  Algonkian,  and 
early  Paleozoic  age.  The  gold  deposits  are  far  from  uniform  in 
character.  Some  of  them  have  quartz  and  garnet  gangue;  others 

1  Op.   cU.,  p.  20. 

2  BECKER,  G.  F. :  Gold  Fields  of  the  Southern  Appalachians.     U.  S.  Geol. 
Survey  Sixteenth  Ann.  RepL,  part  3,  pp.  250-331,  1895. 

GRATON,  L.  C.,  andLiNDGREN,  WALDEMAR:  Reconnaissance  of  Some  Gold 
and  Tin  Deposits  of  the  Southern  Appalachians.  U.  S.  Geol.  Survey  Bull. 
293,  1906. 

McCASKEY,  H.  D.:  Notes  on  Some  Gold  Deposits  of  Alabama.  U.  S. 
Geol.  Survey  Bull.  340,  p.  36,  1908;  U.  S.  Geol.  Survey  Mineral  Resources, 
1908,  part  1,  pp.  645-681,  1909. 

TABER,  STEPHEN:  Geology  of  the  Gold  Belt  in  the  James  River  Basin,  Vir- 
ginia. Va.  Geol.  Survey  Bull.  7,  pp.  1-271,  1913. 


GOLD  425 

are  simple  quartz-pyrite-gold  veins;  but  none  are  of  the  anti- 
mony-silver-gold type  that  is  so  conspicuously  developed  in 
the  West. 

The  Appalachian  gold  veins  are  almost  uniformly  of  low  grade. 
As  stated  by  Lindgren,  they  were  probably  formed,  in  the  main, 
3  or  4  miles  below  the  surface  at  the  time  of  deposition.  Many 
of  them  are  in  mica  schist  and  other  crystalline  rocks,  and  some 
are  closely  associated  with  granitic  intrusions.  Some  of  them 
are  cut  by  diabasic  intrusives,  presumably  later  than  the  ore. 
The  minerals  include  quartz,  sericite,  biotite,  fluorite,  gold,  pyrite, 
galena,  zinc  blende,  pyrrhotite,  chalcopyrite,  and  magnetite. 

Few  of  these  deposits  have  been  extensively  explored  in  depth, 
and  data  respecting  the  vertical  distribution  of  the  gold  are  there- 
fore meager.  Many  of  them  are  profitable  near  the  surface, 
partly  by  reason  of  the  rotten  condition  of  the  rock,  which  ren- 
ders it  more  easily  worked,  and  partly  because  gold  is  accumu- 
lated or  enriched  by  the  removal  of  valueless  material.  At  the 
Haile  mine,  near  Kershaw,  S.  C.,  the  deposits  are  in  quartz-seri- 
cite  schist  and  metamorphosed  igneous  rocks.  Large  diabase  dikes 
cut  the  schist,  and  near  them  the  ore  bodies  are  developed. 
These  dikes,  however,  are  probably  later  than  the  ore.  Accord- 
ing to  Graton,  the  limit  of  profitable  mining  is  in  general  less  than 
200  feet  below  the  limit  of  complete  oxidation.  In  this  zone 
scales  of  pyrite  and  free  gold,  probably  secondary,  are  found  in 
joint  cracks.  At  Dahlonega,  Ga.,  auriferous  veins  of  quartz 
and  garnet,  with  mica  and  hornblende,  are  inclosed  in  schist. 
These  veins,  particularly  the  decomposed  surface  outcrops,  have 
produced  considerable  gold.  They  are  lenticular  and  in.  part 
replace  the  wall  rocks. 

Certain  ore  deposits  of  Alabama  comprise  fissure  veins  in 
granite  and  lenticular  bodies  in  schists.  The  principal  minerals 
are  quartz,  pyrite,  and  gold.  Garnet  is  found  in  the  vein  quartz 
at  Pinetucky.  Weathering  extends  to  water  level,  which  lies 
40  to  80  feet  below  the  surface.  The  ores  are  oxidized  above  this 
level  and  are  generally  free  milling,  but  the  ore  so  far  obtained 
below  this  level  is  not  profitably  amalgamated.  The  deposits 
are  fairly  regular  in  width  and  tenor,  and  no  evidences  of  enrich- 
ment below  the  water  level  are  recorded. 

The  copper  deposits  of  Ducktown,  Tenn.,  carry  a  little  gold 
(page  390). 


426      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Black  Hills,  South  Dakota.— The  Black  Hills  of  South  Dakota1 
yield  annually  about  $7,500,000  in  gold,  considerable  tungsten 
and  silver,  and  some  lead.  The  Homestake  Mining  Co.,  with 
associated  companies,  according  to  Blackstone,2  to  the  end  of  1915 
had  produced  gold  valued  at  $141,610,382.38.  In  1915  it  pro- 
duced 1,573,822  tons  of  ore  valued  at  $6,428,786.56,  or  $4.08 
per  ton. 

ORE  SENT  TO  GOLD  AND  SILVER  MILLS  IN  SOUTH  DAKOTA  IN  1915 


Process 

Ore 
short  tons 

Gold  in  bul- 
lion 
(fine  ounces) 

Silver  in  bul- 
lion 
(fine  ounces) 

Amalgamation  
Cyanidation  .  .      .    .        

1,573,822 
"1,869,248 

228,315.55 
127,901.81 

63,554 
132,806 

356,217.36 

196,360 

0  Includes  1,554,472    tons  of  tailings  from  part  of  ore  amalgamated. 

The  Black  Hills  constitute  a  domical  uplift,  rising  above  the 
Great  Plains.  The  central  peaks  are  of  pre-Cambrian  schists. 
They  slope  gradually  outward  to  a  rim  of  Paleozoic  rocks  that 
dip  away  from  the  hills.  The  sedimentary  rocks  (Fig.  177), 
including  those  as  late  as  Cretaceous,  are  cut  by  many  varieties  of 
igneous  dikes,  stocks,  and  laccoliths.  Alkali-rich  rocks  such  as 
syenite  porphyry  and  phonolite  are  represented.  As  the  Ter- 
tiary (Oligocene?)  beds  at  Lead  contain  pebbles  of  the  porphyries 
the  latter  are  presumably  early  Tertiary. 

The  deposits  are  (1)  gold  deposits  in  pre-Cambrian  schists; 
(2)  ancient  placers  in  the  Cambrian  basal  conglomerate;  (3) 
siliceous  gold  ores  replacing  thin  calcareous  beds  in  the  Cam- 

1  IRVING,  J.  D.,  EMMONS,  S.  F.,  and  JAGGAR,  T.  A. :  Economic  Resources 
of  the  Northern  Black  Hills.     U.  S.  Geol.  Survey  Prof.  Paper  26,  1904. 

SHARWOOD,  W.  J. :  Analyses  of  Some  Rocks  and  Minerals  from  the  Home- 
stake  Mine,  Lead,  S.  D.  Econ.  Geol,  vol.  6,  p.  729,  1911. 

DEVEREUX,  W.  B.:  The  Occurrence  of  Gold  in  the  Potsdam  Formation, 
Black  Hills,  Dakota.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  10,  p.  496,  1882. 

PAIGE,  SIDNEY:  Pre-Cambrian  Structure  of  the  Northern  Black  Hills, 
South  Dakota,  and  Its  Bearing  on  the  Origin  of  the  Homestake  Ore  Body. 
Geol.  Soc.  America  Bull,  vol.  24,  pp.  293-300,  1913. 

2  BLACKSTONE,   RICHARD:  A   History  of  the   Homestake   Mine,   South 
Dakota.     Min.  and  Eng.  World,  July  15,  1916,  pp.  99-102;  South  Dakota 
School  of  Mines  Pahasapa  Quart.,  June,  1916,  pp.  16-30. 


GOLD 


427 


brian;  (4)  gold-bearing  replacement  veins  in  Carboniferous  rocks; 
(5)  silver-lead  replacement  veins  in  Cambrian  and  Carboniferous 
rocks;  and  (6)  recent  placers. 

The  pre-Cambrian  ores  of  the  Homestake  belt  (Figs.  178,  179) 
are  in  a  belt  3  miles  long  and  2,000  feet  wide.  The  rocks  are 
quartzites  and  mica  schists  (metamorphosed  sediments)  and 


FIG.  177. — Generalized  columnar  section  of  northern  Black  Hills,  South 
Dakota.     (After  Jaggar,  U.  S.  Geol.  Survey.) 

amphibolites.  Pre-Cambrian  granite  crops  out  not  far  away. 
The  minerals  of  the  ore  are  quartz,  dolomite,  calcite,  pyrite,  ar- 
senopyrite,  pyrrhotite,  and  gold,  with  which  are  associated  the 
minerals  of  the  schist — quartz,  orthoclase,  hornblende,  biotite, 
garnet,  tremolite,  actinolite,  titanite,  and  graphite.  The  ore 
bodies  are  cut  by  many  dikes  of  porphyry  but  apparently  have 
not  been  much  affected  by  them.  The  ores,  though  of  low 
grade,  are  very  profitable.  Some  of  the  ores  at  the  surface 


428       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

were  below  the  average  in  tenor,  but  other  surface  ores  were  two 
or  three  times  as  rich  as  the  average.  The  valuable  minerals 
extend  downward  as  far  as  exploration  has  gone  and  are  fairly 
uniform  to  depths  2,000  feet  or  more  below  the  surface.  In 
general  enrichment  by  surface  leaching  is  subordinate. 


FIG.  178.— Plan  of  Homestake  ore  bodies  on  300-foot  level,  Black  Hills, 
South  Dakota.     (After  Irving,  U.  S.  Geol.  Survey.} 

The  schists  represent  rocks  of  various  types  intensely  metamor- 
phosed by  pressure.  The  predominating  rock  associated  with  the 
ore  appears  to  be  metamorphosed  calcareous  slate.  The  garnet, 


FIG.  179.— Section  Homestake  ore  body,  Father  de  Smet  mine,  Black  Hills, 
South  Dakota.     (After  Irving,  U.  S.  Geol.  Survey.) 

as  shown  by  Irving,  is  generally  broken  and  shattered,  and  the 
ore-bearing  rock  is  highly  contorted.  The  mineral  association 
suggests  formation  at  great  depths. 

The  deposit  at  the  Clover  Leaf  mine,  7  miles  southeast  of  the 
Homestake  mine,  is  a  "saddle  reef"  in  schists. 

Near  the  Homestake  mine  are  flat-lying  conglomerates  at  the 


GOLD  429 

base  of  the  Cambrian  that  contain  considerable  detrital  gold, 
evidently  derived  from  the  Homestake  deposits.  The  general 
relations  are  shown  by  Fig.  173.  According  to  Irving,  some 
pyrite  has  developed  in  some  of  the  Cambrian  placers  (No. 
5  in  Fig.  177)  since  they  were  formed. 

Valuable  ore  bodies  were  formed  also  in  the  Tertiary  period. 
The  chief  of  these  are  long,  flat-lying  ribbons  of  ore  in  the  Cam- 
brian, not  far  above  its  base.  These  have  been  termed  "re- 
fractory siliceous  ore"  and  "Potsdam"  ore.  Some  of  these 
deposits  are  nearly  1  mile  long;  they  are  from  10  to  300  feet  wide 
and  have  an  average  thickness  of  about  6  feet.  At  the  base  of 
the  Cambrian  series  is  a  conglomerate  which  grades  upward  into 
quartzite.  Above  the  quartzite  is  a  dolomite  bed  about  6  feet 
thick,  and  this  is  succeeded  by  300  feet  of  shale,  sandstone,  and 
limestone.  The  siliceous  gold  ores  occur  extensively  at  two 
horizons — one  in  the  lowest  dolomite  (No.  4,  Fig.  177)  and  one 
about  25  feet  below  the  "Scolithus"  bed  (No.  2,  Fig.  177). 
The  ribbons  of  ore  are  capped  by  blue  shale.  The  minerals  are 
pyrite,  quartz,  fluorite,  gold,  and  some  silver.  As  shown  by 
Irving,  they  are  found  where  the  limestone  beds  are  crossed  by 
small  fractures  called  verticals.  Although  these  are  generally 
very  thin  they  are  closely  spaced,  especially  where  the  ribbons 
are  wide.  These  deposits  are  in  the  areas  intruded  by  the  por- 
phyries and  are  probably  genetically  related  to  them.  They  are 
classic  examples  of  ribbons  of  ore  formed  at  intersections  of 
fissures  with  favorable  beds.  The  fractures  pass  through  the 
quartzite  and  some  of  them  pass  upward  into  shale.  The  metal- 
lizing solutions,  which  spread  out  and  deposited  large  bodies  of 
ore  in  the  limestone,  had  little  effect  on  the  quartzite  and  shale. 

Gold-silver  replacement  veins  in  Carboniferous  rocks  are  found 
near  Ragged  Top,  a  laccolithic  body  of  phonolite.  The  lodes 
are  silicified  brecciated  zones  in  limestone.  The  minerals  are 
pyrite,  opaline  silica,  fluorite,  and  tellurides  (probably  sylvanite). 

Silver-lead  deposits  in  the  Cambrian  and  Carboniferous  rocks 
were  formerly  productive.  Some  are  replacement  deposits  along 
fractures  in  sedimentary  rocks  and  porphyries. 

Near  Camp  Carbonate  irregular  dikes  and  sills  of  porphyry 
are  intruded  in  Carboniferous  limestone.  Irregular  bodies  of 
carbonate  and  galena  ore  have  formed  by  replacement. 

Recent  placers  that  have  developed  from  older  deposits  are 
found  along  the  present  streams.  Tungsten  deposits  are  ex- 


430      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

tensively  mined  in  the  Cambrian  sedimentary  beds.  These  are 
mentioned  on  page  525. 

California  Gold  Belt. — The  California  gold  belt  extends  north- 
westward, with  some  interruptions,  through  the  length  of  the 
State  and  continues  northward  into  Oregon  but  disappears 
beneath  Tertiary  lavas,  to  reappear  in  British  Columbia  and 
Alaska.  The  lodes,  with  associated  gold  deposits  of  the  same 
general  type  and  placers  derived  from  them,  have  produced  over 
$1,300,000,000  in  gold. 

The  geologic  history  of  this  region  is  long  and  varied.  The 
lowest  series  in  the  geologic  column  is  a  greatly  folded  and  meta- 
morphosed complex  consisting  of  Paleozoic  sedimentary  rocks 
and  interbedded  lavas,  called  the  Calaveras  formation.  Above 
this  complex  are  Jurassir  and  Triassic  rocks,  less  intensely  folded 
than  the  Calaveras.  Intruded  into  these  is  an  enormous  grano- 
diorite  batholi'th  which  forms  the  main  mass  of  the  range.  On 
and  near  the  boundary  of  the  central  intrusive  mass  are  many 
smaller  bodies  of  granodiorite,  diorite,  and  gabbro.  These  are 
doubtless  of  about  the  same  age  and  of  similar  origin.  As  the 
Mariposa  formation  (Jurassic)  is  intruded  by  the  granodiorite, 
and  as  the  Chico  (Cretaceous)  is  not  intruded  by  it,  the  age  of 
the  batholith  is  known  to  be  early  Cretaceous. 

The  gold  deposits1  are  notably  sparse  in  the  central  grano- 
diorite belt  but  are  clustered  in  and  around  the  smaller  intruding 
bodies,  especially  on  the  west  slopes  of  the  Sierra.  The  deposits 
are  veins.  Some  of  them  are  arranged  in  conjugated  systems — 
for  example,  those  at  Nevada  City,  Grass  Valley,  and  Ophir. 

The  Mother  Lode  (Fig.  180)  is  a  belt  of  strong,  closely  spaced 
veins  over  100  miles  long  and  not  much  more  than  a  mile  wide. 
It  parallels  the  axis  of  the  range  and  the  general  trend  of  the 
formations.  Many  veins  of  this  group  strike  about  N.  25°  W. 


^INDGREN,  WALDEMAR:  Gold-Silver  Veins  of  Ophir,  Calif.  U.  S.  Geol. 
Survey  Fourteenth  Ann.  Rept. ,  part  2,  pp.  243-284,  1893. — The  Gold-Quartz 
Veins  of  Nevada  City  and  Grass  Valley,  Calif.  U.  S.  Geol.  Survey  Seven- 
teenth Ann.  Rept.  part  2,  pp.  1-262,  1896.— Characteristic  Features  of  the 
California  Gold-Quartz  Veins.  Geol.  Soc.  America  Bull,  vol.  6,  pp.  221- 
240,  1896. 

RANSOME,  F.  L.:  U.  S.  Geol.  Survey  Geol.  Atlas,  Mother  Lode  district, 
folio  (No.  63);  1900. 

STORMS,  W.  H. :  The  Mother  Lode  Region,  California.  Cal.  State  Min. 
Bur.  Bull.  18,  1900. 


GOLD  431 

and  dip  about  60°  E.     Single  deposits  are  developed  for  more  than 
a  mile  along  the  strike. 

As  a  rule  the  deposits  are  of  low  grade,  the  average  tenor  being 
about  $4  a  ton.  Some  mines  exploit  large  bodies  of  ore  that 
carry,  less  than  $3  a  ton,  but  pockets  of  very  rich  ore  have  been 
found.  The  principal  gangue  minerals  are  quartz,  carbonates, 
and  albite.  As  a  rule  the  gold  is  associated  with  sulphides,  in- 
cluding pyrite,  arsenopyrite,  and  pyrrhotite,  with  pyrite,  chalco- 
pyrite,  and  galena.  Specularite,  magnetite,  tetrahedrite,  molyb- 
denite, telluride,  and  scheelite  occur  only  locally.  The  sulphides 
constitute  only  a  small  percentage  of  the  ore.  By  mechanical 
concentration  deposits  of  very  low  grade  are  profitablv  worked. 
The  silver  content  is  small  in  most  of  the  deposits. 


Co—  Carboniferous  Mica  Schist  am  —  Amphibolitic  Schists 

ma—  Altered  Andesitic  Tuffs, Breccias  and  Lavas  ffldt  —  Altered  Quartz  DioritB 

Jm  —  jura  Trial  Slates,  Sandstones,  and  Conglomerate  V=  Quartz  Yein 

FIG.  180. — Cross-section  of  Mother  Lode  region,  California.     (After  Ron- 
some,  U.  S.  Geol.  Survey.) 

There  is  no  evidence  that  tnese  deposits  have  been  enriched 
by  redeposition  of  gold ;  the  outcrops  are  as  rich  as  or  richer  than 
the  ores  in  depth,  and  some  of  the  deposits  have  been  followed 
down  the  dip  nearly  a  mile  without  notable  change  in  value. 
Placers  of  enormous  extent  and  value  occur  in  this  belt  and  have 
yielded  more  than  half  the  gold  derived  from  the  deposits.  At 
some  places  extensive  stream  placers  are  covered  by  Tertiary 
lavas.  Doubtless  many  thousand  feet  of  material  has  been 
eroded  from  these  veins  to  supply  the  placer  gold.  They  are 
nevertheless  classed  with  veins  formed  at  intermediate  depths, 
for  minerals  indicating  high  temperature  and  pressure  are  only 
sparingly  and  locally  developed.  As  a  whole  the  group  forms  a 
transition  type  between  the  ores  found  in  the  deep-vein  zone  and 
those  found  at  moderate  depths.  The  remarkable  persistence 
of  the  ore  in  depth  points  to  ascending  solutions  as  the  agents 
of  deposition,  and  the  geographic  relations  of  the  deposits  to 
intruding  igneous  rocks  suggest  a  close  genetic  relation  between 
the  igneous  rocks  and  the  ore-depositing  solutions. 


432       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


PRODUCTION  OP  CERTAIN   METALS  IN  CALIFORNIA,  1911-1915" 


Year 

Mines 
pro- 
ducing 

Ore, 
(short 
tons) 

Gold 

Silver, 
(fine 
ounces) 

Copper, 
(pounds) 

Lead, 
(pounds) 

Zinc, 

(pounds) 

Total 
value 

1911 
1912 
1913 
1914 
1915 

1,181 
1.041 
796 
658 
608 

2,797,261 
2,641,497 
2,495,958 
2,465,485 
3,002,779 

$19,738,908 
19,713,478 
20,406,958 
20,653,496 
22,442,296 

1,270,445 
1,300,136 
1,378,399 
1,471,859 
1,678,756 

36,316,136 
33,451,672 
34,575,007 
30,507,692 
40,751,625 

1,398,111 
1,144,731 
3,514,342 
4,251,923 
4,579,245 

2,807,035 
4,345,591 
1,057,485 
389,471 
13,094,032 

$25,174,677 
26,383,946 
26,812,487 
25,710,645 
32,263,844 

"Yale,  C.  G.;  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  207,  1916. 

Neva'da  City  and  Grass  Valley,  California. — The  Nevada  City 
and  Grass  Valley  district,1  which  is  one  of  the  most  steadily  pro- 
ductive in  California,  includes  metamorphosed  Carboniferous 
sedimentary  rocks,  compressed  into  isoclines,  and  associated 
igneous  rocks  less  intensely  metamorphosed.  Above  these  are 
slates  with  associated  diabase  and  serpentine.  These  rocks  are 
folded  and  metamorphosed  but  are  not  so  intensely  compressed 
as  the  Carboniferous.  Intruded  into  them  are  great  bodies  of 
granodiorite,  probably  of  early  Cretaceous  age.  The  ore  de- 
posits are  strong  veins,  formed  after  the  granodiorite  intrusions. 
The  gold  content  is  somewhat  higher  than  in  ores  of  the  Mother 
Lode.  The  minerals  are  quartz,  chalcedony,  magnetite,  sericite, 
mariposite,  pyrite,  pyrrhotite,  .chalcopyrite,  galena,  zinc  blende, 
scheelite,  arsenopyrite,  tetrahedrite,  stephanite,  and  cinnabar. 

Near  the  surface  the  upper  part  of  a  vein  is  generally  decom- 
posed, forming  a  mass  of  limonite  and  quartz.  The  decomposi- 
tion does  not  as  a  rule  extend  more  than  200  feet  on  the  incline 
of  a  vein  dipping  45°,  or  more  than  150  feet  below  the  surface. 
The  surface  ore  is  generally  richer  than  the  deeper  ore,  owing  to 
the  liberation  of  gold  from  the  sulphides  and  the  removal  of  sub- 
stances other  than  gold.  In  this  process  silver  also  is  partly  re- 
moved. In  some  of  the  mines  the  lodes  have  been  followed  down 
the  dip  for  over  3,000  feet.  The  unoxidized  ore  shows  no  gradual 
diminution  of  tenor  in  the  pay  shoots  below  the  zone  of  surface 
decomposition.  Valuable  placer  deposits  were  formed  from  these 
veins. 

"  Juneau,  Alaska. — The  California  gold  belt  extends  northward 
into  Oregon  and  Washington,  and  in  much  of  the  area  of  these 

1  LINDGREN,  WALDEMAR  :  The  Gold-Quartz  Veins  of  Nevada  City  and 
Grass  Valley  Districts,  California.  U.  S.  Geol.  Survey  Seventeenth  Ann. 
Rept.  part  2,  pp.  1-262,  1896. 


GOLD  433 

States  it  is  probably  below  later  lavas.  Farther  north  rocks  and 
deposits  nearly  related  to  those  of  the  California  gold  belt  are 
present  in  abundance.  The  gold  deposits  of  the  Juneau  belt, 
Alaska,1  as  pointed  out  by  Spencer,  are  probably  similar  in  age 
and  origin  to  the  California  veins.  The  Treadwell  group  of 
mines,  on  Douglas  Island,  exploits  the  most  productive  lode 
deposits  in  Alaska.  The  district  has  been  producing  gold  almost 
continually  since  1880.  Although  the  ore  as  mined  carries  only 
about  $2  a  ton,  operations  are  highly  profitable  owing  to  the  low 
cost  of  mining  and  milling.  On  the  mainland,  across  the  narrow 


FIG.  181. — Cross-section  through  Alaska  Treadwell  mine  and  north  side  of 
Douglas  Island,  Alaska.     (After  Spencer,  U.  S.  Geol.  Survey.) 

Gastineau  Channel,  a  short  distance  from  the  Treadwell  mines, 
the  Alaska  Gold  Mines  Co.  and  the  Alaska  Juneau  Co.  are 
mining  great  low-grade  deposits  on  a  scale  comparable  to  opera- 
tions in  the  disseminated  ("porphyry")  copper  deposits  of  the 
Southwest. 

The  rocks  of  this  region  consist  of  Paleozoic  greenstones, 
slates,  and  schists,  all  of  which  are  changed  by  pressure  and 
crop  out  as  parallel  belts  trending  northwest.  The  dip  is  in 
general  northeast.  The  Paleozoic  rocks  are  intruded  by  gabbro, 
diorite,  and  diorite  porphyry. 

At  the  Treadwell  mines  (Fig.  181)  great  dikes  of  albite  diorite 
intrude  the  greenstones  and  schist,  and  the  shattered  diorite  has 
been  extensively  replaced  by  mineralizing  solutions  and  cemented 
by  low-grade  gold  ore.  Hydrothermal  alteration  is  attended  by 
the  development  of  albite  and  calcite.  The  minerals  include  also 
quartz,  rutile,  mariposite,  chlorite,  epidote,  siderite,  pyrite, 

1  SPENCER,  A.  C. :  The  Juneau  Gold  Belt,  Alaska.  U.  S.  Geol.  Survey 
Bull.  287,  1906. 

HERSHEY,  O.  H.:  Geology  of  the  Treadwell  Mines.  Min.  and  Sci.  Press, 
vol.  102,  pp.  296-300,  334^335,  1911.  Also  in  BAIN,  H.  F.,  and  others, 
"Types  of  Ore  Deposits,"  pp.  157-171,  San  Francisco,  1911. 


434      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

pyrrhotite,  magnetite,  chalcopyrite,  and  molybdenite.  No 
manganese  minerals  are  reported. 

The  ore  bodies  of  Douglas  Island  are  the  albite  diorite  dikes 
seamed  and  impregnated  with  pyrite,  gold,  and  other  minerals. 
They  are  locally  more  than  200  feet  wide  and  extend  downward 
to  great  depths,  2,000  feet  or  more.  According  to  Spencer  the 
ore  shows  no  progressive  change  in  appearance  or  value  with 
increasing  depth. 

Cripple  Creek,  Colo. — The  Cripple  Creek  district,  Colorado,1 
is  about  10,000  feet  above  sea  level,  in  the  high  country  southwest 
of  Pikes  Peak.  It  has  produced  over  $250,000,000  in  gold. 
The  ore,  which  generally  runs  from  $12  to  $20  a  ton,  is  roasted 
and  treated  by  cyanidation.  Some  lots  much  richer  are  smelted. 
In  1915  Cripple  Creek  produced  948,082  tons  of  ore  containing 
$13,683,494  in  gold.  A  little  silver  is  recovered,  with  insignifi- 
cant quantities  of  other  metals. 

The  oldest  rocks  of  the  reTio1-  •"•  r  3-Cambrian  granite, 
gneiss,  and  schist.  A  volcar '  i  Tertiary  age  2  or  3  miles 

in  diameter  breaks  through  the  p^o  Cambrian  rocks  (page  213). 
This  neck  is  the  core  of  a  volcano  through  which  lavas  were 
thrown  out  upon  the  ancient  rocks,  but  most  of  the  lavas  have 
been  removed  by  erosion.  The  volcanic  neck  is  composed 
mainly  of  tuffs  and  breccias  of  latite-phonolite  which  are  cut  by 
dikes  and  stocks  of  latite-phonolite,  syenite,  and  other  alkali- 
rich  intrusives.  Phonolite  and  basic  dikes  cut  the  Tertiary  rocks 
and  the  pre-Cambrian  rocks  near  the  volcanic  center.  The  ore 
deposits  (Fig.  182)  are  veins  that  were  formed  soon  after  the 
basic  dikes.  Many  of  them  lie  along  the  walls  of  the  dikes  and, 
like  the  dikes,  are  rudely  radial  about  the  volcanic  neck.  The 
veins  were  formed  by  filling  small  openings  along  fissures  and 
sheeted  zones  and  subordinately  by  replacement.  Some  ir- 
regular replacement  deposits  occur  in  shattered  granite.  The 
fissures  are  believed  by  Lindgren  and  Ransome  to  have  been 
produced  by  stresses  attending  the  settling  of  the  volcanic  mass. 

Hydrothermal  action  was  not  particularly  intense  although  it 
was  widespread,  especially  in  the  porous  volcanic  breccia. 

1  PENROSE,  R.  A.  F. :  Mining  Geology  of  the  Cripple  Creek  District,  Col- 
orado. U.  S.  Geol.  Survey  Sixteenth  Ann.  Rept.,  part  2,  p.  123,  1895. 

LINDGREN,  WALDEMAR,  and  RANSOME,  F.  L. :  Geology  and  Gold  Deposits 
of  the  Cripple  Creek  District,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper  54, 
pp.  167-168,  1906. 


GOLD 


435 


Dark  silicates  were  changed  to  pyrite,  carbonates,  and  fluorite, 
and  feldspars  and  feldspathoids  were  changed  to  sericite  and 
adularia.  Calaverite  is  the  chief  primary  constituent  of  the 
ores;  native  gold  is  rarely  present  in  the  unoxidized  ores.  Pyrite 
is  widely  distributed;  tetrahedrite,  stibriite,  sphalerite,  and 
molybdenite  are  sparingly  present.  The  gangue  is  made  up  of 


FIG.  182.— Section  through  Stratton's  Independence  mine,  Cripple  Creek, 
Colorado,  showing  relation  of  veins  to  granite-breccia  contact.  (After 
Lindgren  and  Ransome,  U,  S.  Geol.  Survey.) 

quartz,  fluorite,  adularia,  carbonates  (including  rhodochrosite), 
some  sulphates,  and  other  minerals.  Some  of  the  deposits  were 
workable  at  the  surface,  but  the  placers  formed  are  of  relatively 
little  value. 

In  general,  the  lower  part  of  the  zone  of  oxidation  is  above 
water  level  and  is  usually  less  than  200  feet  deep.  Exploration  is 


436      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

extensive  to  depths  of  1,500  feet  below  the  surface.  Whether 
a  slight  enrichment  of  gold  has  taken  place  in  the  superficial 
zone  is  not  easy  to  demonstrate.  The  oxidized  zone  as  a  whole 
is  probably  somewhat  richer  than  the  corresponding  telluride 


Spearhead  rhyolite. 


Siebe't  formation. 


Siebert  formation. 


Dacite  vitrophyre. 


Chisjmandesite. 


Daclte  vitrophyre. 


Mllltown  andeslte  and  intrusive  doclte. 


Sandstorm  rbyollte  cut  by  dactte  and  Morena  rhyolit 


Kendall  tuff  cut  by  dncite  and  Morena  rhyolit 


Latlte  cut  by  daclte  and  Morena  rhyolite. 


Vindicator  rhyolite 


kite  and  granite  intrusive  Into  Cambrian  shale. 


Long  erosion  Interval. 


FIG.  183. — Generalized  columnar  section  of  the  rocks  of  the  Goldfield  dis- 
trict, Nevada.     (After  Ransome,  U.  S.  Geol.  Survey.) 

zone.     The  enrichment  in  this  zone,  however,  may  have  resulted 

from  the  removal  of  valueless  constituents  of  the  primary  ore. 

If  gold  was  dissolved  in  the  Cripple  Creek  deposits  it  was 

precipitated  again  at  practically  the  same  horizon,  for  in  these 


GOLD 


437 


deposits  the  zone  in  which  solution  usually  takes  place  is  rich. 
The  ground  is  open,  providing  paths  for  downward-circulating 
waters,  but  although  the  ore-bearing  complex  is  very  pervious 
to  water  it  is  surrounded  by  impervious  rocks  After  the  volcanic 
rocks  had  been  drained  in  mining  the  flow  of  water  was  compara- 
tively small.  Lindgren  and  Ransome  have  compared  the 
volcanic  complex  to  a  "sponge  in  a  cup."  As  shown  by  them, 


FIG.  184. — Cross-section    of    January    mine,    Goldfield,    Nevada.     (After 
Ransome,  U.  S.  Geol.  Survey.) 

the  conditions  were  unfavorable  for  the  circulation  of  atmospheric 
water. 

Interesting  experiments  bearing  on  the  downward  migration 
of  gold  at  Cripple  Creek  have  been  made  by  Nishihara,1  who 
showed  that  alkali-rich  minerals  such  as  nepheline  and  leucite 
reduce  the  acidity  of  solutions  very  rapidly. 

Goldfield,  Nev. — The  Goldfield  district,   Nevada,2  is  essen- 

1  NISHIHARA,  G.  S. :  The  Rate  of  Reduction  of  Acidity  of  Descending 
Waters  by  Certain  Ore  and  Gangue  Minerals  and  Its  Bearing  upon  Second- 
ary Sulphide  Enrichment.     Econ.  Geol.,  vol.  9,  pp.  743-757,  1914. 

2  RANSOME,    F.  L.:  The  Geology  and  Ore  Deposits  of  Goldfield,  Nev. 
U.  S.  Geol.  Survey  Prof.  Paper  66,  p.  27,  1909.  • 


438      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


tially  a  low  domical  uplift  of  Tertiary  lavas  and  lake  sediments 
resting  upon  ancient  granitic  and  metamorphosed  sedimentary 
rocks.  Erosion  of  the  flat  dome  has  exposed  the  pre-Tertiary 
rocks  at  several  places,  and  these  are  surrounded  by  wide  zones 
of  younger  formations.  Some  of  the  later  lavas  were  erupted 
after  the  dome  had  been  elevated  and  truncated.  The  Tertiary 
rocks  are  mainly  volcanic,  as  is  indicated  by  Fig.  183.  The  da- 
cite  occupies  a  considerable  area  east  of  Goldfield  and  is  the 
principal  country  rock  of  the  larger  mines.  Some  rich  shoots 
were  found  also  in  the  Milltown  andesite,  in  latite,  and  in  the 
Sandstorm  rhyolite. 

The  region  of  the  deposits  is  complexly  fissured,  but  faults 
are  few  and  of  small  throw,  and  the  Tertiary  beds  are  not  steeply 
tilted.  The  fracture  system  is  very  irregular.  The  rocks, 
especially  the  dacite,  have  undergone  extensive  hydrothermal 
alteration,  attended  by  the  development  of  much  alunite,  kaolin, 
quartz,  and  pyrite.  The  deposits  are  associated  with  iron- 
stained  silicified  craggy  outcrops,  but  a  great  many  of  these  out- 
crops are  barren.  The  deposits  (Fig.  184)  are  related  to  fissures 
and  were  formed  by  replacement;  only  a  small  part  of  the  ore  has 
actually  filled  fractures.  Ransome  terms  them  "ledges,"  using 
the  term  in  a  sense  less  definite  than  "veins"  to  designate  the 
masses  of  silicified  or  otherwise  altered  rock  in  which  the  ore 
bodies  are  found.  The  workable  ore  bodies  are  very  irregularly 
distributed  in  the  ledges. 

METAL  PRODUCTION  OP  GOLDFIELD  DISTRICT,  NEVADA,  1914-1915° 


Year 

Ore 

(short 
tons) 

Gold 

Silver 
(fine 
ounces) 

Copper 
(pounds) 

Lead 

(pounds) 

Total 
value 

1914       

367,166 

$4,705  169 

129,830 

1,069,021 

4,018 

$4,919,302 

1915  

418,935 

4,389,385 

165,306 

1,679,423 

4,767,094 

Total.  1914-1915  

75,700,937 

998,748 

4,819,203 

23,662 

77,022,591 

"HEIKES,  V.  C.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  631,  1916. 

The  sulphide  ores  are  mineralogically  complex  and  are  accom- 
panied by  minerals  containing  copper,  silver,  antimony,  arsenic, 
bismuth,  and  tellurium.  In  some  ores  the  gold  occurs  free,  in 
fine  particles,  so  closely  crowded  in  the  flinty  quartz  gangue  as 
to  form  yellow  bands.  Associated  minerals  are  pyrite,  marcasite, 
bismuthinite,  'famatinitfe  (OugSbS*),  and  gbldfieldite  (a  cxipric 


GOLD  439 

sulphantimonite).  A  concentric  crustification  is  characteristic 
of  the  richest  ores,  fragments  of  silicified,  alunitized,  and  pyrit- 
ized  rock  being  covered  with  shells  of  gold  and  sulphides.  Near 
the  surface  oxidized  ores  predominate.  Among  the  minerals  of 
this  ore  are  kaolin,  alunite,  gypsum,  barite,  alum,  and  iron  oxides. 
Silver  halides  are  not  common,  and  manganese  compounds  are 
practically  unknown.  No  valuable  placers  have  been  formed  in 
this  region. 

The  deposits,  according  to  Ransome,  are  probably  late  Miocene 
or  early  Pliocene.  As  the  Siebert  tuffs  are  alunitized  and  are 
not  known  to  contain  fragments  of  alunitized  dacite,  and  as  the 
Pozo  formation  contains  fragments  of  alunitized  dacite,  the  met- 
allization is  probably  post-Siebert  and  pre-Pozo  (see  Fig.  183). 

The  ledges  were  probably  deposited  near  the  Tertiary  surface. 
They  are  believed  to  have  been  formed  by  hot  ascending  solutions 
from  a  deep  source,  which  mingled  with  descending  sulphate 
water  that  contained  oxygen  derived  from  the  air.  Although 
some  erosion  has  taken  place  since  the  ores  were  deposited,  the 
effects  of  enrichment  are  believed  to  be  subordinate. 


CHAPTER  XXV 

SILVER 
MINERAL  COMPOSITION  OF  SILVE  R  DEPOSITS 


Mineral 


Percentage  of 
silver 


Composition 


Silver  

100.0 

Ag. 

Cerargyrite  
Bromyrite  
Embolite  
lodyrite  

75.3 
57.4 
65.1 
46  0 

AgCl 
AgBr. 
Ag(Cl,  Br). 
Agl. 

Argentite  

87.1 

Ag2S. 

Pyrargyrite  
Proustite 

59.9 
65  4  . 

AgaSbSs  or  3Ag2S.Sb2S». 
AgsAsSs  or  3Ag2S  As2Ss 

Stephanite  
Polybasite  

68.5 
75.6 

Ag5SbS4  or  5Ag2S.Sb2S3. 
Ag9SbS6  or  9Ag2S  Sb2S3. 

Pearceite  
Tetrahedrite  
Tennantite 

78.4 
Variable 
Variable 

Ag9AsS6  or  9Ag2S.As2S3. 
4Cu2S.Sb2S3  or  4(Cu2Ag2)S.Sb2S8. 
4Cu2S  As2S3  or  4(Cu2Ag2)S  A82S3 

Tellurides. 

Unlike  gold,  silver  forms  many  stable  compounds.  Of  these 
only  a  few,  the  more  common  ores,  are  mentioned  above.  The 
silver  ores  are  generally  more  complex  than  those  of  gold  and 
more  difficult  to  treat. 

Silver  in  many  deposits  occurs  in  an  undetermined  state  inter- 
grown  with  other  minerals.  Thus  galena  is  commonly  argen- 
tiferous, and  sphalerite,  pyrite,  or  other  sulphides  may  carry 
enough  silver  to  make  it  the  principal  consideration  in  the  ore. 
Nearly  all  copper  deposits  carry  some  silver,  and  in  smelting 
much  of  it  is  recovered.  The  muds  that  are  obtained  from  the 
electrolytic  refining  of  copper  are  smelted  for  silver  and  gold. 
Nearly  all  the  lead  ores  of  the  West  carry  notable  amounts  of 
silver,  and  gold  ores  are  very  commonly  argentiferous. 

The  gangue  minerals  of  silver  ores  are  similar  to  those  of  cop- 
per and  gold  ores.  Quartz,  carbonates,  and  sericite  are  among 
the  common  minerals  in  deposits  of  both  metals. 

The  primary  deposits  of  silver  are  principally  veins  and  nearly 
related  deposits.  Workable  deposits  formed  by  magmatic  segre- 
gation or  as  pegmatite  veins  are  practically  wanting,  and  silver 
is  relatively  rare  or  subordinate  in  most  contact-metamorphic 

440 


SILVER  441 

deposits  and  veins  of  the  deep  zone.  Silver  deposits  are  formed 
mainly  at  moderate  and  shallow  depths  and  are  characteristically 
developed  in  the  central  belt  mentioned  in  the  discussion  of  gold 
deposits  (page  404)  and  in  the  middle  and  late  Tertiary  group. 
Silver  is  deposited  by  cold  solutions  in  a  few  of  its  primary  de- 
posits, -as  in  the  Red  Beds  of  Colorado,  Arizona,  and  New  Mexico, 
in  which  it  is  associated  with  copper  sulphides.  Syngenetic  de- 
posits of  silver  ores  are  practically  unknown  in  North  America. 

AGE  OF  SILVER  DEPOSITS  IN  NORTH  AMERICA 

The  silver  ores  of  North  America  range  in  age  from  pre-Cam- 
brian  to  late  Tertiary.  Cobalt,  Ontario,  the  most  productive 
silver  district  on  the  continent,  is  in  an  area  of  pre-Cambrian  rocks, 
and  probably  the  ores  are  pre-Cambrian,  but  they  appear  to  have 
been  formed  at  moderate  depths.  The  copper  deposits  of  Kewee- 
naw  Peninsula,  Michigan,  carry  some  silver.  Silver  is  generally 
not  abundant  in  early  Cretaceous  deposits,  such  as  the  California 
gold  lodes.  In  late  Cretaceous  and  early  Tertiary  time  silver 
was  deposited  in  considerable  amounts  in  Montana  (Butte, 
Philipsburg,  Elkhorn),  in  Colorado  (Leadville),  in  Utah  (Park 
City)  and  in  many  other  districts  of  the  so-called  central  belt. 
Silver  was  deposited  in  largest  amounts  in  middle  and  late  Ter- 
tiary time,  especially  in  the  Basin  province,  which  includes 
Nevada  and  parts  of  the  adjoining  States.  During  this  period 
the  great  deposits  of  the  Comstock  lode,  Tuscarora,  Tonopah, 
and  many  other  districts  in  the  United  States  and  in  Mexico 
were  formed. 

ENRICHMENT 

Silver  is  dissolved  in  dilute  sulphuric  acid  and  is  precipitated 
in  a  reducing  environment  by  metallic  sulphides  or  by  hydrogen 
sulphide.  Silver  chloride  is  comparatively  insoluble.  Although 
ferrous  sulphate  and  copper  sulphate  remain  dissolved  even  in 
solutions  of  high  concentration,  ferrous  sulphate  precipitates 
metallic  silver  from  a  dilute  solution  of  silver  sulphate  according 
to  the  reaction  Ag2S04  +  2FeS04  =  2Ag  +  Fe2(SO4)3. 

Silver  sulphate,  which  is  somewhat  soluble,  is  formed  by  the 
action  of  concentrated  sulphuric  acid  on  silver.  According  to 
Cooke,1  silver  sulphide  is  only  slightly  soluble  in  very  dilute  sul- 

1  COOKE,  H.  C. :  The  Secondary  Enrichment  of  Silver  Ores.  Jour.  Geol., 
vol.  21,  p.  17,  1913. 


442      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


phuric  acid,  but  if  a  little  ferric  sulphate  is  added  to  the  solution 
the  solubility  of  the  sulphide  is  considerably  increased.  Silver 
sulphate  is  easily  attacked  by  many  minerals,  the  native  metal 
being  deposited. 

Notwithstanding  the  low  solubility  of  the  silver  halides,  it  sel- 
dom happens  that  all  the  silver  dissolved  in  the  upper  parts  of 
an  argentiferous  deposit  is  fixed  as  halides.  The  formation  of 
the  chloride  near  the  surface  does  not  entirely  inhibit  the  down- 
ward migration  of  silver.  The  secondary  silver  sulphides  are 
numerous  and  in  some  veins  abundant. 

Silver  is  readily  precipitated  as  argentite  below  the  zone  of 
oxidation  on  account  of  the  low  solubility  of  its  sulphide.  It 
stands  near  the  end  of  the  Schuermann  series,  being  preceded 
only  by  mercury,  and  accordingly  it  should  replace  most  other 
metals  in  sulphide  combinations.  With  silver  sulphate  (Ag2SO4) 
hydrogen  sulphide,  which  is  generated  by  acid  reacting  upon 
zinc  blende,  galena,  or  other  sulphides,  gives  argentite.  The 
reaction  is 

Ag2SO4  +  H2S  =  Ag2S  +  H2SO4. 

If  the  reaction  is  with  galena,  or  if  lead  sulphide  is  precipitated 
simultaneously  with  argentite,  argentiferous  galena  may  be 

formed.  If  arsenic  and  anti- 
mony are  present  in  solution, 
the  complex  sulphosalts  of  these 
metals  may  be  formed. 

In  many  silver  deposits  second- 
ary pyrargyrite,  proustite,  steph- 
anite,  and  polybasite  are  the 
most  valuable  minerals.  These 
minerals  are  probably  formed 
only  in  the  lower  part  of  the 

,,  secondary   sulphide    zone   (Fig. 

Fio.  185. — Ideal  section  showing  J  \    e 

distribution  of  minerals  in  a  silver  185),  where  alkaline  conditions 
deposit  that  has  been  superficially  generally  prevail  (see  Fig.  66, 

page  134).    If  stibnite  is  treated 

with  a  dilute  alkaline  solution  such  as  sodium  carbonate  or 
sodium  hydroxide,  sodium-antimony  sulphide  is  formed.  With 
silver  sulphate  the  salt  forms  a  compound  having  the  composi- 
tion of  stephanite,  (Ag2S)5Sb2S3. 

(Na2S)5Sb2S3  +  5Ag2S04  =  5Na2SO4  +  (Ag2S)5.Sb2S3 


SILVER  443 

It  has  been  shown  that  there  is  below  the  surface  a  zone  of 
alkaline  waters  where  oxygen  is  excluded  and  where  underground 
waters  react  for  long  periods  on  alkaline  rocks.  Above  the  alka- 
line zone  the  waters  are  acid.  After  a  series  of  heavy  rains  or 
during  a  wet  period  the  water  level  is  elevated,  and  the  acid 
waters  descend  rapidly  on  account  of  increased  pressure  of  the 
water  above  the  acid  zone.  The  descending  acid  waters  will 
mingle  with  the  alkaline  waters.  Acid  waters  containing  silver 
sulphate,  reacting  with  alkaline  waters  containing  sodium-anti- 
mony sulphide,  can  form  stephanite  and  pyrargyrite,  as  has  been 
shown  by  Grout1  and  by  Ravicz.2 

In  many  deposits  containing  silver  chloride,  native  silver, 
silver  sulphide,  and  the  arsenic  and  antimony  sulphosalts,  these 
minerals  occur  at  fairly  well  defined  horizons.  The  chloride  is 
most  abundantly  developed  above  the  argentite  ore;  the  anti- 
mony and  arsenic  sulphosalts  are  found  with  and  below  the 
argentite;  the  native  metal  is  found  with  the  chloride,  and, 
overlapping  the  zone  of  chloride  ores,  it  extends  downward  with 
argentite.  In  sulphide  deposits  of  silver  and  copper  the  richer 
silver  ores  are  generally  nearer  the  surface  than  the  richer  copper 
ores.  Some  of  the  great  copper  lodes  of  Butte,  Mont.,  were 
worked  for  silver  to  depths  of  200  to  400  feet  below  the  surface, 
where  the  deposits  changed  to  rich  copper  ore.  Like  gold,  silver 
would  be  driven  from  sulphate  solutions  in  an  environment  where 
chalcocite  forms. 

Native  silver  is  a  primary  mineral  in  some  deposits,  as  in  the 
zeolitic  copper  ores  of  Lake  Superior,  but  in  sulphide  deposits  it 
is  generally  or  invariably  secondary.  In  some  districts  it  is 
among  the  most  valuable  ore  minerals.  It  commonly  occurs  as 
thin  flakes  or  as  sheets  plastered  on  the  older  minerals  or  as  vein- 
lets  filling  cracks  in  the  ore,  and  presumably  it  has  been  formed 
at  many  places  through  the  reduction  of  silver  sulphides  or  other 
silver-bearing  minerals. 

Cerargyrite  is  unknown  as  a  primary  constituent  of  ores  de- 
posited by  ascending  hot  waters  but  is  frequently  developed  by 
weathering,  at  or  near  the  outcrops  of  silver-bearing  sulphide 

1  GROUT,  F.  F. :  On  the  Behavior  of  Cold  Acid  Sulphate  Solutions  of  Cop- 
per, Silver,  and  Gold  with  Alkaline  Extracts  of  Metallic  Sulphides.     Econ. 
Geol.,  vol.  8,  pp.  407-433,  1913. 

2  RAVICZ,  L.  G.:  Experiments  in  the  Enrichment  of  Silver  Ores.     Econ. 
Geol,  vol.  10,  pp.  378-384,  1915. 


444      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

lodes.  At  many  places,  even  where  the  primary  sulphide  ores 
are  not  profitable,  the  superficial  chloride  ores  may  be  very  rich. 
The  chloride  ores  generally  pass  into  the  sulphides  below,  and 
the  bottom  of  the  zone  of  ore  carrying  horn  silver  is  generally 
above  the  bottom  of  the  zone  of  secondary  silver  sulphides. 

Argentite  is  one  of  the  commonest  secondary  silver  minerals, 
but  it  occurs  also  as  a  primary  mineral.  If  sulphuric  acid  in  its 
descent  encounters  a  soluble  sulphide  like  zinc  blende,  hydrogen 
sulphide  and  zinc  sulphate  are  formed.  The  hydrogen  sulphide 
would  precipitate  silver  sulphide  from  a  solution  containing 
Ag2SO4.  Argentite  could  be  precipitated  in  any  of  the  following 
reactions: 

H2S  +  Ag2S04  =  H2S04  +  Ag2S 
ZnS  +  Ag2SO4  =  ZnS04  +  Ag2S 
PbS  +  Ag2S04  =  PbS04  +  Ag2S 

Pyrargyrite  (dark  ruby  silver),  is  probably  the  most  abundant 
secondary  silver  mineral  in  a  large  number  of  silver  mines  in  the 
United  States.  It  is  confined  to  epigenetic  deposits  and  occurs 
in  many  deposits  of  early  and  middle  Tertiary  age  in  the  Ameri- 
can Cordillera.  It  is  not  known  as  a  primary  mineral  of  contact- 
metamorphic  deposits  and  veins  of  the  deep  zone.  In  some  mines 
ruby  silver  is  found  at  considerable  depths,  however,  possibly 
below  the  zone  of  secondary  alteration. 

Proustite  (light  ruby  silver)  is  similar  to  pyrargyrite  in  its  oc- 
currences and  is  generally  secondary. 

Stephanite  is  common  in  some  districts.  It  generally  accom- 
panies ruby  silver  and  polybasite,  and  in  places  it  occurs,  like 
them,  in  cracks  cutting  the  primary  ore. 

Polybasite  is  commonly  a -secondary  mineral.  It  occurs  in 
several  mining  districts  in  cracks  cutting  the  primary  ore  and  in 
the  main  is  related  to  the  present  surface. 

Pearceite  is  less  common  than  the  corresponding  antimony 
salt,  polybasite.  It  is  generally  secondary. 

Freibergite,  the  argentiferous  variety  of  tetrahedrite,  is  an  im- 
portant source  of  silver  in  many  deposits.  It  is  primary  in  most 
districts  but  is  probably  secondary  in  some. 

Tennantite  has  the  same  range  of  occurrence  as  the  correspond- 
ing antimony  sulphide,  tetrahedrite. 


SILVER 


445 


SILVER-BEARING  DISTRICTS 

Cobalt,  Ont. — Cobalt,1  in  the  Nipissing  district,  northern 
Ontario,  is  at  present  the  most  productive  silver-bearing  district 
in  North  America.  The  ore  is  very  rich  and,  shipments  carrying 
1,000  ounces  or  more  to  the  ton  are  not  uncommon.  Much  of 
the  ore  is  sent  directly  to  smelters,  but  in  re  jent  years  some  of  the 
larger  mining  companies  have  built  mills,  and  the  lower-grade 
material  is  concentrated. 


COBALT  SERIES 

Veins  j  Hypothetical  veins 

FIQ.  186. — Generalized  vertical  section  through  the  productive  part  of  the 
cobalt  area,  Ontario. 

The  section  shows  the  relations  of  the  Nipissing  diabase  sill  to  the  Keewatin  and  Cobalt 
series  and  to  the  veins.  The  eroded  surface  is  restored  in  the  section.  The  sill  is  less  regular 
than  the  illustration  shows  it  to  be.  B  and  C  represent  a  large  number  of  veins  that  are  in 
the  fragmental  rocks  (Cobalt  series),  in  the  lower  or  footwall  of  the  eroded  sill.  N  repre- 
sents a  type  of  vein,  such  as  No.  26  on  the  Nipissing,  in  the  Keewatin  below  the  eroded  sill, 
and  L  a  type  such  as  one  under  Peterson  lake,  in  the  Keewatin  footwall,  but  not  extending 
upward  into  the  sill;  K,  a  vein  in  the  sill  itself,  such  as  No.  3  on  the  Kerr  Lake  property: 
T,  a  .vein,  such  as  that  on  the  Temiskarning  or  Beaver  properties,  in  the  Keewatin  hanging 
wall  and  extending  downward  into  the  sill.  (After  Miller.) 

Cobalt  is  on  the  great  ancient  peneplain  which  extends  over 
much  of  Ontario  and  the  surrounding  region.  The  country  is 
hilly,  but  the  relief  is  not  great.  The  recent  glaciation  is  clearly 
evident,  but  drift  is  generally  thin  or  absent. 

The  basement  rocks  are  the  Keewatin  series,  a  complex  of 
metamorphosed  basic  igneous  rocks,  usually  known  as  green- 
stones, which  includes  also  some  rock  of  sedimentary  origin. 
The  eroded  surface  of  the  Keewatin  is  overlain  by  Huronian 
conglomerates,  graywacke,  and  other  metamorphosed  sedi- 

1  MILLER,  W.  G. :  The  Cobalt-Nickel  Arsenides  and  Silver  Deposits  of 
Temiskarning,  3d  ed.  Ontario  Bur.  Mines  Rept.,  vol.  16,  part  2,  1908:— 
4th  ed.  Idem.,  vol.  19,  part  2,  1913. 


446      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

mentary  rocks.  A  quartz  diabase  sill  some  500  feet  thick  was 
intruded  into  both  Huronian  and  Keewatin  rocks.  This  dips 
about  17°S. 

The  deposits  are  short,  narrow  veins,  very  numerous  and 
rich.  They  are  found  in  the  Huronian,  in  the  diabase,  and  in  the 
Keewatin,  but  the  more  productive  deposits  are  in  the  Huronian 
near  the  diabase  sill,  or  they  were  below  the  foot  wall  of  the  sill 
before  the  sill  was  eroded  (Fig.  186).  The  deposits  are  probably 
genetically  related  to  the  diabase,  and  the  fractures  have  been 
assumed  to  represent  cooling  cracks  formed  in  connection  with  the 
intrusion.  Post-mineral  fracturing  and  faulting  have  taken  place 
extensively. 

The  principal  sulphides  of  earlier  age  include  smaltite,  cobaltite, 
chloanthite,  and  bismuth  sulphide,  with  some  arsenopyrite  and 
tetrahedrite.  Pyrite,  galena,  and  sphalerite  are  present  in  the 
wall  rock  near  the  vein.  The  silver  occurs  as  native  metal, 
proustite,  pyrargyrite,  dyscrasite,  and  argentite.  The  gangue 
minerals  include  calcite  and  quartz. 

The  zone  of  oxidation  is  exceedingly  shallow  or  altogether 
lacking,  but  certain  exceptionally  rich  superficial  deposits,  a  few 
feet  thick,  are  directly  connected  with  the  zone  of  weathering. 
This  is  called  the  "nugget  horizon,"  and  in  it  the  smaltite  and 
cobaltite  have  been  largely  altered  to  secondary  minerals  or 
leached  out.  In  this  zone  erythrite  and  annabergite  are  charac- 
teristic minerals. 

Extending  downward  200  or  300  feet  or  more  below  the  surface 
are  rich  silver  minerals,  largely  in  veinlets  in  earlier  sulphides. 
The  minerals  of  the  veinlets  include  native  silver,  argentite,  and 
calcite.  The  change  from  rich  to  low-grade  ore  is  very  abrupt 
both  in  depth  and  on  the  strike.  Some  writers1  have  attributed 
these  richer  silver  ores  to  processes  of  sulphide  enrichment,  but 
Miller  is  inclined  to  the  belief  that  this  feature  of  the  genesis  has 
been  too  much  emphasized.  Notwithstanding  its  late  age  and 

*VAN  HISE,  C.  R.:  The  Ore  Deposits  of  the  Cobalt  District,  Ontario. 
Canadian  Min.  Inst.  Jour.,  vol.  10,  pp.  43-53,  1907. 

EMMONS,  S.  F.:  Cobalt  District,  Ontario,  in,  BAIN,  H.  F.,  and  others: 
"Types  of  Ore  Deposits,"  pp.  140-156,  San  Francisco,  1911. 

CAMPBELL,  W.,  and  KNIGHT,  C.  W.:  Microscopic  Examination  of  the 
Cobalt-Nickel  Arsenides  and  Silver  Deposits  of  Temiskaming.  Econ.  Geol., 
vol.  1,  p.  757,  1906. 

HORE,  R.  E.:  Geology  of  the  Cobalt  District.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  42,  p.  480,  1911. 


SILVER  447 

its  occurrence  in  cracks  of  older  shattered  remnants,  he  believes 
that  much  of  the  native  silver  is  primary. 

Eureka,  Nev. — The  Eureka  district,  in  Eureka  County, 
eastern  Nevada,  from  1869  to  1883  yielded  about  $60,000,000 
in  silver  and  gold  and  225,000  tons  of  lead.  After  that  the  pro- 
duction declined  greatly,  but  in  late  years  considerable  ferru- 
ginous gold  ore  has  been  shipped  to  Utah  smelters,  where  it  is 
in  demand  on  account  of  its  fluxing  properties.  Most  of  the 
Eureka  ore,  however,  was  beneficiated  in  lead  smelteries  at 
Eureka.  In  1915  it  produced  109,320  tons  of  ore  containing 
$500,040  in  gold,  silver,  lead,  zinc,  and  copper. 

The  district1  is  an  area  of  Paleozoic  quartzites,  limestones, 
and  shales,  which  were  intruded,  probably  in  late  Mesozoic  time, 
by  granite,  granite  porphyry,  and  quartz  porphyry.  Later, 
probably  in  the  Tertiary  period,  the  sedimentary  rocks  were 
intruded  by  great  igneous  bodies  of  andesitic  composition  and 
covered  in  places  by  rhyolite  and  basalt.  The  beds  are  thrown 
into  open  folds,  and  the  dominant  structure  is  that  of  a  fault 
mosaic,  the  principal  faults  being  of  the  normal  type. 

The  ores  occur  in  Paleozoic  sedimentary  rocks.  The  ore  bodies 
have  been  formed  chiefly  by  the  replacement  of  fractured  lime- 
stones2 and  include  lodes,  stocks,  and  bedding-plane  deposits. 
Near  the  deposits  great  masses  of  limestone  are  impregnated 
with  magnetite. 

The  larger  ore  bodies  are  capped  by  caves  developed  by  solu- 
tion, and  the  fall  of  rock  into  openings  has  caused  further  fissur- 
ing.  Subsequently  the  metals  have  in  many  places  been  redis- 
tributed by  underground  waters.  The  ore  above  the  water  level 
is  composed  principally  of  galena,  anglesite,  cerusite,  mimetite, 
and  wulfenite,  with  much  limonite,  quartz,  and  calcite.  It 
carries  also  considerable  gold  and  silver  and  some  zinc.  The  ore 
below  the  water  level  is  composed  chiefly  of  pyrite,  arsenopyrite, 
galena,  zinc  blende,  and  other  sulphides,  with  silver  and  gold. 
At  some  places  altered  ore  is  found  below  the  water  level,  and 
Curtis  supposed  that  the  water  level  had  been  recently  elevated. 
Large  bodies  of  low-grade  limonitic  gold  ore  have  been  mined  in 
the  shattered  and  altered  limestones  that  surround  the  old  silver 
stopes. 

1  HAGUE,  ARNOLD:  Geology  of  the  Eureka  District,  Nevada.     U.  S.  Geol. 
Survey  Mon.  20,  1892. 

2  CURTIS,  J.  S.:  Silver-Lead  Deposits  of  Eureka,   Nev.     U.  S.  Geol. 
Survey  Mon.  7,  1884, 


448       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Boulder-Leadville  Belt,  Colorado. — Lying  30  or  40  miles  west 
of  Denver,  Colo,  and  extending  from  Boulder  southwest  to 
Leadville,  a  distance  of  about  80  miles  (Fig.  187)  is  one  of  the 
most  richly  mineralized  belts  in  North  America.  This  belt  is 
occupied  by  pre-Cambrian  schists,  gneisses,  granites,  and  other 
igneous  rocks  overlain  by  Paleozoic  and  Mesozoic  sedimentary 
rocks.  It  contains  many  porphyry  dikes,  sills,  and  stocks,  which 
are  believed  to  be  of  late  Cretaceous  or  early  Tertiary  age. 
Closely  associated  with  the  intrusive  rocks  and  probably  of  about 
the  same  age  are  numerous  deposits  containing  silver,  gold,  lead, 
and  other  metals.  In  the  igneous  rocks  and  schists  the  principal 
deposits  are  normal  veins.  In  the  limestones  irregular  replace- 
ment and  bedding-plane  deposits  predominate.  Some  sub- 
districts  exhibit  parallel  coordinated  vein  systems. 


FIG.  187. — Sketch  showing  metallized   areas  in   Colorado.     (After  Spurr 
and  Garrey,  U.  S.  Geol.  Survey.) 

The  ores  vary  from  place  to  place  but  normally  contain  a  fairly 
high  proportion  of  sulphides,  including  pyrite,  sphalerite,  galena, 
chalcopyrite,  and  tetrahedrite.  Normally  the  gangue  is  quartz, 
but  some  deposits  contain  also  carbonates  and  barite.  In  the 
wall  rock  near  the  veins  sericite  and  carbonates  occur.  There 
is  very  little  contact  metamorphism  near  the  intruding  rocks,  and 
no  andradite-amphibole  border  zones  are  developed.  The  deposits 
have  been  formed  mainly  at  moderate  depths,  probably  between 
1  and  2  miles  below  the  surface;  those  formed  at  the  greater 
depths  are  most  abundant  in  the  southwestern  part  of  the  area. 


SILVER  .  449 

Although  the  deposits  occur  in  rocks  of  varied  composition  and 
character,  no  relation  between  their  composition  and  that  of  the 
containing  rocks  is  recognized.  Ores  of  similar  composition  may 
be  found  in  sedimentary,  metamorphic,  and  igneous  rocks. 
Differences  in  vein  composition  appear  to  be  related  rather  to 
differences  in  the  character  of  the  associated  intruding  rocks. 
In  the  Georgetown  quadrangle1  the  areas  containing  veins  coin- 
cide with  areas  containing  porphyry  dikes.  In  the  Georgetown 
district,  characterized  by  argentiferous  galena-blende  veins,  the 
dikes  consist  of  alaskite  porphyry,  granite  porphyry,  quartz 
monzonite  porphyry,  and  dacite.  In  the  Idaho  Springs  district, 
characterized  by  pyritic  gold  veins,  the  dikes  are  bostonite, 
alaskitic  quartz  monzonite,  biotite  latite,  and  alkali  syenite. 
The  dikes  near  Georgetown  belong  to  the  early  Tertiary 
monzonitic  magma;  the  rocks  of  the  Idaho  Springs  district 
are  referable  to  an  alkaline  magma.  Similarly,  there  are  two 
classes  of  ore  deposits  in  the  quadrangle,  the  deposition  of 
which  has  in  general  followed  the  eruption  of  the  different 
groups  of  magmas. 

Many  deposits  of  this  belt  have  been  enriched  by  superficial 
alteration,  but  commonly  the  veins  become  unprofitable  a  few 
hundred  feet  below  the  surface.  Some  of  the  districts  of  this 
belt  are  discussed  in  the  following  paragraphs. 

Breckenridge. — The  oldest  rocks  in  the  Breckenridge  region2 
are  pre-Cambrian  granites,  gneisses,  and  schists.  The  oldest 
sedimentary  rocks,  which  rest  directly  on  the  pre-Cambrian,  are 
red  sandstones  and  shales,  supposed  to  be  of  Triassic  or  Permian 
age.  Apparently  conformable  above  them  is  the  Dakota  quartz- 
ite,  with  some  gray  shale,  which  is  overlain  by  a  thick  formation 
of  Upper  Cretaceous  shales.  The  sediments  and  the  pre-Cam- 
brian rocks  are  intruded  by  monzonitic  porphyries,  which  occur 
mainly  as  sills. 

The  ore  deposits  include  veins  of  a  zinc-lead-silver-gold  series, 
stockworks  and  veins  of  a  gold-silver-lead  series,  and  the  gold 
veins  of  Farncomb  Hill.  Gold  placers  also  are  present. 

In  the  Wellington  veins,  which  belong  to  the  zinc-lead-silver- 


1  SPURR,  J.  E.,  and  GARRET,  G.  H. :  Economic  Geology  of  the  Georgetown 
Quadrangle,  Colorado.     U.  S.  Geol.  Survey  Prof.  Paper  63,  p.  157,  1908. 

2  RANSOME,  F.  L. :  Geology  and  Ore  Deposits  of  the  Breckenridge  Dis- 
trict, Colorado.    U.  S.  Geol.  Survey  Prof.  Paper  75,  pp.  25-26, 1911. 

29 


450      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

gold  series,  the  filling  consists  of  sulphides,  mainly  galena,  sphale- 
rite, and  pyrite,  without  much  quartz. 

A  notable  feature  of  the  oxidized  ores  is  their  general  high  con- 
tent of  lead  and  silver  as  compared  with  the  sulphides  beneath. 
In  some  mines  the  deep  ores  are  not  profitable.  Here  and  there 
the  oxidized  ores  show  also  a  noteworthy  concentration  of  gold, 
where  the  sulphide  ores  below  contain  only  small  quantities. 

Silver  Plume. — The  Silver  Plume  district,1  near  Georgetown, 
is  an  area  of  pre-Cambrian  gneisses,  schists,  granites,  and  diorites 
intruded  by  early  Tertiary  or  late  Cretaceous  porphyry  dikes. 
These  trend  northeast  and  southeast  and  cross  the  schistosity  at 
high  angles.  The  ore  deposits  are  veins,  which  also  strike  north- 
east and  southeast.  Many  of  them  follow  contacts  of  porphyry 
dikes.  The  veins  contain  silver  and  lead  without  much  gold. 
The  principal  minerals  are  pyrite,  sphalerite,  and  galena,  with 
some  polybasite,  pyrargyrite,  and  argentite.  Sulphide  enrich- 
ment has  enhanced  the  value  of  the  ore  near  the  surface. 

Idaho  Springs.2 — The  Idaho  Springs  district  is  mainly  in  the 
northeast  corner  of  the  Georgetown  quadrangle,  extending  beyond 
its  limits.  The  rocks  are  principally  schists  of  the  Idaho  Springs 
formation,  belonging  to  the  pre-Cambrian  complex  that  is  present 
at  Silver  Plume.  These  are  intruded  by  porphyry  dikes.  Both 
schists  and  dikes  are  cut  by  veins,  many  of  which  parallel  the 
dikes.  Some  of  the  veins  carry  gold,  others  silver,  still  others 
both  metals.  Some  of  the  ore  shoots  are  located  at  junctions  of 
veins.  By  superficial  alteration  many  of  the  deposits  are  en- 
riched near  the  surface.  Gold  placers  have  been  formed. 

Gilpin  County. — Gilpin  County,3  northeast  of  Georgetown,  is 
one  of  the  most  steadily  productive  regions  in  Colorado.  The 
country  rock  is  a  pre-Cambrian  complex  cut  by  porphyry  dikes. 
The  ore  deposits  of  Gilpin  County  and  adjacent  areas  include 
gold-silver  ores,  uranium  ores,  and  tungsten  ores.  The  tungsten 
ores  are  prominent  also  in  the  adjoining  Boulder  County.  Most 
of  the  ore  bodies  occupy  zones  of  fracturing  and  minor  faulting. 

1  SPURR,  J.  E.,  and  GARRET,  G.  H. :  Economic  Geology  of  the  Georgetown 
Quadrangle,  Colorado.  U.  S.  Geol.  Survey  Prof.  Paper  63,  pp.  176-243, 
1908. 

2 SPURR,  J.  E.,  and  GARRET,  G.  H.:  Op.  cit.,  pp.  314-382. 

3  BASTIN,  E.  S.,  and  HILL,  J.  M. :  Economic  Geology  of  Gilpin  County  and 
Adjacent  Parts  of  Clear  Creek  and  Boulder  Counties,  Colorado.  U.  S. 
Geol.  Survey  Prof.  Paper  94,  pp.  190-280,  1917, 


SILVER 


451 


Leadville  — Leadville1  stands  about  10,000  feet  above  sea  level, 
on  a  high  terrace  at  the  foot  of  a  spur  of  the  Mosquito  Range. 
The  district  is  one  of  the  most  productive  in  the  West  and  has 
yielded  large  quantities  of  lead  and  silver,  considerable  gold, 
zinc,  and  copper,  and  some  iron,  manganese,  and  bismuth.  In 
1915  it  produced  470,808  tons  of  ore  which  yielded  $2,003,866 
in  gold,  2,389,371  ounces  of  silver,  1,782,235  pounds  of  copper, 
20,808,407  pounds  of  lead,  and  72,424,873  pounds  of  zinc,  with 
a  total  value  of  $13,485,847. 

The  normal  sequence  of  rocks  in  the  district  from  the  top  down 
is  as  follows: 


Local  name 

Age 

Character 

Average 
thickness 
(feet) 

i 

, 

Weber  shales            Lower  Carboniferous 

Shales  and  grit 

0  to  2,500 

White  porphyry 

Pre-Cretaceous 

White  rhyolite  porphyry 

800 

Blue  limestone 

Lower  'Carboniferous 

Blue-gray  dolomite 

200 

Gray  porphyry 

Pre-Cretaceous 

Gray  monzonite  porphyry 

50 

Parting  quartzite 

Devonian 

Coarse  quartzite 

30 

White  limestone 

Silurian 

Drab  siliceous  dolomitic  lime- 

stone. 

160 

Lower  quartzite 

Cambrian. 

Mostly  white  quartzite 

160 

Granite 

Basement   complex   or   pre- 

Granite  and  gneiss 

Cambrian. 

-! 

The  porphyries  are  intruded  mainly  as  sheets,  but  locally  they 
cut  the  sedimentary  rocks.  They  range  from  a  few  feet  to  several 
hundred  feet  in  thickness.  They  are  generally  separated  by  the 
blue-gray  dolomite,  as  indicated  in  the  table,  but  at  several  places 
they  are  in  contact  with  each  other.  At  some  places  there  is 
more  than  one  sheet  of  the  gray  porphyry.  Over  much  of  the 
area  erosion  has  removed  the  top  of  the  white  porphyry.  The 

1  EMMONS,  S.  F. :  Geology  and  Mining  Industry  of  Leadville,  Colo.  U.  S. 
Geol.  Survey  Mon.  12,  1886. 

EMMONS,  S.  F.,  and  IRVING,  J.  D. :  The  Downtown  District  of  Leadville, 
Colo.  U.  S.  Geol.  Survey  Bull.  320,  1907. 

ARGALL,  PHILIP:  The  Zinc  Carbonate  Ores  of  Leadville:  Min.  Mag.; 
vol.  10,  pp.  282-288,  1914. 

RICKETTS,  L.  D.:  "The  Ores  of  Leadville,"  Princeton,  1883. 

BLOW,  A.  A.:  The  Geology  and  Ore  Deposits  of  Iron  Hill,  Leadville, 
Colo.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  18,  pp.  145-181,  1890. 

BUTLER,  G.  M.:  Some  Recent  Developments  at  Leadville.  Econ.GeoL, 
vol.  7,  pp.  315-323,  1912;  vol.  8,  pp.  1-18,  1913. 

BOEHMER,  MAX:  The  Genesis  of  Leadville  Ore  Deposits.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  41,  pp.  162-165,  1911. 


452      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

most  valuable  deposits  are  found  in  Carboniferous  limestone  at 
or  near  its  contact  with  an  overlying  porphyry.  Thus  the  ore 
bodies  constitute  sheets,  the  upper  surfaces  of  which,  being 
formed  by  the  bases  of  the  porphyry  bodies  are  comparatively 
regular,  while  the  lower  surfaces  are  ill  denned  and  irregular, 
there  being  a  gradual  transition  from  ore  to  limestone.  The  ore 
extends  to  varying  depths  below  the  surface,  occupying  in 
places  the  entire  thickness  of  the  Leadville  ("Blue")  limestone. 
Other  deposits  include,  however,  steeply  dipping  veins,  some  of 
them  in  fault  fissures,  and  irregular  masses  or  sheets  in  lime- 
stone near  the  "Gray"  or  other  porphyries  (Fig.  188). 


FIG.  188. — Geologic  section  of  the  Downtown  district.  Leadville,  Colorado. 
(After  Emmons  and  Irving.) 

The  most  valuable  ore  consists  of  argentiferous  galena  and  its 
secondary  products,  cerusite  and  cerargyrite.  Lead  is  found  also 
as  anglesite  and  pyromorphite  and  occasionally  as  oxide.  Silver 
occurs  commonly  as  chloriodide  and  is  very  rare  in  the  native 
state.  The  gangue  minerals  include  quartz,  chert,  barite,  sider- 
ite,  and  clay,  the  clay  being  commonly  charged  with  iron  and 
manganese  oxides  or  with  sulphates. 

Alteration  products  of  mixed  pyrite  and  galena  ore  include 
limonite,  jarosite,  anglesite,  and  pyromorphite.  Manganiferous 
siderite  on  oxidation  yields  manganese  oxides. 

Gold  occurs  in  small  flakes.  Other  minerals  are  zinc  blende, 
calamine,  arsenic  and  antimony  (probably  as  sulphides),  wulfen- 
ite,  copper  carbonate  and  silicate,  and  bismuth  sulphide.  Nod- 
ules of  galena  surrounded  by  lead  carbonates  are  locally  numerous 
in  the  oxidized  zone. 


SILVER  453 

In  depth  the  ores  consist  of  pyrite,  sphalerite,  and  galena,  with 
some  chalcopyrite  and  other  minerals. 

In  certain  veins  of  sulphide  ores  below  the  porphyry  contacts, 
some  small  masses  of  sphaleritic  manganiferous  ores  are  very  rich 
in  gold.1  Silver  is  found  as  chloride  and  in  general  diminishes  in 
quantity  with  increasing  depth.  The  upper  contact  bodies  as  a 
whole  were  richest  in  silver;  the  "second  contact"  bodies  were 
slightly  lower  in  tenor;  and  at  lower  horizons  the  ore  is  of  low 
grade. 

Recently  large  bodies  of  iron-stained  smithsonite  and  of  mon- 
heimite  have  been  found  in  the  oxidized  zones  below  lead-carbon- 
ate ores  (see  page  477). 

Aspen,  Colo. — The  Aspen  district,  Colorado,2  which  lies 
southwest  of  Leadville,  is  an  area  of  granite  overlain  by  Paleozoic 
limestones,  sandstones,  and  shales,  which  are  intruded  by  dikes 
and  sills  of  diorite  porphyry  and  quartz  porphyry.  Structurally 
the  district  is  a  fault  mosaic  of  folded  beds,  and  the  principal  ore 
deposits  have  been  formed  by  the  replacement  of  limestone  in  and 
along  fault  fissures.  The  primary  ore  deposition  was  effected  by 
ascending  magmatic  waters  and  took  place  in  a  relatively  brief 
period,  but  according  to  Spurr3  it  had  three  successive  stages,  pro- 
ducing (1)  barite  veins,  (2)  silver  sulphides,  sulphantimonites, 
and  sulpharsenites,  (3)  galena  and  zinc  blende — each  stage 
being  preceded  by  slight  fracturing  of  the  rocks.  The  maximum 
deposition  was  below  shale  beds. 

Near  the  surface  the  ores  occur  as  oxides,  sulphates,  and  car- 
bonates, mixed  with  sulphides.  With  increase  in  distance  from 
the  surface  the  oxides,  sulphates,  and  carbonates  give  place  to 
pure  sulphides.  Argentiferous  galena  and  blende  are  abundant 
in  the  deeper  ore;  other  sulphides  are  of  less  common  occurrence. 
Pyrite,  chalcopyrite,  and  locally  bornite  also  are  found.  Tetra- 
hedrite  and  tennantite  are  common  and  contain  a  large  propor- 
tion of  silver.  The  gangue  is  quartz  and  barite. 

In  the  Mollie  Gibson  and  Smuggler  mines  there  is  much  poly- 
basite,  which  generally  occurs  in  flesh-colored  barite,  the  color 

1  BUTLER,  G.  M.:  Some  Recent  Developments  at  Leadville.     Econ.  Geol, 
vol.  7,  p.  318,  1912. 

2  SPURR,  J.  E.:  Geology  of  the  Aspen  Mining  District,  Colorado.     U.  S. 
Geol.  Survey  Mon.  31,  1898. 

3  SPURR,  J.  E. :  Ore  Deposition  at  Aspen,  Colorado.     Econ.  Geol,  vol.  45, 
p.  303,  1909. 


454      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

being  due  to  a  small  amount  of  iron  oxide.  Along  watercourses 
the  polybasite  is  reduced  to  native  silver,  so  that  the  ore  consists 
of  pink  and  gray  barite  bound  together  by  irregular  wires  and 
masses  of  silver. 

San  Juan  Region,  Colorado. — The  San  Juan  region,1  in  south- 
western Colorado,  embraces  a  lofty  plateau  from  which  rise 
rugged  mountains.  The  central  group  is  termed  the  San  Juan 
Mountains,  and  on  its  border  are  the  Rico,  La  Plata,  Engineer, 
Needle,  and  other  mountains  and  mountain  groups.  The  rocks 
in  this  region  range  in  age  from  pre-Cambrian  to  Recent,  and  all 
the  great  subdivisions  are  represented.  The  dominant  structural 
feature  is  a  great  dome,  on  the  margin  of  which  are  smaller  domes 
and  laccoliths.  Mesozoic  and  older  rocks  were  folded  and  eroded, 
and  on  their  steeply  tilted  edges  the  Telluride  conglomerate,  prob- 
ably of  early  Tertiary  age,  was  deposited.  Above  this  con- 
glomerate lie  thousands  of  feet  of  volcanic  tuffs,  rhyolite,  and 
andesite  flows.  Extensive  sills  and  laccoliths  of  diorite  porphyry 
have  been  thrust  between  the  beds.  Intruding  the  sedimentary 
rocks  and  lavas  and  locally  cutting  across  the  laccolithic  sheets 
are  great  bodies  of  diorite  and  monzonite,  which  occur  princi- 
pally as  stocks.  Volcanic  rocks  predominate  especially  in  the 
central,  more  elevated  part  of  the  area.  The  volcanic  activity, 
which  probably  began  in  early  Tertiary  time,  appears  to  have 
continued  through  most  of  that  period.  At  several  places  within 
the  region  are  hot  springs,  which  are  regarded  as  features  of  a 
declining  volcanic  era. 

After  the  latest  volcanic  rocks  were  erupted  the  region  was 
complexly  faulted.  Some  of  the  faults  are  mineralized,  among 
them  the  Amethyst  fault  at  Creede,  which  carries  the  most  pro- 
ductive silver  deposit  in  the  San  Juan  region. 

The  ore  deposits  of  the  San  Juan  region  are  in  the  main  veins 
and  related  deposits.  The  central  part  of  the  region  is  noted  for 
strong,  persistent  veins  with  bold  outcrops.  Many  of  them 
may  easily  be  followed  on  the  surface  for  thousands  of  feet. 
Not  all  the  deposits  are  simple  veins.  The  pipe-like  deposits  on 
Red  Mountain,  between  Silverton  and  Ouray,  and  the  ribbons  of 
ore  replacing  limestone  at  Rico  and  Ouray  are  noteworthy. 

1  CROSS,  WHITMAN,  and  HOWE,  ERNEST:  U.  S.  Geol.  Survey  Geol.  Atlas, 
Silverton  folio  (No.  120),  1905. 

CROSS,  WHITMAN:  U.  S.  Geol.  Survey  Geol.  Atlas,  Telluride  folio  (No.  57), 
1899. 


SILVER  455 

Nearly  all  the  deposits  exhibit  features  of  veins  formed  at  moder- 
ate depths.  A  few  veins  near  Rico  and  Ouray  and  in  the  Tellu- 
ride  quadrangle  contain  specularite,  magnetite,  and  other  min- 
erals normally  found  at  great  depths,  and  a  few  deposits  appear 
to  have  been  formed  at  shallow  depths.  At  least  two  periods  of 
vein  formation  are  indicated.  The  principal  metals  produced 
are  silver,  gold,  and  lead,  with  some  zinc  and  a  little  copper. 
The  production  of  the  region,  including  Creede,  is  estimated  at 
about  $200,000,000.  The  principal  districts  are  described  below. 

Silverton. — The  ore  bodies  at  Silverton1  include  lodes,  stocks, 
and  subordinate  replacement  deposits.  Of  these  the  lodes  are 
the  most  valuable.  The  system  of  fissuring  is  of  the  coordinated 
type.  The  veins  have  steep  dips;  some  strike  northeast  and 
others  northwest.  Some  of  them  occupy  fault  fissures.  The 
gangue  consists  of  quartz,  calcite,  dolomite,  rhodochrosite,  rhodo- 
nite, kaolin,  and  fluorite.  The  metallic  minerals  include  pyrite, 
tetrahedrite,  sphalerite,  chalcopyrite,  galena,  polybasite,  ruby 
silver,  argentite,  and  in  some  deposits  tellurides  of  gold  and  silver 
in  small  amounts.  As  the  lodes  cut  all  the  volcanic  rocks  of  the 
region,  their  formation  is  regarded  as  not  earlier  than  late 
Tertiary. 

The  Camp  Bird  vein,1  which  is  at  present  the  most  productive 
deposit,  is  in  the  northwest  corner  of  the  Silverton  quadrangle, 
but,  because  of  transportation  facilities,  is  tributary  to  Ouray 
It  is  near  the  Telluride  district  and  generically  is  to  be  grouped 
with  the  Telluride  lodes. 

The  Yankee  Girl  and  associated  deposits  of  Red  Mountain 
are  pipe-like  masses  in  volcanic  rocks.  The  ores  down  to  about 
200  feet  from  the  surface  were  mainly  silver  and  lead,  galena  and 
pyrite  being  the  principal  minerals.  The  ore  below  this  zone  is 
composed  mainly  of  stromeyerite,  bornite,  chalcocite,  and  some 
gray  copper  and  barite,  yielding  about  30  per  cent,  of  copper  and 
little  or  no  lead.  In  depths  below  600  or  700  feet  the  rich  ore 
changed  to  pyritic  material,  that  was  too  low  grade  to  work. 
The  ores  in  the  copper-bearing  zone  were  exceptionally  rich, 
carrying  several  thousand  ounces  of  silver  to  the  ton. 

1  RANSOME,  F.  L. :  A  Report  on  the  Economic  Geology  of  the  Silverton 
Quadrangle,  Colo.  U.  S.  Geol.  Survey  Bull.  182,  1901. 

WOODS,  T.  H.,  and  DOVETON,  G.  D.:  The  Camp  Bird  Mine,  Ouray. 
Colorado.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  33,  pp.  499-550.  1903. 


456      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

South  of  Silverton,  in  the  Needle  Mountains  quadrangle1  is 
the  Bear  Creek  district,2  a  small  area  with  thin  quartz  veins  con- 
taining tellurides  and  other  minerals. 

Telluride. — The  Telluride  quadrangle3  adjoins  the  Silverton 
quadrangle  on  the  west,  and  the  principal  deposits  are  in  the 
northeast  corner  of  the  quadrangle.  The  district  is  one  of  the 
most  steadily  productive  in  the  San  Juan  region,  yielding  annu- 
ally about  $3,000,000  in  gold  and  silver.  The  ore  is  concen- 
trated, and  tailings  are  treated  in  cyanide  plants.  The  district 
includes  the  Liberty  Bell,  Smuggler-Union,  and  Tomboy  mines. 
The  Camp  Bird  mine,  in  the  Silverton  quadrangle,  is  essentially 
in  the  same  group.  The  deposits  are  strong  veins  in  rhyolites, 
andesites,  and  volcanic  tuffs.  The  principal  veins  trend  north- 
west; a  smaller  number  have  an  east  or  northeast  strike.  The 
deposits  contain  gold  and  silver,  and  some  carry  both  metals. 
There  is  no  evidence  of  more  than  one  period  of  mineralization. 
The  ore  minerals  include  quartz,  calcite,  siderite,  adularia,  rhodo- 
chrosite,  barite,  and  fluorite,  with  gold,  pyrite,  chalcopyrite, 
sphalerite,  and  galena. 

Ouray. — At  Ouray,4  north  of  the  Silverton  quadrangle,  the 
deposits  include  veins  and  replacement  deposits  in  quartzite 
and  limestone.  In  the  American  Nettie  mine  thin  veins  in 
flat-lying  quartzite  extend  upward  to  beds  of  shale.  In  the 
quartzite  they  form  pockets  of  rich  gold  ores  that  lie  parallel  to 
the  beds.  About  600  feet  below  the  horizon  of  the  American 
Nettie  deposits,  in  limestone  below  shale,  are  bedding-plane 

1  CBOSS,  WHITMAN,  HOWE,  ERNEST,  IRVING,  J.  D.,  and  EMMONS,  W.  H.: 
U.  S.  Geol.  Survey  Geol.  Atlas,  Needle  Mountains  folio  (No.  131),  1905. 

2  EMMONS,  W.  H. :  Ore  Deposits  of  Bear  Creek,  near  Silverton,  Colo.   U.  S. 
Geol.  Survey  Butt.  285,  pp.  25-27,  1906. 

3  CROSS,  WHITMAN,  and  PURINGTON,  C.  W.:   U.  S.  Geol.  Survey  Geol. 
Atlas,  Telluride  folio  (No.  57),  1899. 

PURINGTON,  C.  W. :  Preliminary  Report  on  the  Mineral  Industries  of  the 
Telluride  Quadrangle,  Colorado.  U.  S.  Geol.  Survey  Eighteenth  Ann.  Rept., 
part  3,  pp.  745-850,  1898. 

WINSLOW,  ARTHUR:  The  Liberty  Bell  Mine.  Am.  Inst.  Min.  Eng.  Trans., 
vol.  29,  pp.  285-307,  1899. 

RICKARD,  T.  A.:  Ore  Deposits,  A  Discussion  (reprinted  from  Eng.  and 
Min.  Jour.),  p.  88,  1903. 

4  CROSS,  WHITMAN,  HOWE,  ERNEST,  and  IRVING,  J.  D. :  U.  S.  Geol.  Sur- 
vey Geol.  Atlas,  Ouray  folio  (No.  153),  1907. 

IRVING,  J.  D.:  Ore  Deposits  of  the  Ouray  Quadrangle.  U.  S.  Geol. 
Survey  Bull.  260,  pp.  54-77,  1905. 


SILVER  457 

deposits  containing  pyrite  and  magnetite  with  garnet  and  actino- 
lite,  carrying  about  $12  gold  to  the  ton.  The  ore  body,  though 
small,  is  an  interesting  illustration  of  a  deposit  containing  min- 
erals characteristic  of  the  deep  zone  developed  at  moderate 
depths  below  flat-lying  shale  under  conditions  that  presumably 
did  not  favor  free  circulation  to  the  surface. 

Rico. — The  rocks  of  Rico1  include  Paleozoic  limestones, 
sandstones,  and  shales,  which  rest  on  pre-Cambrian  schists  and 
are  cut  by  dikes  and  laccolithic  sheets  of  monzonite  porphyry. 
The  ore  deposits  include  veins  and  ribbon-like  masses  that  make 
out  in  limestone  from  the  veins  where  they  cross  the  contact  of 
the  limestone  with  overlying  shale.  In  the  lower  levels  of  the 
mines,  some  200  feet  below  the  "contact,"  the  ore  is  mainly 
quartz,  pyrite,  and  chalcopyrite.  In  raising  on  the  veins 
rhodochrosite,  galena,  sphalerite,  and  tetrahedrite  become 
prominent.  Upward,  toward  the  "contact,"  the  proportion  of 
metallic  minerals  steadily  increases,  and  the  ore  becomes  much 
richer  in  gold  and  silver.  The  ore  minerals  include  argentite, 
polybasite,  stephanite,  pyrargyrite,  proustite,  and  native  silver. 

La  Plata  Mountains. — South  of  Rico  are  the  La  Plata  Moun- 
tains,2 a  laccolithic  group  composed  of  Jurassic  and  Triassic 
sandstones  and  shales,  intruded  by  huge  sheets  and  dikes  of 
monzonite  porphyry.  Both  igneous  and  sedimentary  rocks  are 
cut  by  many  veins  that  carry  tetrahedrite,  tennantite,  pyrite, 
galena,  sphalerite,  chalcopyrite,  and  locally  gold  and  silver 
tellurides.  In  the  Neglected  mine3  a  little  native  mercury  occurs 
in  the  altered  ore. 

Lake  City. — The  ore  deposits  of  Lake  City,4  east  of  Ouray,  are 

1  CROSS,  WHITMAN,  and  SPENCER,  A.  C. :  Geology  of  the  Rico  Moun- 
tains, Colorado.     U.  S.  Geol.  Survey  Twenty-first  Ann.  Rept.,  part  2,  pp. 
7-165,  1900. 

RANSOME,  F.  L.:  The  Ore  Deposits  of  the  Rico  Mountains,  Colorado. 
U.  S.  Geol.  Survey  Twenty-second  Ann.  Rept.,  part  2,  pp.  229-398,  1901. 

RICKARD,  T.  A.:  The  Enterprise  Mine,  Rico,  Colo.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  26.  pp.  906-980,  1896. 

PARISH,  J.  B.:  The  Ore  Deposits  of  Newman  Hill.  Colo.  Sci.  Soc.  Proc., 
vol.  4,  pp.  151-161,  1892. 

2  CROSS,  WHITMAN,  SPENCER,  A.  C.,  and  PURINGTON,  C.  W. :  U.  S.  Geol. 
Survey  Geol.  Atlas,  La  Plata  folio  (No.  60),  1899. 

3  EMMONS,  W.  H. :  The  Neglected  Mine  and  Near-by  Properties,  Durango 
Quadrangle,  Colorado.     U.  S.  Geol.  Survey  Butt.  260,  pp.  121-127,  1905. 

4  IRVING,  J.  D.,  and  BANCROFT,  ROWLAND:  Geology  and  Ore  Deposits 
near  Lake  City,  Colo.     U.  S.  Geol.  Survey  Butt.  478,  1911. 


458      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

in  Tertiary  flows  and  tuffs  which  consist  of  andesites,  rhyolites, 
latites,  and  basalts.  These  are  cut  by  intrusive  rhyolite,  quartz 
latite,  and  quartz  monzonite  porphyry.  The  deposits  are  narrow 
veins,  and  some  fill  conjugated  fissures.  Their  vertical  range, 
according  to  Irving  and  Bancroft,  is  over  5,000  feet,  and  in  their 
lower  levels  the  primary  minerals  are  chiefly  quartz,  galena,  zinc 
blende,  and  pyrite.  The  ores  formed  at  shallower  depths  in- 
clude also  tetrahedrite,  rhodochrosite,  barite,  and  jasperoid. 
The  mineralization  was  probably  effected  at  moderate  depths  by 
solutions  connected  genetically  with  the  quartz  monzonite  in- 
trusion or  with  some  closely  related  deeper  rock. 

Many  of  the  lodes  are  greatly  fractured.  Erosion  is  rapid, 
and  the  oxidized  zone  is  not  deep,  extending  generally  not  more 
than  100  or  200  feet  below  the  surface.  This  zone  contains  iron 
oxides  and  sulphates,  copper  carbonates,  and  considerable 
anglesite,  with  some  native  copper  and  silver.  Minerals  below 
the  oxidized  zone  include  pyrargyrite,  galena,  and  chalcocite. 
Native  gold  is  present  in  the  upper  part  of  the  zone  of  sulphide 
enrichment. 

Creede. — The  Creede  district1  is  in  Mineral  County,  near  the 
headwaters  of  the  Rio  Grande.  Mining  operations  began  in  1891, 
and  for  several  years  the  district  produced  over  $3,000,000 
annually.  Altogether  it  has  produced  about  $42,000,000. 
In  order  of  importance  the  metals  are  silver,  lead,  gold,  and  zinc. 
Most  of  the  ore  was  smelted  without  previous  concentration, 
low  rates  being  charged  on  account  of  its  highly  siliceous  character. 
In  late  years  mechanical  concentration  has  been  practiced.  The 
district  lies  within  the  great  Tertiary  volcanic  area  of  the  San 
Juan  Mountains,  and  no  rocks  other  than  Tertiary  flows  and 
intrusives  are  exposed  within  a  radius  of  several  miles.  Eleven 
formations  are  recognized,  each  of  which  has  been  made  up  of  one 
or  more  flows  of  rhyolite,  andesite,  or  basalt,  or  of  tuffs.  Lake 
beds  consisting  of  water-laid  fragments  of  rhyolite  and  extensive 
deposits  of  travertine  are  found  south  of  Creede.  The  lavas 
are  intruded  by  dikes  of  quartz  porphyry,  quartz  monzonite 
porphyry,  and  basalt. 

The  lava  flows  are  deformed  by  complicated  block  faulting, 
and  some  of  the  faults  have  throws  of  more  than  1,400  feet. 

1  EMMONS,  W.  H.  and  LARSEN,  E.  S. :  A  Preliminary  Report  on  the  Geol- 
ogy and  Ore  Deposits  of  Creede,  Colo.  U.  S.  Geol.  Survey  Bull.  530,  pp. 
42-65,  1913, 


SILVER  459 

The  deposits  are  veins,  for  the  most  part  occupying  normal  fault 
fissures;  some  of  them  follow  porphyry  dikes. 

The  Amethyst  lode,  the  most  productive  deposit,  has  been  ex- 
ploited for  about  9,500  feet  along  the  strike  and  to  depths  of 
1,00.0  to  1,400  feet  below  the  surface.  It  strikes  about  N.  22° 
W.  and  dips  50°  to  65°  SW.  It  occupies  a  fault  fissure  between 
the  rhyolite  formations.  At  several  places  along  the  strike  the 
vein  splits  to  inclose  horses  of  the  country  rock,  and  a  number  of 
small  veins  make  off,  especially  in  the  hanging  wall  of  the  main 
fissure. 

The  subordinate  fissures  are  especially  well  developed  in  the 
Last  Chance  mine.  On  level  6  a  zone  100  feet  wide  includes  six 
nearly  parallel  fissures,  each  from  6  inches  to  4  feet  wide.  All  of 
them  dip  westward,  but  the  foot  wall  has  the  lowest  dip,  and  the 
fissures  projected  join  it  below.  Above  this  level  the  ground  be- 
tween the  six  fissures  was  highly  altered  and  mineralized,  and 
the  ore  body  for  a  width  of  100  feet  was  stoped. 

The  minerals  that  constitute  the  unoxidized  ore  in  the  lower 
levels  of  the  Amethyst  vein  include  zinc  blende,  galena,  pyrite, 
chalcopyrite,  gold,  barite,  and  amethystine  and  white  quartz. 
In  the  country  rock  along  the  vein  secondary  quartz,  chlorite, 
adularia,  and  some  sericite  occur.  Thuringite,  a  chlorite  rich  in 
iron,  appears  in  the  filled  portion  of  the  vein,  as  well  as  in  the 
country  rock  near  it.  This  mineral,  which  is  unusual  in  western 
silver  deposits,  is  one  of  the  most  abundant  gangue  minerals  of 
the  Amethyst  vein.  Except  near  the  vein  hydrothermal  meta- 
morphism  is  not  pronounced. 

At  the  outcrop,  in  the  oxidized  zone,  and  ih  the  zone  of  partly 
oxidized  sulphides,  numerous  minerals  have  been  formed  as  a 
result  of  the  weathering  of  the  sulphide  ores.  These  include 
limonite,  hematite,  pyrolusite,  kaolin,  jarosite,  cerusite,  smith- 
sonite,  anglesite,  pyromorphite,  cerargyrite,  native  silver,  mala- 
chite, and  jasper.  Barite  is  more  abundant  in  the  upper  levels 
than  in  the  lower. 

The  most  valuable  deposits  were  silver-lead  ores  that  were 
found  200  to  700  feet  below  the  surface.  Here  and  there  in  and 
below  this  zone  occurs  rich  gold  ore,  consisting  of  sulphide  ore 
locally  oxidized,  cut  by  veinlets  of  manganous  oxide,  some  of 
which  carry  a  high  content  of  gold.  A  considerable  body  of  this 
ore  yielded  $20  a  ton  in  gold,  which  indicates  notable  enrich- 
ment, as  the  average  content  of  gold  in  the  mine  is  about  $2  a  ton. 


460      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Park  City,  Utah. — Park  City,1  a  heavy  producer  of  silver  and 
lead,  is  about  25  miles  southeast  of  Great  Salt  Lake,  Utah.  The 
sedimentary  rocks  of  the  district  are  of  Pennsylvanian,  Permian 
(?),  Triassic,  Jurassic  (?),  and  Eocene  age  and  consist  of  quartzite, 
limestone,  shale,  and  sandstone.  The  Eocene  rocks  grade  up- 
ward into  andesite  tuff  and  are  overlain  by  andesite.  The 
sedimentary  rocks  show  no  discordance  in  bedding.  They  are 
intruded  by  dikes,  sills,  stocks,  and  laccolithic  masses  of  quartz 
diorite  and  quartz  diorite  porphyry.  The  andesite  carries  frag- 
ments of  porphyry  and  is  therefore  younger  than  the  porphyry. 
Boutwell  concludes,  for  these  reasons,  that  the  quartz  diorite  and 
quartz  diorite  porphyry  are  as  late  as  Triassic  and  probably  older 
than  the  Eocene  rocks  on  which  the  andesite  rests. 

MINE  PRODUCTION  IN  PARK  CITY  MINING  REGION,  UTAH" 


Year 

Gold 

Silver 
(fine  ounces) 

Copper 
(pounds) 

Lead 

(pounds) 

Zinc 
(pounds) 

Total 
value 

1914  

$99,183 

2,955,008 

1,559,953 

32,323,066 

3,173,313 

$3,363,216 

1915  

86,980 

3,754,598 

2,287,172 

49,350,377 

7,771,350 

5,673,936 

1870-1915. 

4,145,295 

129,461,654 

28,230,670 

1,185,495.505 

55,853,076 

158,128,516 

•  HEIKES,  V.  C.:  U.  S.  Geol.  Survey  Mineral  Resources.  1915,  part  1,  p.  414,  1916. 

The  intrusion  of  igneous  rocks  was  attended  by  the  develop- 
ment of  zones  of  garnet  in  limestone  around  the  igneous  bodies, 
and  in  some  of  these  zones  there  are  chalcopyrite,  sphalerite,  and 
other  ores.  The  principal  ore  bodies,  however,  are  not  garnet- 
iferous.  The  dominating  structural  feature  is  a  great  anticline, 
broken  by  many  faults. 

The  ore  deposits  (Fig.  189)  are  replacement  veins  in  limestone, 
quartzite,  and  porphyry.  Although  the  limestones  are  inter- 
stratified  with  extensive  beds  of  shale,  the  shale  is  not  mineralized. 
The  lode  deposits  are  extensive,  strong,  and  valuable.  They  lie 
in  a  few  continuous  master  zones  rather  than  in  many  small 
fissures.  Examples  are  the  Ontario,  Daly  West,  Silver  King,  and 
Kearns-Keith  fissure  zones. 

The  earliest  deposits  were  formed  as  large  tabular  bodies  par- 
allel to  the  beds.  Later  the  great  crosscutting  fissure  zones  were 
formed  and  metallized.  Some  of  these  cut  across  the  earlier  de- 

1  BOUTWELL,  J.  M. :  The  Geology  and  Ore  Deposits  of  the  Park  City  Dis- 
trict, Utah,  with  Contributions  by  L.  H.  Woolsey.  U.  S.  GeoL  Survey  Prof, 
Paper  77,  1912, 


SILVER 


461 


Thaynes  Woodside  Park  City  Webar  Or. 

formation  shale  formation          quarti.te 


300  «00 


FIG.  189.— Geologic  section  through  Silver  King  mine,  Park  City  district, 
Utah.     (After  Boutwell,  U.  S.  Geol  Survey.) 


462      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

posits,  and  at  many  places  ore  shoots  of  contemporaneous  age 
make  out  from  them  parallel  to  the  beds.  Thus  there  are  bed- 
ding-plane deposits  of  two  periods  of  metallization,  but  all  the 
deposits  are  believed  to  be  genetically  related  to  intrusive  rocks, 
for  they  are  not  found  more  than  a  few  hundred  feet  from  the 
intrusives. 

The  ore  minerals  are  galena,  pyrite,  chalcopyrite,  sphalerite, 
tetrahedrite,  and  the  usual  oxidation  products.  The  gangue 
minerals  are  quartz,  jasper,  fluorite,  calcite,  and  rhodochrosite. 
Barite  is  practically  unknown  in  the  deposits,  the  sulphates  being 
represented  only  by  alteration  products.  Where  porphyry  lies 
along  the  walls  it  is  silicified  and  sericitized  and  contains  pyrite. 
The  bedded  ores  are  generally  richer  than  the  lode  ores. 

Tintic,  Utah. — The  Tin  tic  district,1  in  central  Utah,  yields 
complex  smelting  ores  containing  gold,  silver,  lead,  and  copper. 
Recently  valuable  zinc  deposits  have  been  developed.  The 
total  production  from  1869-1915  is  valued  at  $160, 149,227.  The 
area  is  occupied  by  a  thick  series  of  Paleozoic  quartzite,  slate, 
limestone,  and  sandstone,  which  are  overlain  by  Tertiary  rhyolite 
and  andesite.  These  rocks  are  intruded  by  great  masses  of 
monzonite  and  by  basalt  dikes,  and  an  andesite,  equivalent  to  the 
monzonite,  caps  the  rhyolite.  The  rocks  are  folded  and  exten- 
sively fractured  and  faulted. 

After  the  folding  of  the  Paleozoic  sedimentary  rocks  erosion, 
which  began  with  the  Mesozoic  uplift  and  continued  into  the 
Tertiary  period,  produced  a  surface  with  great  relief.  Upon 
this  eroded  surface  volcanic  material  was  poured  out,  the  earlier 
rhyolitic  lava  filling  deep  canyons,  on  whose  slopes  talus  was 
cemented  by  the  rhyolite  and  later  andesite.  The  more  com- 
pact igneous  and  sedimentary  rocks  were  fissured,  and  ores 
were  deposited  in  them.  Subsequently  great  masses  of  igneous 
rocks  have  been  removed  by  erosion,  with  only  slight  changes  in 
the  topography  of  the  limestone  ridges,  which  had  been  buried 
by  the  lavas,"  thus  exhuming  the  ancient  topography. 

The  ore  deposits  are  (1)  large  fractured  zones  in  sedimentary 
rocks,  chiefly  in  the  limestone;  (2)  veins  in  igneous  rocks;  and 

1  TOWER,  G.  W.,  JR.,  and  SMITH,  G.  O. :  Geology  and  Mining  Industry  of 
the  Tintic  District,  Utah.  U.  S.  Geol.  Survey  Nineteenth  Ann.  Rept.,  part 
3,  pp.  603-767,  1899. 

LOUGHLIN,  G.  F.:  The  Oxidized  Zinc  Ores  of  the  Tintic  District,  Utah. 
Econ.  Geol.,  vol.  9,  p.  1,  1914. 


SILVER  463 

(3)  contact-metamorphic  deposits  in  sedimentary  rocks  near 
intrusive  igneous  rocks,  mainly  in  limestone  near  monzonite. 

The  primary  ore  minerals  include  pyrite,  galena,  enargite, 
chalcopyrite,  and  tennantite.  The  gangue  includes  quartz, 
barite,  carbonates,  chalcedony,  and  gypsum.  Oxidation  prod- 
ucts are  limonite,  hematite,  anglesite,  cerusite,  cerargyrite, 
native  sulphur,  jarosite,  copper  carbonates,  cuprite,  native  copper, 
and  a  large  number  of  rare  arsenic  compounds  that  have  resulted 
from  the  decomposition  of  enargite.  Chalcocite  and  bornite 
become  increasingly  abundant  in  the  lower  parts  of  the  oxidized 
zone. 

Valuable  oxidized  zinc  ores  have  recently  been  developed. 
Apparently  they  have  been  deposited  by  ground  water  that 
dissolved  zinc  from  the  sulphide  bodies  and  migrated  into  the 
limestone  wall  rock.  Zinc  sulphate  reacting  on  lime  carbonate 
has  precipitated  smithsonite.  In  genesis  these  deposits  are 
probably  similar  to  the  great  zinc  carbonate  deposits  recently 
developed  at  Leadville,  Colo,  (see  page  476). 

Lindgren1  recognizes  zones  of  deposition  both  horizontal  and 
vertical. 

1.  In   veins  in   monzonite   quartz   occurs  in  well-developed 
crystals  with  much  pyrite  and  some  galena,  enargite,  zinc  blende, 
and  chalcopyrite. 

2.  In  the  sedimentary  rocks  to  a  distance  of  1  to  1^  miles 
north  of  the  contact  with  the  monzonite  the  gangue  consists  of 
fine-grained  replacement  quartz  and  much   barite.     The  ores 
contain  much  enargite,  with  a  little  pyrite,  tetrahedrite,  and 
famatinite.     There  are  a  few  shoots  of  lead  ore,  and  the  shoots 
of  copper  ore  contain  a  little  lead.     The  ores  carry  gold  and 
about  20  ounces  of  silver  to  the  ton. 

3.  Farther  north  in  the  same  vein  zones  the  deposits  in  the 
sedimentary  rocks  contain  principally  galena,  with  a  little  zinc 
blende  and  pyrite.     The  silver  content  is  higher  than  in  deposits 
farther  south,  the  average  of  the  ores  being  30  or  40  ounces  to 
the  ton.     There  is  practically  no  gold.     The  gangue  consists  of 
a  fine-grained  cherty  material  formed  by  the  replacement   of 
limestone  and  barite.     This  zone  continues  1  to  1J^  miles  north 
of  the  end  of  the  copper  zone. 

4.  Farther  north  and  east,  beyond  the  lead-silver  shoots,  the 
1  LINDGREN,  WALDEMAR:  Processes  of  Mineralization  and  Enrichment  in 

the  Tintic  Mining  District.     Econ.  Geol,  vol.  10,  pp.  225-240,  1915. 


464      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


mineralization  becomes  more  feeble.  The  gangue  minerals 
consist  of  calcite,  dolomite,  and  a  little  quartz;  the  ore  minerals 
comprise  galena  and  zinc  blende,  with  a  few  ounces  of  silver  to  the 
ton. 

Gold  and  copper  thus  occur  on  the  whole  near  the  monzonite; 
lead  and  silver  mainly  farther  away.  This  arrangement,  accord- 
ing to  Lindgren,  may  correspond  to  deposition  in  successively 
cooler  zones  and  a  gradual  spreading  of  the  ore-forming  solutions 
toward  the  north  until  they  became  so  mingled  with  surface 
waters  that  ore  deposition  declined. 

The  variation  in  ore  with  difference  in  depth  in  individual 
mines  is  less  marked,  yet  noteworthy.  Ore  shoots  of  gold,  silver, 
and  zinc  have  clearly  been  segregated  by  oxidation.  The  district 
is  remarkable  for  an  unusually  low  water  level  and  great  depth  of 
oxidation.  The  water  level  in  igneous  rocks  is  200  to  700  feet 
below  the  surface;  in  sedimentary  rocks  1,650  to  2,400  feet  below 
the  surface.  In  the  Gemini  mine  partly  oxidized  ores  have  been 
found  200  feet  below  the  present  water  level.  The  ore  as  deep 
as  level  21  of  the  Mammoth  mine  is  oxidized  and  honeycombed 
like  a  gossan. 

The  oxysalts  formed  in  lead  and  zinc  mines  consist  of  anglesite, 
cerusite,  plumbojarosite,  smithsonite,  calamine,  and  hydrozincite ; 
the  copper  mines  yield  many  copper  arsenates,  malachite,  and 
azurite,  more  rarely  cuprite  and  native  copper.  Silver  is  present 
as  argentite,  cerargyrite,  and  native  metal,  and  some  rich  oxidized 
ores  show  native  gold.  Covellite  and  chalcocite  are  found  in  the 
oxidizing  copper  ores,  though  nowhere  in  great  masses. 

PRODUCTION  OF  TINTIC  DISTRICT  UTAH" 


Year 

Gold 
(value) 

Silver 
(fine  ounces) 

Copper 
(pounds) 

Lead 
(pounds) 

Zinc 
(spelter) 
(pounds) 

Total  value 

1914  
1915  

$953,790 
916,755 

4,666,944 
4,370,984 

5,290,471 
5,357,932 

36,510,911 
32,657.018 

758,217 
3,845,058 

$5,700,837 
6,082,169 

1869-1915. 

30,832,935 

114,599,353 

145,269,267 

677,676.126 

11,909,556 

$160,149,227 

«  HBIKBS,  V.  C.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  402,  1916. 

Philipsburg,  Mont.1  —  Philipsburg  is  in  the  western  part  of 
Montana,  about  40  miles  northwest  of  Butte.     During  the  most 


,  W.  H.,  and  CALKINS,  F.  C.:  The  Geology  and  Ore  Deposits 
of  the  Philipsburg  Quadrangle,  Montana.  U.  S.  Geol.  Survey  Prof.  Paper 
78,  1913. 


SILVER  465 

active  period  of  mining  on  the  Granite-Bimetallic  lode  (1881- 
1893)  it  was  one  of  the  most  productive  silver-bearing  districts 
in  the  United  States.  In  recent  years  its  yield  has  been  smaller. 
The  Hope  mine,  one  of  the  oldest  in  Montana,  is  also  at  Philips- 
burg.  The  Philipsburg  quadrangle  includes  the  Cable  mine,  at 
Cable  and  the  Southern  Cross,  near  by;  also  the  Combination 
and  Henderson  mines,  northwest  of  Philipsburg.  The  total  pro- 
duction of  the  quadrangle  is  estimated  at  over  $50,000,000,  about 
four-fifths  of  which  is  in  silver  and  all  but  a  small  amount  of  the 
remainder  in  gold. 

The  Philipsburg  quadrangle  is  an  area  of  sedimentary  rocks 
ranging  in  age  from  pre-Cambrian  to  Pliocene,  with  intrusions  of 
granodiorite  and  related  rocks,  probably  belonging  to  the  same 
period  of  intrusion  as  that  of  the  Butte  quartz  monzonite  and 
other  batholiths  in  Montana. 

The  igneous  masses,  which  intrude  rocks  as  young  as  the  Cre- 
taceous, have  caused  pronounced  contact  metamorphism  of  the 
sedimentary  beds,  converting  the  calcareous  members  to  garnet 
and  tremolite  zones  and  the  argillaceous  members  to  andalusite 
and  scapolite  rocks.  Tourmaline  is  formed  at  considerable  dis- 
tances from  the  intrusives.  Silica,  iron,  alkalies,  fluorine,  chlo- 
rine, and  boron  have  been  added  to  the  sedimentary  rocks 
surrounding  the  intrusive,  probably  by  magmatic  solutions.. 

No  igneous  intrusion  or  extensive  deformation  took  place  be- 
tween Cambrian  and  late  Cretaceous  time,  but  the  region  was 
greatly  deformed  by  folding  and  faulting  in  early  Tertiary  time, 
about  contemporaneous  with  the  period  of  igneous  activity. 

The  ore  bodies  are  contact-metamorphic  deposits  in  sedimen- 
tary rocks,  veins  in  various  rocks,  and  bedding-plane  deposits,  in- 
cluding saddle  deposits,  in  sedimentary  rocks.  All  are  in  or  near 
igneous  bodies  and  probably  were  formed  through  the  agency  of 
magmatic  waters.  The  deposits  were  probably  formed  in  early 
Tertiary  time  at  moderate  depths— about  6,000  or  7,000  feet 
below  the  surface.  Some  of  the  veins  carry  pyrrhotite  and  specu- 
larite,  and  others  tourmaline,  indicating  conditions  approaching 
those  of  the  deep-vein  zone.  The  largest  and  most  persistent 
veins,  however,  are  free  from  these  and  other  high-temperature 
minerals.  They  probably  filled  openings  that  connected  freely 
with  the  surface  at  the  time  of  deposition  (see  page  56). 

In  calcareous  rocks  the  veins  and  bedding-plane  deposits  have 
formed  largely  by  replacement  of  the  walls.  In  shales  and  ig- 

30 


466      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

neous  rocks  replacement  has  been  less  pronounced,  but  in  igneous 
rocks  there  has  been  strong  sericitization  accompanied  by  the 
formation  of  carbonates  and  some  sulphides. 

The  Granite-Bimetallic  mine  is  working  a  strong  vein  in  grano- 
diorite,  which  carries  chiefly  silver  but  also  considerable  gold. 
It  fills  a  wide  fissure,  which  is  joined  by  subordinate  mineralized 
fractures.  There  is  conclusive  evidence  of  the  enrichment  of 
silver  below  the  water  level,  and  the  rich  silver  ore  contains  also 
more  gold  than  the  low-grade  silver  ore  in  the  bottom  of  the  mine. 
The  outcrop  of  this  deposit  carried  some  silver  but  very  little 
gold,  and  after  the  discovery  the  location  was  allowed  to  lapse 
because  of  the  small  assay  returns  from  the  gossan.  Richer  ore 
with  secondary  cerargyrite,  native  silver,  and  ruby  silver,  in 
cracks  across  the  older  sulphides,  appeared  in  considerable 
amounts  200  to  400  feet  below  the  surface  and  extended  to  depths 
of  800  or  900  feet.  The  shoot  of  high-grade  ore,  which  extended 
for  about  a  mile  along  the  strike  of  the  deposit,  followed,  in  a 
broad  way,  the  present  rugged  surface  (see  Page  131).  The 
gangue  is  rich  in  manganese.  Zinc  blende  is  abundant  at 
several  places  in  the  primary  ore  below  the  richer  sulphides. 
Some  migration  of  gold  has  undoubtedly  taken  place.  No  asso- 
ciated placers  have  been  formed. 

The  deposits  of  the  Cable  mine  are  included  in  a  long,  thin 
block  of  limestone,  in  contact  on  each  side  with  quartz  monzon- 
ite.  Garnet  and  amphiboles  occur  in  the  limestone,  but  the  ores 
are  only  locally  associated  with  garnet.  The  principal  minerals 
are  calcite,  quartz,  pyrrhotite,  pyrite,  magnetite,  and  chalcopy- 
rite,  with  chlorite,  muscovite,  and  other  silicates.  The  deposits 
were  formed  by  replacement  under  contact-metamorphic  condi- 
tions. Valuable  placers  were  worked  near  the  outcrop.  Good 
ore  was  found  at  or  very  near  the  surface,  and  the  tenor  increased 
somewhat  for  a  short  distance  below  the  surface.  Some  con- 
centration has  taken  place  by  the  removal  of  calcite  and  other 
valueless  material  more  rapidly  than  gold,  but  there  is  no 
evidence  of  enrichment  in  gold  below  the  water  table. 

At  the  Southern  Cross  mine1  the  deposits  of  oxidized  gold  ore 
fill  a  complex  of  fissures,  of  which  some  follow  the  calcareous  beds 
and  others  cut  across  them. 

The  ore  bodies  of  the  Hope  mine  are  extensive  replacement  de- 

1  BILLINGSLY,  PAUL:  The  Southern  Cross  Mine,  Georgetown,  Mont.  Am 
Inst.  Min.  Eng.  Trans.,  vol.  46,  pp.  128-136,  1913. 


SILVER  467 

posits  in  folded  limestone,  and  most  of  them  follow  bedding 
planes.  Some  become  wider  at  the  crests  of  folds.  A  number  of 
thin,  nearly  vertical  veins  cross  the  beds.  Although  the  country 
rock  is  locally  tremolitized,  the  ore  bodies  do  not  carry  heavy 
silicate  minerals.  Extensive  normal  faulting  followed  the  de- 
position of  the  ore  bodies. 

The  Combination  lode  is  an  extensive  bedding-plane  deposit 
in  tilted  quartzite  that  dips'  about  15°.  The  great  blanket-like 
deposit  is  broken  into  many  blocks  by  normal  faults  (see  Fig.  54, 
page  111).  The  ore  is  highly  siliceous  and  carries  silver  and  some 
copper.  The  grade  decreases  with  increasing  depth. 

Comstock  Lode,  Nev. — The  Comstock  lode  (Washoe  district) 
is  in  Storey  County,  Nevada,  about  20  miles  southeast  of  Reno. 
At  the  end  of  1912  it  had  produced  about  $380,000,000  in  gold 
and  silver,  which  is  more  than  the  production  of  any  other  pre- 
cious-metal camp  in  the  United  States.  About  60  per  cent,  of 
this  sum  is  in  silver  and  40  per  cent,  in  gold.  Much  of  the  ore  was 
rich,  over  $30  a  ton,  but  in  recent  years  ore  running  below  $14 
a  ton  has  been  treated.  In  the  early  history  of  the  district,  when 
production  was  at  its  zenith — that  is,  in  the  seventies  and  eighties 
of  the  last  century — the  "  Washoe"  process  (pan  amalgamation) 
was  extensively  employed.  In  recent  years  the  ore,  as  well  as 
much  of  the  old  tailings,  has  been  treated  by  cyanidation.  Pro- 
duction has  declined  greatly,  however,  and  is  now  less  than 
$500,000  annually.  The  workings  in  depth  are  hot.  Large  vol- 
umes of  hot  sulphate  water  (170°F.)  rise  from  the  deepest  work- 
ings and  greatly  hinder  mining.  At  some  places  where  the  cir- 
culation is  poor  the  men  work  15-minute  shifts  in  a  blast  of  cold 
air  and  a  spray  of  cold  water.  The  Comstock  lode  has  a  certain 
distinction,  also,  as  'the  place  where  Philip  Deidesheimer  invented 
"square-set"  timbering,  since  then  employed  extensively  else- 
where for  supporting  large  chambers  in  heavy  ground. 

The  lode1  lies  along  a  broad  fault  in  late  Tertiary  rocks  (Figs. 
190  and  191).  It  strikes  a  few  degrees  east  of  north  and  dips 

KING,  CLARENCE:  The  Comstock  Lode,  in  HAGUE,  J.  D.,  Mining 
Industry.  U.  S.  Geol.  Expl.  Fortieth  Par.  Rept.,  vol.  3,  pp.  11-96, 
1870. 

CHURCH,  J.  A.:  "The  Comstock  Lode,"  New  York,  1879. 

BECKER,  G.  F. :  Geology  of  the  Comstock  Lode  and  the  Washoe  District. 
U.  S.  Geol.  Survey  Mon.  3,  1882. 

REID,  J.  A. :  The  Structure  and  Genesis  of  the  Comstock  Lode  Cal.  Univ., 

Dept.  Geol.  Bull.,  vol.  4,  pp.  177-199,  1905. 


468      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Wile 


FIG.  190. — Geologic  sketch  map  of  Comstock  lode  and  vicinity,  Washoe 
district,  Nevada.  Black  is  quartz  and  vein  matter.  (Based  on  map  by 
Becker,  U.  S.  Geol.  Survey.) 


SILVER  469 

about  45°  E.  The  foot  wall  is  diorite,  and  the  hanging  wall  is 
mainly  diabase.  This  fault  may  be  traced  about  13,000  feet 
along  the  strike.  The  hanging  wall  was  apparently  shattered  as 
it  fell,  and  many  nearly  vertical  fractures  in  it  join  the  lode  in 
depth.  The  structural  relations  (see  p.  193)  are  somewhat 
similar  to  those  of  the  Amethyst  vein  at  Creede,  Colo,  and  of 
the  lodes  at  Tonopah,  Nev. 

The  country  rock  is  greatly  altered  by  hydrothermal  processes. 
Chlorite,  sericite,  and  pyrite  are  developed  and  probably  sec- 
ondary orthoclase.  Propylitization  (page  249)  is  extensive. 
Along  the  fault  is  a  body  of  quartz  and  vein  matter  several  hun- 
dred feet  wide.  The  ore  shoots  are  found  here  and  there  in  this 
quartzose  material,  and  some  of  them  make  off  in  the  hanging 
wall  along  secondary  fractures.  Much  of  the  quartz  is  barren. 
The  ore  is  composed  of  native  gold,  native  silver,  argentite, 
stephanite,  and  rich  galena,  with  a  little  pyrargyrite,  polyba- 


FIG.  191. — Section   through   Comstock  lode,    Nevada,   on   Sutro   tunnel. 
Black  is  quartz  and  vein  matter.     (After  Becker,  U.  S.  Geol.  Survey.) 

site,  horn  silver,  and  sternbergite.  Intimately  associated  with 
those  minerals  are  iron  and  copper  pyrites  and  zinc  blende.  The 
gangue  is  quartz  with  some  calcite.  Oxidation  of  the  ore 
yields  abundant  manganese  oxide,  probably  from  the  calcite. 

After  deposition  much  of  the  ore  was  fractured,  and  locally 
quartz  by  movement  was  reduced  to  powder.  Since  the  period 
of  crushing  additional  quartz  and  ore  have  been  introduced  into 
the  fissure.  In  a  few  places,  as  in  the  800-foot  level  of  the 
Yellow  Jacket  mine,  broken  fragments  of  quartz,  themselves 
containing  ore,  have  been  recemented  by  sheets  of  stephanite 
that  have  penetrated  the  cracks,  and  over  the  stephanite  a 
secondary  growth  of  quartz  crystals  has  taken  place.1 

Placers  of  subordinate  importance  were  worked  in  streams  that 
head  near  the  lode.  These  yielded  electrum,  a  gold  alloy  rich  in 
silver.  It  is  said  that  the  alloy  near  the  lode  carried  only  about 

1  KING,  CLARENCE:  Op.  cit.,  p.  81, 


470      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

25  per  cent,  of  gold,  but  farther  away  it  was  richer.  It  was 
through  the  exploitation  of  these  placers  that  the  lode  was 
discovered. 

The  Comstock  lode  was  probably  deposited  by  ascending  hot 
waters  at  comparatively  shallow  depths.     In  many  of  its  fea- 


FIG.  192. — Geologic  sketch  map  of  part  of  Tonopah  district,   Nevada. 
(After  Spurr,  U.  S.  Geol.  Survey.) 

tures  it  resembles  deposits  at  Tonopah  and  Tuscarora,  Nev., 
and  at  Pachuca  and  Esperanza,  Mexico. 
Tonopah,  Nev. — Tonopah1  is  in  the  desert  region  of  western 

1  SPURR,  J.  E.:  Geology  of  the  Tonopah  Mining  District,  Nevada.  U.  S. 
Geol.  Survey  Prof.  Paper  42,  1905;  Geology  and  Ore  Deposition  at  Tono- 
pah, Nev.  Econ.  Geol.,  vol.  10,  p.  713,  1915. 

BURGESS,  J.  A. :  Geology  of  the  Producing  Part  of  the  Tonopah  District. 
Econ.  Geol,  vol.  4,  p.  681,  1909. 


SILVER 


471 


Nevada,  about  160  miles  southeast  of  Reno.  All  the  rocks  near 
Tonopah  are  of  Tertiary  age,  probably  Miocene  and  later,  and 
all  are  eruptive  except  a  series  of  water-laid  tuffs  which  repre- 
sent the  accumulations,  mainly  of  fine  volcanic  detritus,  in  a 
Tertiary  lake  (Figs.  192,  193). 

METAL  PRODUCTION  OP  TONOPAH  DISTRICT,  NEVADA,  1914-1915° 


Year 

Pro- 
ducers 

Ore 
(short  tons) 

Gold 

Silver 
(fine  ounces) 

Copper 
(pounds) 

Lead 
(pounds) 

Total 
value 

1914     

23 

531,278 

$2,648,833 

11,388,452 

2,284 

924 

$8,946,987 

1915  

22 

516,337 

2,228,983 

10,171,374 

7,385,870 

Total,  1904-1915 

3,858,464 

21,491,950 

98,162,762 

12,113 

214,523 

77,280,916 

HEIKES,  V.  C.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  647,  1916. 


othet-   Earli* 
deep-    leal  deep 
ated         seated 
granit 


Tonopah    Lake  beds.    Faults. 


. 

rhyolite  (lesser  veins 

intru-  belonging  to 

•ions.  other  periods 


FIG.  193. — Ideal  section  to  illustrate  relations  in  the  Tonopah  district, 
Nevada.     (After  Spurr,  U.  S.  Geol.  Survey.) 

The  first  eruption  of  the  volcanic  period  was  the  Mizpah 
trachyte  (formerly  called  the  Earlier  andesite).  Later  an 
andesite  (somewhat  more  basic  and  formerly  called  the  later 
andesite)  was  formed.  Subsequently  rhyolite  and  dacite  were 
erupted  and  produced  the  volcanoes  whose  necks,  left  in  relief 
by  the  erosion  of  the  surrounding  softer  material,  now  form 
the  hills  around  Tonopah. 


472      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  area  occupied  by  the  dacitic  volcanic  necks  is  coextensive 
with  the  region  of  observed  complicated  faulting.  'This  faulting 
was  probably  initiated  chiefly  by  the  intrusion  of  the  necks. 
After  the  intrusion  and  subsequent  eruption  there  was  a  collapse, 


Montana     e 
Tonopah  shaft  _' 


Fia.  194. — Cross-section  of  Montana  Tonopah  workings,  Tonopah,  Nevada. 
(After  Spurr,  U.  S.  Geol.  Survey.) 

or  sinking  of  various  vents.     The  lava  in  sinking  dragged  down 
with  it  adjacent  blocks  of  the  intruded  rock. 

The  principal  mineral  veins,  as  shown  by  Spurr,  occur  in  the 
Mizpah  trachyte,  and  do  not  extend  into  the  overlying  rocks  (Fig. 


SILVER  473 

194).  These  veins  have  been  formed,  chiefly  by  replacement, 
along  sheeted  zones.  Transverse  fractures  have  determined  the 
position  of  cross  walls  and  ore  shoots  by  limiting  and  concentrat- 
ing the  circulation.  The  mineralization  was  probably  caused 
by  hot  ascending  waters  immediately  after  the  earlier  trachyte 
eruptions. 

The  zone  of  oxidation  extends  to  greater  depth  in  the  more 
highly  fractured  places,  and  for  this  reason  the  brittle  and  more 
broken  lodes  are  more  deeply  oxidized  than  the  wall  rock.  The 
Mizpah  vein  is  for  the  most  part  oxidized  to  a  depth  of  700  feet. 
The  oxidized  ore  contains  limonite  and  manganese  dioxide,  with 
horn  silver  and  bromides  and  iodides  of  silver.  The  oxidized 
ore  from  the  outcrop  down  is,  according  to  Spurr,  a  mixture  of 
original  sulphides  and  selenides,  together  with  secondary  sul- 
phides, chlorides,  and  oxides.  At  a  depth  of  500  feet  in  the  Mon- 
tana Tonopah  mine  good  crystals  of  argentite,  polybasite,  and 
chalcopyrite  have  been  formed  freely  in  cracks  and  druses  of  the 
sulphide  ore.  These  minerals  are  later  than  the  massive  ore. 
Pyrargyrite  is  formed  in  cracks  in  the  oxidized  ore,  and  some 
argentite  fringes  minute  particles  of  horn  silver  as  if  secondary 
to  it. 

A  series  of  veins,  of  little  commercial  importance,  was  formed 
after  the  eruption  of  the  Tonopah  rhyolite-dacite.  These  veins 
are  characterized  by  irregularity  and  by  lack  of  definition  and 
persistence,  though  some  are  large.  They  may  disappear  by 
scattering  and  pass  into  a  silicified  wall  rock.  These  veins  are 
usually  barren  or  contain  only  very  small  quantities  of  gold  and 
silver.  Barite,  which  is  present  in  some  of  them,  is  not  known 
in  the  veins  in  the  earlier  andesite. 


CHAPTER  XXVI 

ZINC  AND  LEAD 
ZINC 


Mineral 

Percentage 
of  zinc 

Composition 

Goslarite 

22  6 

ZnSO*  7H2O 

Smithsonite  . 

52  0 

ZnCO3 

Calamine  
Willemite  
Hydrozincite    
Zincite 

54.2 
58.5 
60.0 
80  3 

Zn2H2SiO5  or  2ZnO.SiO4.H2O. 
Zn2SiO4. 
ZnCO3.2ZnO2H2  or  3ZnO.CO2.2H2O. 
ZnO 

Frank!  inite  . 

16  0 

(FeZnMn)O.(Fe,  Mn)2O». 

Sphalerite  (zinc  blende)  . 
Wurtzite   

67.0 
67.0 

ZnS. 
ZnS. 

Zinc  Minerals. — Sphalerite  is  by  far  the  most  abundant  zinc 
mineral,  although  smithsonite  and  calamine  are  mined  in  con- 
siderable quantities  in  deposits  that  are  altered  near  the  surface. 
Nearly  all  zinc  sulphide  ores  carry  some  lead;  the  western  ores 
of  zinc  generally  carry  silver,  and  some  of  them  carry  both  silver 
and  gold.  Pyrite,  pyrrhotite,  chalcopyrite,  and  galena  are  very 
commonly  associated  with  sphalerite  in  sulphide  deposits.  The 
most  common  gangue  minerals  are  quartz,  calcite,  and  dolomite. 
Rhodochrosite  and  rhodonite  are  abundant  in  zinc  ores  at  Butte, 
Mont.  In  some  deposits  the  heavy  silicates,  such  as  garnet  and 
actinolite,  are  intergrown  with  sulphides  of  zinc  and  lead. 

Goslarite  occurs  on  the  walls  of  some  mine  workings  as  a  white 
efflorescence.  Owing  to  the  high  solubility  of  zinc  sulphate  in 
water,  goslarite  is  comparatively  rare  in  most  districts. 

Smithsonite  is  commonly  found  in  the  oxidized  zones  of  zinc- 
bearing  veins.  It  is  most  abundant  in  deposits  in  limestone. 
It  has  not  been  reported  as  a  primary  mineral  in  deposits  formed 
by  hot  ascending  waters.  Where  zinc  sulphate  waters  attack 
limestone,  smithsonite  is  deposited,  calcium  sulphate  going  into 
solution.  The  reaction  is  stated  as  follows: 


ZnSO4  +  CaC03 


2H20  = 

474 


CaS04.2H2O  +  ZnC03. 


ZINC  AND  LEAD  475 

The  calcium  sulphate  precipitated  as  gypsum  may  remain  with 
smithsonite  or  may  be  carried  away  in  solution,  as  it  is  fairly 
soluble  in  cold  water. 

Calamine  is  commonly  associated  with  smithsonite  in  the 
oxidized  zones  of  zinciferous  ores.  It  is  not  known  as  a  primary 
mineral  of  ore  veins  deposited  from  hot  solutions. 

Willemite,  the  anhydrous  silicate,  is  much  less  common  than 
calamine.  It  is  abundant  in  the  primary  ores  of  Franklin  Fur- 
nace, N.  J.,  but  is  not  reported  as  occurring  in  the  secondary  ores 
at  many  zinc  deposits. 

Hydrozincite  the  basic  carbonate,  occurs  commonly  in  altered 
ores. 

Aurichalcite,  a  basic  carbonate  of  zinc  and  copper,  is  deposited 
by  cold  solutions,  generally  as  drusy  incrustations. 

Sphalerite  is  the  most  abundant  primary  ore  of  zinc.  At  some 
places  it  has  been  shown  to  be  secondary  also.  Wurtzite,  as 
shown  by  Butler,  is  secondary  in  the  San  Francisco  district, 
Utah.  It  is  probably  secondary  at  Butte,  Mont. 

Genesis  of  Zinc  Deposits. — Zinc  is  rare  in  deposits  formed  by 
magmatic  segregation  but  occurs  in  deposits  formed  at  consider- 
able depths  and  in  contact-metamorphic  deposits.  In  some 
deposits  of  the  last-named  class  sphalerite  is  abundant.  The 
great  deposits  of  zinc  in  North  America,  which  are  associated 
with  igneous  intrusives,  were  formed  for  the  most  part  at  moder- 
ate depths.  Zinc  offers  some  puzzling  problems  in  its  concentra- 
tion by  enrichment.  It  dissolves  very  readily  in  sulphuric  acid, 
and  as  a  rule  outcrops  of  zinc-bearing  ores  are  leached  of  zinc. 
In  calcareous  rocks  the  zinc  which  is  leached  out  of  sulphide  ores 
is  deposited  as  carbonate  by  replacement  of  the  lime  carbonate 
around  the  older  deposit.  Some  zinc  that  is  dissolved  in  sul- 
phate solutions  descends  and  is  deposited  as  secondary  sphal- 
erite and  wurtzite. 

The  principal  primary  zinc  sulphide  is  the  isometric  sphalerite. 
Wurtzite,  the  hexagonal  zinc  sulphide,  is  comparatively  rare. 
In  some  occurrences  the  primary  zinc  minerals  are  oxides,  such 
as  franklinite,  zincite,  and  gahnite,  but  these  are  rare  or  absent 
in  sulphide  deposits. 

As  zinc  sulphide  is  easily  dissolved  in  sulphuric  acid  sphalerite 
would  not  form  in  a  highly  acid  solution.  If,  through  reactions 
with  the  wall  rock,  the  solution  should  become  feebly  acid  or 
neutral,  then  zinc  sulphide  could  be  precipitated.  Allen  and 


476      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Crenshaw1  state  that  sphalerite  is  precipitated  from  acid  as  well 
as  from  alkaline  solutions,  but  that  wurtzite  is  precipitated  only 
from  acid  solutions  and  is  probably  always  secondary.  As  zinc 
sulphide  is  one  of  the  most  soluble  of  the  common  sulphides  it 
could  not  replace  sulphides  of  copper,  silver,  or  lead.  Possibly 
it  could  replace  pyrite,  marcasite,  or  pyrrhotite  under  certain 
conditions,  but  no  examples  of  its  pseudomorphs  after  these 
minerals  are  known. 

There  is  every  reason  to  suppose  that  the  zinc  sulphide  in  car- 
bonate rocks  in  the  zinc  deposits  of  southwestern  Wisconsin  has 
been  dissolved  by  ground  waters  and  has  been  reprecipitated 


FIG.  195. — Ideal  diagram  showing  a  large  mass  9f  sulphide  ore  in  lime- 
stone. The  lead  ore  is  partly  oxidized  but  remains  in  place.  By  oxidation 
and  leaching  iron  and  zinc  sulphates  are  formed  and  move  downward. 
Zinc  carbonate  (black)  is  precipitated  around  the  original  ore  body  and  in 
joints  and  bedding  planes  near  it. 


as  sulphide  in  large  amounts  where  the  solutions  were  in  contact 
with  reducing  agents. 

Secondary  zinc  blende  has  been  found  in  western  ore  deposits 
in  rocks  other  than  limestone,  but  so  far  as  the  records  show  its 
occurrence  in  western  deposits  is  rare.  Large  quantities  of  zinc 
are  undoubtedly  dissolved  from  the  outcrops  and  oxidized  zones 
of  deposits  in  which  it  occurs  as  sulphide  and  are  passed  as  sul- 
phate downward,  below  the  water  level.  Considerable  amounts, 
however,  may  be  carried  in  fissures  and  fractures  that  join  the 
original  deposit  and  be  redeposited  at  some  distance  from  the 

1  ALLSN,  E.  T.,  and  CRENSHAW,  J.  L. :  The  Sulphides  of  Zinc,  Cadmium, 
and  Mercury;  Their  Crystalline  Forms  and  Genetic  Conditions.  Am.  Jour. 
Sci.,  4th  ser.,  vol.  34,  p.  359,  1912. 


ZINC  AND  LEAD 


477 


primary  deposit.1  On  account  of  the  greater  solubib'ty  of  their 
salts  in  an  alkaline  and  reducing  environment,  zinc  and  iron 
migrate  more  extensively  and  farther  from  the  original  sources 
than  gold  and  silver.  At  Tintic,  Utah,  valuable  bodies  of  oxi- 
dized zinc  ore2  are  found  on  the  borders  of  older  sulphide  deposits. 
In  this  district  and  at  Leadville  Colo.,3  and  in  the  Cerro  Gordo 
vein,  California,4  oxidized  iron-stained  zinc  carbonate  ores,  un- 
noticed for  many  years,  have  lately  become  prominent  sources 
of  zinc  (Fig.  195). 

Joplin  Region. — The  Joplin  region,  which  is  mainly  in  south- 
western Missouri,  extends  into  neighboring  portions  of  Kansas 
and  Oklahoma.  Although  zinc  and  lead  are  found  over  an  area 
of  about  3,100  square  miles,  more  than  four-fifths  of  the  output 
of  the  region  comes  from  an  area  of  100  square  miles  centering 
about  Joplin  and  Webb  City,  Mo.  It  is  the  most  productive 
zinc-bearing  region  in  the  United  States.  The  deposits  have 
been  known  since  1850,  and  the  mines  have  produced  over 
1,000,000  tons  of  lead  concentrates  and  5,000,000  tons  of  zinc 
concentrates.  The  ore  in  general  is  of  low  grade,  and  enormous 
tonnages  are  treated,  especially  of  ore  from  the  "sheet 
ground."  Considering  the  nature  of  the  deposits,  the  low 
cost  of  mining  in  this  region  is  noteworthy. 


LEAD  AND  ZINC  PRODUCED  IN  MISSOURI  IN  1915 


L 

ead 

2 

inc 

District 

Quantity 
(short  tons) 

Value 

Quantity 
(short  tons) 

Value 

Southwestern  Missouri  .  .  . 
Central  and  southeastern 
Missouri  

26,534 
183,906 

$2,494,196 
17,287,164 

135,928 
372 

$33,710,144 
92,256 

State  total,  1915  

210,440 

19,781,360 

136,300 

33,802,400 

1  PENROSE,  R.  A.  F.,  JR.  :  Certain  Phases  of  Superficial  Diffusion  in  Ore 
Deposits.  Econ.  Geol,  vol.  9,  p.  20,  1914. 

"LouGHLiN,  G.  F.:  The  Oxidized  Zinc  Ores  of  Tintic,  Utah.  Econ.  Geol, 
vol.  9,  p.  1,  1914. 

3  BUTLER,  G.  M. :  Some  Recent  Developments  at  Leadville,  The  Oxidized 
Zinc  Ore.     Econ.  Geol,  vol.  8,  p.  1,  1913. 

4  KNOPF,  ADOLPH:  Mineral  Resources   of  the  Inyo  and  White  Moun- 
tains, Cal.    U.  S.  Geol.  Survey  BuU.  540,  p.  97,  1914. 


478      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

All  the  rocks1  of  the  region  are  sedimentary.  They  dip  south- 
westward  at  very  low  angles  away  from  the  Ozark  uplift  (Fig. 
196).  The  surface  is  a  rolling  prairie.  Carboniferous  rocks 
only  are  exposed.  These  are  for  the  most  part  Mississippian, 
but  here  and  there  small  remnants  of  Pennsylvanian  rocks  are 
found.  The  following  formations  are  represented: 

Cherokee  formation  (Pennsylvanian):  Shale,  sandstone,  and 
cool  beds,  top  eroded;  at  some  places  it  rests  on  Carterville,  at 
others  on  Boone  chert;  at  many  places  entire  formation  has  been 
removed  by  erosion 0  to  150  + 

Unconformity. 

Carterville  formation  (Pennsylvanian):  Shale  and  sandstone. 
Rests  on  eroded  surface  of  Boone ;  not  everywhere  present  .  .  0  to  50 

Unconformity,  marked  by  an  erosion  surface  of  the  Boone 
with  valleys  and  ridges. 

Boone  formation  (Mississippian):  A  thick  cherty  limestone. 
It  contains  the  Grande  Falls  chert  member,  from  15  to  120  feet 
thick.  The  top  is  an  erosion  surface  subsequently  covered  by 
Pennsylvanian  shale  and  sandstone.  The  Boone  is  the  principal 
ore-bearing  formation.  The  "sheet  ground "  is  in  the  Grand 
Falls  chert 145  to  485 

Pre-Boone  limestone,  sandstone,  and  locally  shale. 

During  the  two  periods  of  erosion  represented  by  the  uncon- 
formities above  and  below  the  Carterville  formation  the  Boone 
limestone  was  deeply  trenched  and  a  topography  was  developed 
characterized  by  underground  drainage.  Caves  were  formed, 
perhaps  of  the  same  order  of  magnitude  as  the  Mammoth 
Cave  of  Kentucky,  and  limestone  sinks  were  numerous.  The 
country  was  near  sea  level,2  and  solution  greatly  exceeded 

1  BAIN,  H.  F. :  Preliminary  Report  on  the  Lead  and  Zinc  Deposits  of  the 
Ozark  Region.  U.  S.  Geol.  Survey  Twenty-second  Ann.  Rept.,  part  2,  pp. 
23-228,  1901. 

SMITH,  W.  S.  T.,  and  SIEBENTHAL,  C.  E. :  U.  S.  Geol.  Survey  Geol.  Atlas, 
Joplin  District  folio  (No.  148),  1907. 

SIEBENTHAL,  C.  E.:  Structural  Features  of  the  Joplin  District.  Econ. 
Geol.,  vol.  1,  pp.  119-128,  1906. 

BUCKLEY,  E.  R.,  and  BUEHLER,  H.  A.:  The  Geology  of  the  Granby 
Area.  Mo.  Bur.  Geol.  and  Mines,  vol.  4,  2d  ser.,  1909. 

HA  WORTH,  ERASMUS:  Relation  between  the  Ozark  Uplift  and  Ore  Depos- 
its. Geol.  Soc.  America  Bull,  vol.  11,  pp.  231-240,  1900. 

SIEBENTHAL,  C.  E.:  Origin  of  the  Zinc  and  Lead  Deposits  of  the  Joplin 
Region,  Missouri,  Kansas,  and  Oklahoma.  U.  S.  Geol.  Survey  Bull.  606, 
1916. 

2 BUCKLEY,  E.  R..  BAIN  H.  F.  and  others:  "Types  of  Ore  Deposits,"  p. 
118,  1911. 


ZINC  AND  LEAD  479 

stream  erosion.  On  the  surface  there  accumulated  great 
bodies  of  residual  chert,  especially  on  hillsides  and  along  cliffs 
bordering  streams.  This  chert  is  typically  shown  in  the 
Granby  district,  where  it  has  been  termed  the  "Granby"  forma- 
tion by  Buckley  and  Buehler.  The  "Granby"  and  Boone  were 
covered  by  the  Carterville.  Later  the  Carterville  was  eroded  in 
places,  and  the  Cherokee  was  deposited  on  the  eroded  surface  of 
the  Boone  or,  where  it  was  present,  on  the  Carterville.  After  the 
Boone  had  been  buried  below  later  beds  it  contained,  at  and  near 
its  top,  water  channels,  such  as  solution  cavities  and  buried  talus 
of  chert.  In  places  the  beds  above  solution  cavities  slumped 
down,  producing  solution  faults.1  The  cavities  and  breccia  were 
later  cemented  with  ores. 

The  principal  minerals  are  galena  and  sphalerite  and  their  al- 
teration products,  smithsonite,  calamine,  etc.  Some  pyrite, 
marcasite,  and  chalcopyrite  are  also  present,,  with  their  alteration 
products.  The  gangue  consists  of  chert,  calcite,  aragonite,  and 
quartz.  Associated  with  the  ore  at  many  places  is  a  hydrocarbon 
"tar,"  derived  from  organic  matter  originally  in  the  sedimentary 
beds.  Chert  is  abundant.  The  older  residual  chert  is  cemented 
by  a  later  variety,  presumably  deposited  from  the  ore-bearing 
solutions. 

The  deposits  in  the  "sheet  ground"  are  extensive  and  lie  flat 
along  certain  horizons,  mainly  in  the  Grand  Falls  chert  member. 
The  ore  zones  are  about  15  feet  thick,  and  the  ore  has  evidently 
been  deposited  around  chert  or  in  old  solution  cavities. 

It  is  believed  that  ground  waters  dissolved  the  metals,  that 
the  solutions  circulated  in  the  breccia  and  ancient  caves,  and  that 
the  metals  were  deposited  in  them  in  part  by  reduction  through 
the  agency  of  organic  material  in  the  rocks.  Buckley  and  Bueh- 
ler held  that  the  solutions  moved  downward  from  higher  rocks, 
now  generally  eroded.  Bain  and  Van  Hise  maintained  that  the 
solutions  circulated  down  the  southwest  slope  of  the  Ozark 
uplift  and  rose  along  faults  in  the  Joplin  region.  Siebenthal 
found  that  some  of  the  faults  were  formed  by  caving  of  beds 
above  solution  cavities,  and  as  they  did  not  extend  below  the 
cavities  they  could  not  offer  deep  channels.  Siebenthal,  however, 
attributes  the  metallization  to  ascending  waters  which  circulated 
through  cracks  and  joints.  He  reports  many  analyses  of  muds 

1  SIEBENTHAL,  C.  E.:  Structural  Features  of  the  Joplin  District.  Econ. 
Geol,  vol.  1,  p.  119,  1906. 


480      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


that  settled  from  artesian  waters  in 
this  region  and  that  were  found  to 
contain  iron,  zinc,  and  lead  sulphides 
and  a  little  copper. 

The  sedimentary  rocks  slope  south- 
westward  from  the  Ozark  uplift. 
The  waters  following  down  the  dip  of 
the  limestone  beds  dissolved  the 
•metals  that  were  present  in  small 
amounts.  West  of  Joplin  the  lime- 
stones are  covered  by  impermeable 
shale,  and  this  dammed  back  the 
waters,  which  rose  through  joints  or 
other  openings  to  the  surface  (see  Fig. 
196).  In  the  limestone  beds  the 
waters,  which  carried  carbonates,  dis- 
solved the  metals  and  liberated  hy- 
drogen sulphide.  As  long  as  excess 
carbon  dioxide  was  present  the  metals 
could  remain  in  solution.  Artesian 
waters,  rising  to-day  from  below  the 
shale  capping,  contain  lead,  zinc,  and 
hydrogen  sulphide,  and  when  these 
waters  issue  at  the  surface  the  carbon 
dioxide  and  hydrogen  sulphide  escape 
and  sulphides  are  deposited.  It  is 
believed  that  the  ores  were  deposited 
by  the  solutions  rising  to  the  surface 
through  the  limestone.  Precipitation 
was  accomplished  by  organic  matter 
in  the  limestones  and  cherts  and  was 
facilitated  by  escape  of  gases  to  the 
surface.  This  hypothesis,  as  pointed 
out  by  Siebenthal,  appears  to  be 
strongly  supported  by  the  position 
of  the  deposits  with  respect  to  struc- 
tural features.  They  are  grouped  in 
the  limestone  near  the  edge  of  the 
Pennsylvanian  shale,  and  in  the  re- 
gion containing  lenses  of  the  Devonian 
shale  they  are  practically  absent  ex- 


ZINC  AND  LEAD 


481 


cept  just  under  the  edge  of  the  shale,  where  there  is  a  return 
flow  of  solutions. 

Since  deposition  some  of  the  ores  have  been  oxidized,  and  some 
solution  and  reprecipitation  have  taken  place.  By  removal  of 
zinc  and  calcite  galena  has  been  concentrated  near  the  surface; 
anglesite,  smithsonite,  and  cerusite  have  been  developed;  and 
greenockite  has  been  deposited  on  crystals  of  older  sulphides. 

Wisconsin  Region. — In  the  upper  Mississippi  Valley,  in  south- 
western Wisconsin,  northwestern  Illinois,  and  northeastern  Iowa, 
are  numerous  deposits  of  zinc  and  lead.1  The  rocks  of  this  area 
are  limestone,  sandstone,  and  shale.  They  dip  very  gently  to 

TENOR  OF  LEAD  AND  ZINC  ORE  AND  CONCENTRATES  PRODUCED  IN  WISCON- 
SIN, 1914  AND  1915° 


1914 

1915 

Total  crude  ore,  short  tons  

1,387,490 

1,934,000 

Total  concentrates  in  crude  ore: 
Lead,  per  cent  
Zinc,  per  eent  

0.14 
8.02 

0.16 
7.32 

Metal  content  in  crude  ore: 
Lead  per  cent                                             .    .    . 

0  11 

0.12 

Zinc,  per  cent                              

2.80 

2.65 

Average  lead  content  of  galena  concentrates,  per 

73.70 

73.50 

Average  zinc  content  of  sphalerite  concentrates, 

35.05 

36.50 

Average  zinc  content  of  zinc  carbonate  concen- 

27.50 

24.90 

a  DUNLOP,  J.  P.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  126,  1916. 

1  CHAMBERLIN,   T.   C. :  The   Ore   Deposits  of  Southwestern   Wisconsin. 
Wis.  Geol.  Survey,  vol.  4,  pp.  378-575,  1882. 

VAN  RISE,  C.  R.:  Some  Principles  Controlling  the  Deposition  of  Ores. 
Am.  Inst.  Min.  Eng.  Trans.,  vol.  30,  pp.  102-108,  1900. 

GRANT,  U.  S.:  Lead  and  Zinc  Deposits  of  Wisconsin.     Wis.  Geol.  Survey 
Bull.  9,  1903. 

BAIN,  H.  F.:  Zinc  and  Lead  Deposits  of  the  Upper  Mississippi  Valley. 
U.  S.  Geol.  Survey  Bull.  294,  1906. 

Cox,  G.  H.:  Lead  and  Zinc  Deposits  of  Northwestern  Illinois.     Illinois 
Geol.  Survey  Bull.  4,  1910. 

Cox,  G.  H.:  The  Origin  of  Lead  and  Zinc  Ores  of  the  Upper,  Mississippi 
Valley  District.     Econ.  Geol,  vol.  6,  pp.  427-448,  582-603,  1911. 

LEONARD,  A.  G. :  Lead  and  Zinc  Deposits  of  Iowa.    Iowa  Geol.  Survey, 
vol.  6,  pp.  9-66,  1897. 
31 


482      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  southwest,  and  in  places  there  are  small  shallow  synclines. 
The  rocks  are  fissured  and  heavily  jointed.  In  some  places,  as 
in  the  Federal  mine,  faults  with  vertical  displacements  of  over  a 
foot  occur,  but  there  are  no  great  faults  in  this  region.  Outcrops 
of  igneous  rocks  likewise  are  lacking. 

The  rock  section  is  shown  below.  The  ores  are  principally 
in  the  Galena  limestone,  though  some  are  in  the  Platteville  lime- 
stone and  some  in  the  Maquoketa  shale.  Most  of  the  workable 
deposits  are  near  the  base  of  the  Galena,  though  some  are  in  the 
upper  part. 

Feet 

Quaternary:  Alluvium,  terrace  deposits,  loess,  residual  clays 5  to  70 

Silurian:  Niagara  dolomite 150 

Ordovician : 

Maquoketa  shale 160 

Galena  dolomite 240 

Platteville  limestone  and  dolomite 55 

St.  Peter  sandstone 80 

Shakopee  dolomite 50 

New  Richmond  sandstone , 10  to  40 

Oneota  dolomite 200 

Cambrian:  Pottsdam  sandstone  with  minor  shale  and  dolomite. .  .  800 
Pre-Cambrian :  Quartzite  with  various  igneous  rocks. 

The  generalized  section  at  the  main  ore  horizon  is  as  follows : 

Feet 

Dolomitic  limestone  (Galena),  free  from  chert 50 

Oil  rock  (Galena) ^  to  6 

Shale  or  blue  clay,  called  the  "clay  seam"  (Platteville) ^  to  4 

Brittle  limestone,  "glass  rock"  (Platteville). 
Magnesian  limestone  (Platteville), 

The  shale  or  clay  seam  is  at  the  top  of  the  Platteville.  The  oil 
rock  is  an  impure  shaly  limestone  rich  in  organic  matter,  which 
according  to  David  White,1  is  chiefly  microscopic  algae.  The 
oil  rock,  according  to  Rollin  Chamberlin,  is  very  porous  and  light, 
having  a  specific  gravity  of  1.98  and  yielding  gas  bubbles  when 
placed  in  water.  One  volume  of  the  rock  gave  57.46  volumes  of 
gas  when  heated  in  a  vacuum  for  two  hours.  Analysis  of  this 
material  gave  the  following  results: 

IBAIN,  H.  F.:  Op,  cit.,  p.  26. 


ZINC  AND  LEAD  483 

ANALYSIS  OF  GAS  FROM  OIL  ROCK  OF  DUGDALB  PROSPECT,  WISCONSIN" 

Hydrocarbon  vapors 11.11 

Heavy  hydrocarbons 4  00 

CH4 35.98 

H2S ' 6.79 

CO2 18.12 

CO 8.40 

0 0.26 

H2 13.18 

N2 2.21 


100.05 

0  BAIN,  H.  F. :  Op.  tit.,  p.  26. 

The  ore  deposits  are  in  "crevices,"  in  "runs,"  disseminated  in 
beds,  and  in  "flats  and  pitches."  The  crevices,  termed  by  J.  D. 
Whitney  "gash  veins,"  are  fissures  and  joints  enlarged  somewhat 
by  solution  and  cemented  with  ore.  Certain  beds  appear  to  be 
especially  favorable  to  concentration  of  the  ore,  and  where  these 
are  cut  by  crevices,  flat-lying  irregular  ribbons  of  ore  are  de- 
veloped at  and  near  the  intersections.  Such  ore  bodies  are  termed 
"runs." 

The  ore  in  the  "flats"  follows  the  flat  beds,  and  the  ore  in  the 
"pitches"  follows  crevices  that  pitch  or  dip  away  about  45°  from 
either  side  of  the  vertical  crevices.  The  pitches  in  a  deposit  join 
at  the  end,  making  in  plan  a  long,  slender  ellipse  where  they  inter- 
sect the  oil  rock.  The  form  of  the  whole  body  has  been  com- 
pared by  Chamberlin  to  the  domestic  flatiron  (see  page  81). 
As  shown  by  Grant,  this  is  a  very  common  structural  type,  and 
frequently  the  interior  of  the  ellipse  is  filled  with  low-grade  dis- 
seminated ore,  so  that  long,  relatively  narrow  masses  are  worked. 
Deposits  that  are  largely  workable  are  commonly  as  much  as 
1,000  feet  long,  75  feet  wide,  and  40  or  50  feet  high. 

The  genesis  of  these  deposits,  as  stated  by  Chamberlin,  Grant, 
and  Bain,  is  essentially  as  follows:  The  lead,  zinc,  and  iron  were 
originally  deposited  on  the  sea  bottom  at  the  time  the  Galena 
dolomite  was  laid  down.  The  metals  were  probably  in  solution 
as  sulphates  and  chlorides  and  were  reduced  by  organic  matter  to 
sulphides  at  the  time  of  their  deposition.  Later,  when  the  beds, 
containing  small  amounts  of  metals,  were  elevated  and  the 
Maquoketa  shale1  was  removed,  a  more  active  circulation  was 

1  According  to  Cox,  the  Maquoketa  shale  also  carried  original  sulphides 
and  was  an  important  source  of  ore. 


484      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

initiated.  The  waters  were  oxidizing,  and  the  metals  were  then 
dissolved,  probably  as  sulphates  or  as  carbonates.  The  crevices, 
or  enlarged  joints,  and  the  flats  and  pitches,  opened  by  stresses 
and  by  settling  of  the  limestone  above  the  shrinking  oil  rock, 
were  filled  with  ore,  and  the  rock  near  by  was  replaced  by  the  ore- 
depositing  solutions.  Locally  the  oil  rock  was  replaced  also. 
There  was  some  brecciation  of  the  ore  itself  as  a  result  of  further 
settling,  and  the  fragments  of  ore  were  cemented  by  calcite  and 
other  minerals. 

It  is  noteworthy  that  the  clay  seam  at  the  top  of  the  Platte- 
ville  seals  off  an  artesian  circulation  in  sandstone  beds  below  the 
Platteville.1  The  ores,  which  are  mainly  above  the  clay  seam, 
must  have  been  deposited  by  superficial  waters.  The  gentle 
synclines  which  many  of  the  deposits  occupy  are  believed  to  be 
the  original  troughs  of  the  sea  bottom  subsequently  accentuated 
by  moderate  compressive  stresses.  In  such  troughs  the  oil  rock 
is  thickest  and  ground  waters  converge.  The  oil  rock  would 
shrink  most  where  it  was  thickest,  and  fractures  would  be  local- 
ized in  thick  parts  of  the  oil  rock,  especially  where  thick  parts 
were  bounded  by  thin  parts,  thus  permitting  differences  in 
amounts  of  settling. 

The  mineral  composition  of  the  ores  is  simple.  They  are 
made  up  of  sphalerite,  galena,  marcasite,  calcite,  and  rarely  ba- 
rite.  Precious  metals  and  cadmium  are  lacking.  Quartz-  is 
practically  absent.  The  wall  rock  is  essentially  unchanged  near 
the  ore  deposits  and  shows  no  evidence  of  hydrothermal  action. 
To  a  large  extent  the  deposits  fill  fissures,  though  some  replace 
the  limestone  wall  rock.  As  a  rule  marcasite  was  formed  first 
and  was  followed  by  sphalerite  and  that  by  galena.  Near  the 
surface  sphalerite  has  been  changed  to  smithsonite  and  calamine, 
and  much  zinc  has  been  removed  by  solution,  leaving  a  concen- 
tration of  galena  associated  with  limonite,  anglesite,  and  cerusite. 
The  lead  sulphide  persists  at  the  very  surface,  and  some  deposits 
have  been  discovered  by  farmers  when  they  plowed  up  galena  in 
their  fields. 

Eastern  Tennessee. — In  eastern  Tennessee  zinc  deposits  are 
found  at  many  widely  separated  places,  but  the  principal  deposits 
are  between  Knoxville  and  Morristown,  in  a  belt  about  40  miles 
long  and  a  mile  wide,  and  in  Claiborne  County  about  35  miles 

1  BAIN,  H.  F.:  "Types  of  Ore  Deposits,"  p.  82,  San  Francisco,  1911. 


ZINC  AND  LEAD  485 

north  of  Knoxville.1  In  southwestern  Virginia2  similar  ores  are 
found  in  similar  rocks  and  possibly  in  an  extension  of  the  same 
belt.  The  zinc  is  of  unusual  purity,  carrying  little  iron  or  lead, 
and  commands  a  high  premium  in  the  market.  Recently  the 
American  Zinc  Co.  has  built  three  mills,  two  of  which  have  a 
combined  capacity  of  more  than  2,500  tons  daily.  In  1915 
Tennessee  produced  32,912,902  pounds  of  zinc,  valued  at 
$7,318,405. 

The  rocks  of  this  region  are  limestones,  dolomites,  and  calca- 
reous shales.  They  are  closely  folded  and  faulted.  They  strike 
northeast  and  dip  from  10°  to  45°.  No  igneous  rocks  are  known 
in  this  vicinity.  The  deposits  are  in  the  Knox  dolomite  (Cambro- 
Ordoyician) ;  in  Virginia  this  formation  is  called  the  Shenandoah 
limestone.  Although  they  are  not  confined  closely  to  any  particu- 
lar layer,  in  Tennessee  the  deposits  are  principally  near  the  top 
and  bottom  of  that  formation.  They  are  in  the  main  bedding- 
plane  deposits,  but  at  some  places  the  ores  are  developed  along 
fault  planes  or  brecciated  zones,  and  locally  their  deposition  ap- 
pears to  have  been  influenced  by  the  folds  in  the  limestone. 

The  ore  minerals  are  sphalerite,  galena,  and  pyrite.  A  little 
cadmium  is  present.  The  sphalerite  is  light-colored  and  nearly 
free  from  iron.  The  gangue  is  calcite  and  dolomite.  In  the  Fall 
Branch  group  a  hydrocarbon  compound  has  been  noted.3  Oxi- 
dation has  yielded  the  usual  alteration  products — smithsonite, 
calamine,  anglesite,  etc.  There  is  not  much  pyrite  in  the  ore, 
and  in  general  oxidation  has  not  extended  to  great  depths. 

Mineralogically  these  deposits  resemble  those  of  southwestern 
Missouri  and  southwestern  Wisconsin.  They  are  believed  to 
have  been  formed  by  cold  solutions. 

Butte,  Mont. — The  Butte  district  is  one  of  the  largest  pro- 
ducers of  zinc  in  the  United  States — a  distinction  only  recently 
achieved.  In  1915  it  produced  186,514,375  pounds  of  spelter. 
The  geology  of  the  district  is  discussed  on  page  357.  It  is  an 

1  PURDUE,  A.  H. :  The  Zinc  Deposits  of  Northeastern  Tennessee.     Tenn. 
Geol.  Survey  Bull.  14,  1912. 

KEITH,  ARTHUR:  Recent  Zinc  Mining  in  Eastern  Tennessee.  U.  S.  Geol. 
Survey  Bull.  225,  pp.  208-213,  1904. 

NASON,  F.  L. :  The  Zinc  Deposits  of  Eastern  Tennessee.  Eng.  and  Min. 
Jour.,  vol.  99,  p.  734,  1915. 

2  WATSON,   T.  L. :  Lead  and  Zinc  Deposits  of  the  Virginia-Tennessee 
Regions.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  36,  p.  681,  1905. 

3  PURDUE,  A.  H.:  Op.  tit.,  p.  60. 


486      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

area  of  quartz  monzonite  crossed  by  several  fracture  systems, 
two  of  which  are  heavily  mineralized.  The  oldest  or  easterly 
system  of  fractures  includes  the  principal  copper  lodes,  among 
them  the  Anaconda  and  Bell-Speculator  vein  systems.  Of  simi- 
lar age  and  trend  is  the  Rainbow  lode,  which  lies  only  a  few  hun- 
dred feet  north  of  the  Bell-Speculator  group.  This  lode,  which 
received  its  name  from  its  crescentic  outline,  is  the  largest  and 
most  productive  silver  lode  of  the  district.  It  is  made  up  of 
many  closely  spaced  anastomosing  veins  of  the  replacement  type. 
These  crop  out  boldly  and  are  heavily  stained  with  iron  and  man- 
ganese. The  upper  portions  have  been  worked  successfully  for 
silver.  The  most  valuable  silver  deposits  were  found  in  the 
Alice  mine,  in  which  silver  decreased  with  increasing  depth  and 
sphalerite  appeared  in  great  quantities  around  the  1,000-foot  level. 
On  the  east  end  of  the  Rainbow  lode,  for  about  2,000  feet  along  its 
strike,  great  zinc  deposits  have  been  developed  in  the  Elm  Orlu 
and  Black  Rock  mines.  In  their  upper  levels  both  these  mines 
were  worked  for  silver.  Zinc  appeared  in  quantity  at  depths  be- 
tween 400  and  700  feet  below  the  surface.  Huge  deposits  are 
developed  to  the  bottoms  of  these  mines,  which  are  opened  to 
depths  of  about  1,700  to  1,900  feet  below  the  surface. 

The  deposits  are  replacement  veins  and  fractured  zones,  locally 
over  100  feet  wide.  The  minerals  include  sphalerite,  pyrite, 
galena,  rhodochrosite,  rhodonite,  and  quartz.  Appreciable  quan- 
tities of  silver  are  present.  Chalcopyrite,  bornite,  chalcocite, 
and  other  copper  sulphides  typical  of  the  copper  veins  are  found 
locally  in  the  zinc  deposits.  Like  the  copper  deposits  of  the  Butte 
district,  the  zinc  deposits  appear  to  have  been  formed  at  moderate 
depths  by  deposition  from  ascending  hot  waters  genetically  re- 
lated to  igneous  activities. 

A  noteworthy  feature  of  these  deposits  is  the  absence  of  carbo- 
nates and  silicate  of  zinc  in  the  zone  of  oxidation.  This  is  proba- 
bly due  to  thorough  leaching  by  sulphuric  acid  generated  by 
the  oxidation  of  abundant  pyrite. 

Coeur  d'Alene  District,  Idaho. — The  Coeur  d'Alene  region  of 
Idaho  has  for  many  years  produced  considerable  zinc  as  a  by- 
product of  the  concentration  of  lead  ore,  and  recently  some  of 
the  mines  have  encountered  in  depth  large  bodies  of  zinc  ore. 
The  Interstate-Callahan  mine,  one  of  the  most  productive  zinc 
mines  in  the  United  States,  yielded  45,226,949  pounds  of  spelter 
in  1915.  It  is  about  7  miles  northeast  of  Wallace.  The  rocks  of 


ZINC  AND  LEAD  487 

the  area  are  pre-Cambrian  quartzite  and  slate  cut  by  monzonite 
and  related  igneous  intrusives.1  The  ore  deposits  are  veins. 
The  Interstate-Callahan  mine  is  in  the  Prichard  slate.  The 
vein  strikes  about  N.  58°  W.  and  carries  in  places  14  feet  of  high- 
grade  ore.  The  ore  minerals  are  sphalerite,  galena,  pyrite,  and 
quartz.  Little  or  no  siderite  is  present.  Considerable  silver  is 
contained  in  the  galena  concentrate. 

Franklin  Furnace,  N.  J.— The  Franklin  Furnace  district 
is  in  Sussex  County,  New  Jersey,  about  50  miles  northwest 
of  Jersey  City.  Although  discovered  as  early  as  1650,  the  de- 
posits were  not  actively  exploited  until  1860.  In  1915  the  mines 
produced  272,084,000  pounds  of  zinc,  figured  as  zinc  oxide  and 
spelter.  This  was  recovered  from  737,310  tons  of  ore.  The 
larger  part  of  the  ore  is  concentrated,  partly  by  magnetic  proc- 
esses, but  a  considerable  amount  is  smelted  directly.  The 
residuum  obtained  from  smelting  some  of  the  zinc  ores  carries 
12  per  cent,  of  manganese  and  40  per  cent,  of  iron.  Much  of 
this  material  is  utilized  for  the  manufacture  of  spiegeleisen,  a 
product  added  to  iron  in  making  high-grade  steel. 

The  rocks  of  the  Franklin  Furnace  area2  are  pre-Cambrian 
gneiss  and  limestone  and  Cambrian  limestone  and  quartzite. 
The  ore  deposits  are  in  the  southwest  end  of  a  band  of  limestone 
that  extends  northeastward  22  miles  into  Orange  County, 
New  York  (Fig.  197).  Both  limestone  and  gneiss  are  bounded 
by  later  Cambrian  limestone.  The  main  deposits  are  at  Mine 
Hill  and  at  Sterling  Hill,  about  2  miles  apart.  Both  are  spoon- 
shaped,  or  synclinal,  and  pitch  about  20°  NE.  The  ore  layer 
is  from  1  to  100  feet  thick,  and  the  total  length  of  the  "keel"  of 
the  syncline  at  Mine  Hill  is  3,500  feet.  The  Sterling  Hill  deposit 
is  a  great  mass  of  low-grade  zinc-bearing  material  250  feet  wide. 

1  RANSOME,  F.  L.,  and  CALKINS,  F.  C. :  The  Geology  and  Ore  Deposits 
of  the  Coeur  d'Alene  District,  Idaho.     U.  S.  Geol.  Survey  Prof.  Paper  62, 
1908. 

2  SPENCER,  A.  C.,  KUMMEL,  H.  B.,  WOLFF,  J.  E.,  and  PALACHE,  CHARLES: 
U.  S.  Geol.  Survey  Geol.  Atlas,  Franklin  Furnace  folio  (No.  161),  1908. 

SPENCER,  A.  C. :  The  Mine  Hill  and  Sterling  Hill  Zinc  Deposits  of  Sussex 
County,  New  Jersey.  N.  J.  State  Geologist  Ann.  Rept.  for  1908,  pp.  23-52, 
1909. 

NASON,  F.  L.:  The  Franklinite  Deposits  of  Mine  Hill,  Sussex  County, 
New  Jersey.  Amer.  Inst.  Min.  Eng.  Trans.,  vol.  24,  pp.  121-130,  1895. 

KEMP,  J.  F.:  The  Ore  Deposits  at  Franklin  Furnace  and  Ogdensburg, 
New  Jersey.  N.  Y.  Acad.  Sci.  Trans.,  vol.  13,  p.  76,  1894. 


488      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


The  ore  minerals  are  unusual  species:  franklinite  constitutes 
50  per  cent,  of  the  ore,  willemite  20  to  30  per  cent.,  and  zincite 
about  4  per  cent.  Other  minerals  are  calcite,  tephroite,  zinc 
pyroxene,  zinc  spinel,  zinc  garnet,  and  axinite.  Still  other 


Cambrian  Franklin 

Quartzite  Limestone 

and  Lime-          (White 

stone  Crystalline 

(dlue  Limestone)  Limestone) 


Gneiss 


Zinc  Ore 
Bodies 


Magnetite 


FIG.  197. — Geologic  map  and  sections  of  Franklin  Furnace  region,  New 
Jersey.     (Based  on  map  by  Spencer,  U.  S.  Geol.  Survey.) 

minerals,  including  sphalerite,  have  been  deposited  locally,  espe- 
cially near  pegmatite  veins  that  cross  the  ore  bodies  here  and 
there. 
The  zinc  deposits  grade  into  limestone  and  doubtless  were 


ZINC  AND  LEAD 


formed  by  replacement  of  limestone.  The  origin  of  these  deposits 
is  a  vexed  problem,  not  yet  solved.  Nason  suggests  an  origin 
connected  genetically  with  irhe  gneiss,  which  was  formerly  granite. 
Both  Kemp  and  Spencer  consider  the  probability  of  deposition 
by  ascending  magmatic  waters.  Such  a  deposit  might  be  pro- 
duced by  close  folding  of  an  extensive  bedding-plane  deposit 
after  oxidation.  •  It  is  noteworthy  that  the  granite  now  shows 
gneissic  structure  and  has  been  metamorphosed  by  dynamic 
processes  since  it  was  formed.  The  ore  minerals  are  somewhat 
metamorphosed  but  less  extensively  than  the  granite.  The 
association  of  minerals  formed  ordinarily  at  high  temperatures 
suggests  processes  of  contact  or  dynamic-metamorphism. 

LEAD 


Mineral 

Percentage  of 
lead 

Composition 

Lead                   .    . 

100.0 

Pb. 

Minium  

90.6 

2PbO.PbO2. 

92  8 

PbO. 

Plattneritc 

86.6 

PbO2. 

Pyromorphite 

76.3 

3Pb3P2O8.PbClI. 

Cotunnite  

74.5 

PbCl2. 

Anglesite  

68.3 

PbSO4. 

Cerusite  

77.5 

PbCO3. 

Galena  

86.6 

PbS. 

Lead  Minerals. — Of  the  lead  minerals  galena,  cerusite,  and 
anglesite  are  the  most  abundant.  The  gangue  minerals  quartz, 
chert,  siderite,  and  calcite  are  their  common  associates.  Most 
galena  ores  connected  genetically  with  igneous  rocks  contain 
silver  or  gold  or  both.  The  silver  in  argentiferous  galena  is 
commonly  supposed  to  be  present  as  silver  sulphide.  Some  zinc 
is  generally  found  in  lead  deposits,  and  in  many  zinc  ores  lead 
is  a  valuable  by-product. 

In  its  primary  deposits  lead  is  restricted  to  fewer  classes  than 
copper.  Economic  concentrations  in  igneous  rocks  are  unknown. 
Some  lead  is  found  in  contact-metamorphic  deposits  and  in  veins 
formed  at  considerable  depths,  but  in  only  a  few  of  these  is  it 
abundant.  It  is  characteristically  developed  at  moderate  depths 
and  in  deposits  formed  by  cold  solutions  in  calcareous  rocks. 

In  North  America  deposits  of  lead  associated  with  igneous 
rocks  have  been  formed  principally  in  late  Cretaceous  and  early 
Tertiary  time.  These  are  characteristic  of  the  "central  belt" 


490      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

(page  404).  Some  lead  is  obtained,  however,  from  deposits  of 
Miocene  age.  Galena  is  present  in  a  few  depo«its  of  the  deep  zone. 

The  percentage  of  lead  in  lead  ores-  is  variable.  In  the  Coeur 
d'Alene  region,  Idaho,  the  ore  averages  about  7.8  per  cent.  In 
the  disseminated  deposits  of  southeastern  Missouri  the  ore  carries 
3.6  per  cent.  In  many  regions  either  the  lead  is  obtained  as 
a  by-product,  or  the  ore  carries  besides  lead,  several  other  metals. 

All  natural  salts  of  lead  have  low  solubilities.  The  native 
metal  and  the  oxides  of  lead  (minium,  plattnerite,  and  massicot) 
are  of  rare  occurrence.  Lead  chloride  is  moderately  soluble,  so 
cotunnite  does  not  accumulate  in  the  oxidized  zones  of  lead  de- 
posits. Lead  chlorophosphate,  pyromorphite,  is  much  more 
common.  Lead  carbonate  is  very  sparingly  soluble,  so  cerusite 
is  a  comparatively  stable  mineral.  Although  the  solubility  of  the 
sulphate  anglesite  is  also  low,  it  is  nevertheless  appreciable. 

The  transfer  of  lead  as  sulphate  in  small  quantities  was  shown 
in  experiments  of  Buehler  and  Gottschalk.  As  would  be  sup- 
posed from  consideration  of  the  relatively  low  solubility  of  its 
sulphate,  lead  is  not  extensively  transferred  in  cold  sulphuric 
acid  waters.  The  salts  that  form  under  natural  conditions  coat 
the  sulphides,  retarding  further  action.  Consequently  lead 
sulphide  dissolves  slowly.  It  is  dissolved  in  acid  to  only  a  slight 
extent  and,  like  copper  sulphide,  would  be  deposited  in  an  acid 
environment.  Although  several  primary  minerals  contain  lead, 
galena  is  the  only  one  of  these  that  is  common.  The  chlorides, 
oxides,  sulphates,  and  carbonates  are  probably  formed  as  second- 
ary minerals  only.  Lead,  like  gold,  migrates  very  slowly  in 
cold  solutions. 

Although  native  lead  is  found  in  a  number  of  lead  deposits,  it  is 
in  very  few  so  abundant  as  to  become  an  important  ore  mineral. 
It  is  probably  formed  by  reduction  of  oxygen  salts  of  lead. 

Pyromorphite  is  the  principal  metalliferous  phosphate.  It  is 
an  alteration  product  of  lead  ores  that  are  exposed  to  waters 
carrying  chlorine  and  phosphoric  acid.  In  many  deposits  it  is 
associated  with  limonite  and  commonly  carries  silver,  possibly 
as  finely  divided  cerargyrite. 

Anglesite  is  known  only  as  an  alteration  product,  generally 
from  galena.  In  many  districts  it  is  a  valuable  ore  mineral  in  the 
oxidized  zones  of  sulphide  deposits. 

Cerusite,  unknown  as  a  primary  mineral,  is  common  as  an  alter- 
ation product.  It  is  an  important  ore  mineral  in  many  districts, 


ZINC  AND  LEAD 


491 


Galena  is  formed  under  many  natural  conditions.  It  is  the 
principal  primary  ore  of  lead.  The  stability  of  galena  in  the  oxi- 
dized zone  has  already  been  mentioned.  Galena  is  commonly 
found  in  sluice  boxes  of  placer  mines  and  is  plowed  up  in  the 
fields  of  the  southwestern  Wisconsin  zinc  district.  Even  in  dis- 
tricts where  the  climate  is  comparatively  moist,  galena  is  often 
found  at  the  very  outcrops  of  some  ore  veins. 

Southeastern  Missouri. — The  disseminated  lead  deposits  of 
southeastern  Missouri1  are  about  50  miles  south  of  St.  Louis,  in 
St.  Francis  and  adjoining  counties.  These  deposits  produce 
about  one-third  of  the  lead  of  the  United  States. 

TENOR  OF  CRUDE  LEAD  ORE  AND  CONCENTRATES  IN  SOUTHEASTERN 
MISSOURI  DISSEMINATED  LEAD  ORE  IN  1914  AND  1915" 


1914 

1915 

Total  crude  lead  ore  short  tons 

4,718,300 

5,067,800 

Galena  concentrates  in  crude  ore,  per  cent  
Lead  content  in  crude  ore,  per  cent  

5.27 
3.56 

5.47 
3.62 

Average  lead  content  of  galena  concentrates,  per 

67  60 

66  50 

Average  value  per  ton  of  galena  concentrates  

$38.96 

$45.89 

a  DUNLOP,  J.  P.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  85,  1916. 

The  geologic  section,  after  Buckley,  is  stated  below: 

Cambrian:  Feet 

Potosi  dolomite 300+ 

Doe  Run  argillaceous  dolomite 60  to  100 

Derby  dolomite 40 

Davis  formation;  shale,  limestone,  and  limestone  conglom- 
erate   170 

Bonneterre    magnesian   limestone;   sandy    dolomite   and 

shale  (principal  ore-bearing  formation) 365  ± 

La  Motte  sandstone 20° 

Unconformity. 

Pre-Cambrian: 

Granite  and  rhyolite  with  intruding  diabase  dikes. 

1  BUCKLEY,  E.'  R. :  Geology  of  the  Disseminated  Lead  Deposits.  Mo. 
Bur.  Geol.  and  Mines,  vol.  9,  parts  1  and  2,  1909;  also  in  BAIN,  H.  F.,  and 
others:  "Types  of  Ore  Deposits,"  pp.  110-130,  San  Francisco,  1911. 

WINSLOW,  ARTHUR:  Lead  and  Zinc  Deposits.  Mo.  Geol.  Survey,  vols. 
6  and  7,  1894.— The  Disseminated  Lead  Ores  of  Southeastern  Missouri, 
U.  S.  Geol.  Survey  Bull.  132,  1896. 


492      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  sedimentary  beds,  though  generally  flat,  dip  locally  as 
much  as  45°.  They  are  cut  by  many  faults  that  strike  about 
northwest  or  northeast.  Normal  faults  prevail,  and  single 
faults  have  vertical  slips  of  less  than  100  feet,  but  some  fault 
zones  have  vertical  shifts  of  as  much  as  700  feet.  A  little  ore 
occurs  in  the  Potosi  dolomite,  but  most  of  it  is  in  the  Bonneterre 
limestone,  especially  near  the  base.  Galena  occurs  also  in  the 
La  Motte  sandstone  and  in  pre-Cambrian  rocks  but  has  not 
been  mined  successfully  from  the  latter.  The  ore  occurs  as 


FIG.  198. — Mine  map  and  cross-section  of  Desloge  mine,  St.  Francis  County, 
Missouri.     (After  Buckley.) 

crystals  and  masses  of  galena  disseminated  in  limestone  or  shale, 
as  horizontal  sheets  along  the  bedding  (Fig.  198),  in  small  cavities 
or  filling  the  small  joints,  and  in  shale  and  clay  along  faults.  One 
ore  body  was  9,000  feet  long,  5  to  100  feet  thick,  and  25  to  500 
feet  wide. 

The  limestone  carries  considerable  organic  material,  and  chlo- 
rite is  extensively  developed  in  it.  The  minerals  of  the  deposits 
are  galena  with  a  little  pyrite  and  at  some  places  a  little  chalcopy- 
rite  or  sphalerite.  The  gangue  consists  of  calcite,  chlorite,  and  a 
little  quartz.  The  galena  carries  only  about  2  ounces  of  silver 


ZINC  AND  LEAD  493 

to  the  ton  of  concentrates.  At  Mine  LaMotte  and  Fredericktown 
cobalt  and  nickel  sulphide  are  present,  and  formerly  small  amounts 
of  these  metals  were  recovered.  Winslow  names  pyrrhotite  as  an 
associate  of  the  ores,  but  Buckley  does  not  mention  it.  Barite 
is  associated  with  the  ores  in  the  Potosi  formation. 

Both  Buckley  and  Winslow  attribute  the  metallization  to 
ground  water.  The  lead  was  formerly  widely  dispersed  in  the 
Bonneterre  and  probably  in  other  formations.  It  was  dissolved 
and  concentrated  in  fractures  through  which  the  solutions  moved, 
being  precipitated  on  contact  with  reducing  agents  in  limestone 
or  associated  shale  beds.  Buckley  considered  the  solutions  to 
have  been  in  part  descending  and  in  part  ascending.  Waters 
collected  by  him  in  mines  contained  alkalies  and  alkali  earth 
carbonates,  sulphates,  and  chlorides.  Some  carried  as  much  as 
15  parts  per  million  of  lead  sulphate. 

Although  neither  of  these  investigators  has  seemed  to  consider 
as  probable  any  genetic  connection  with  igneous  rocks,  Buckley1 
has  noted  the  presence  of  basic  igneous  rocks  in  two  places  in 
St.  Genevieve  County,  which  joins  the  ore-bearing  region  on  the 
east.  These  rocks  appear  to  be  intrusive  in  the  Bonneterre  for- 
mation and  carry  galena  and  sphalerite. 

Coeur  d'Alene  District,  Idaho.— The  Coeur  d'Alene  district2 
of  northern  Idaho  is  in  a  mountainous  country  near  the  Montana 

MINE  PRODUCTION  IN  THE  COEUR  D'ALENE  REGION,  SHOSHONE  COUNTT, 
IDAHO,  1884-1915° 


Year 

Gold 

Silver 
(fine  ounces) 

Copper 
(pounds) 

Lead 
(tons) 

Zinc 
(spelter), 
(pounds) 

Total 
value 

1914  
1915  
1884-1915  

$64,157 
46,433 
5,476,257 

12,178,194 
11,158,955 
142,222,795 

4,242,662 
1,941,296 
62,806,908 

169,849 
164,199 
2,283,256 

41,523,383 
69,685,003 
170,688,937 

$22,728,903 
30,119,424 
315,456,316 

•GERRY,  C.N.:  U.S.  Geol.  Survey  Mineral  Resouces,  1915,  part  1,  p.  551,  1916. 


1  BUCKLEY,  E.  R.,  in  BAIN,  H.  F.,  and  others:  "Types  of  Ore  Deposits," 
p.  105,  San  Francisco,  1911. 

2RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  The  Geology  and  Ore  Deposits 
of  the  Coeur  d'Alene  District,  Idaho.  U.  S.  Geol.  Survey  Prof.  Paper  62, 
1908. 

FINLAY,  J.  R.:  The  Mining  Industry  of  the  Coeur  d'Alene  District, 
Idaho.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  33,  pp.  233-271,  1903. 

HERSHEY,  O.  H.:  Genesis  of  the  Lead-Silver  Ores  in  the  Wardner  Dis- 
trict, Idaho.  Min.  and  Sti.  Press,  June  1,  8,  15,  1912. 


494      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

border.     It  produces  about  one-third  of  the  lead  output  of  the 
United  States  and  considerable  silver,  copper,  gold,  and  zinc. 

The  lead-silver  ores,  which  carry  about  8  per  cent,  of  lead  and 
6  ounces  of  silver  to  the  ton,  are  concentrated  in  the  district  to  a 
product  containing  about  50  per  cent,  of  lead,  and  the  concen- 
trates are  shipped  to  western  smelters  where  high-grade  lead  ores 
are  in  demand  for  smelting  siliceous  ores  of  silver  and  gold. 


Slates,  Quartzites,  Impure 
Limestones,  aud  Sandstonei 
Probably  of  Algonkian  Age 


ilonzonite 
and  other 

ntrusive  Kocka 


FIG.  199. — Sketch  map  showing  position  of  lodes  in  Coeur  d'Alene  district, 
Idaho.     (After  Ransome,  U.  S.  Geol.  Survey.) 


The  copper  ores,  though  of  low  grade,  receive  favorable  rates 
from  smelters  that  use  them  for  lining  converters. 

The  Coeur  d'Alene  district  (Fig.  199)  is  an  area  of  pre-Cam- 
brian  quartzites  and  siliceous  slates  that  are  intruded  by  large 
masses  of  monzonite  and  monzonite  porphyry,  with  dikes  of  dia- 
base and  lamprophyre.  The  sedimentary  section,  as  stated  by 
Calkins,  is  given  below. 


ZINC  AND  LEAD 


495 


Feet 

Striped  Peak :  Shales  and  sandstone,  red  and  green 1,000 

Wallace:  Shales,  more  or  less  calcareous,  interbedded  with  thin 
layers  of  siliceous  and  ferruginous  limestones  and  calcareous 

sandstone 4,000 

St.  Regis:  Shales  and  sandstones,  purple  and  green 1,000 

Revett:  White  quartzite,  partly  sericitic 1,200 

Burke:  Indurated  siliceous  shales  with  sandstones  and  quartzites, 

prevailingly  gray-green 2,000. 

Prichard:  Argillite,  blue-gray  to  black,  with  distinct  and  regular 
banding,  interbedded  with  a  subordinate  amount  of  gray  sand- 
stone. Uppermost  part  arenaceous  and  marked  with  shallow- 
water  features.  Base  not  exposed 8,000 

The  larger  intrusive  bodies  are  surrounded  by  aureoles  of  con- 
tact-metamorphosed sediments.  The  quartzites  are  altered  to 
hornfels,  and  the  uniform  sandy  layers  and  argillites  to  rocks  com- 


FIG.  200.— Section  through  Bunker  Hill  and  Sullivan  lode,  Coeur  d'Alene 
district,  Idaho,  showing  relation  of  ore  bodies  to  footwall  fissure.  (After 
Ransome,  U.  S.  Geol.  Survey.) 

posed  of  andalusite,  garnet,  sillimanite,  mica,  quartz,  and  feld- 
spar. In  the  calcareous  beds  of  the  Wallace  formation  amphi- 
boles  and  pyroxene  are  developed.  Tourmaline  and  siderite 
are  extensively  deposited  in  quartzites  at  considerable  distances 
from  the  intruding  bodies. 

The  rocks  are  extensively  folded,  and  the  folds  are  locally 
overturned.  Slaty  cleavage  is  developed  in  argillaceous  mem- 


496       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

bers.  The  rocks  are  separated  into  many  small  blocks  by  normal 
and  reverse  faults  many  of  which  trend  nearly  northwest. 

The  most  valuable  deposits  of  the  district  are  lead-silver  lodes, 
most  of  which  strike  northwest,  in  the  direction  of  the  principal 
faults.  These  lodes,  however,  do  not  occupy  the  principal  fault 
planes.  Only  one,  the  Bunker  Hill  and  Sullivan  lode,  is  formed 
along  a  fault  of  notable  displacement,  and  this  fault  is  one  of 
less  than  200  feet  throw.  The  deposits  of  this  lode,  though 
along  the  fault,  are  principally  in  subordinate  hanging-wall  frac- 
tures (Fig.  200).  The  ores  were  formed  partly  by  filling  open 
spaces,  but  largely  by  replacement  along  zones  of  fissuring  or 
shearing.  The  deposits,  which  have  a  vertical  range  of  4,000 
feet,  were  probably  formed  under  several  thousand  feet  of  rock, 
which  has  since  been  removed  by  erosion. 

The  ore  minerals  are  galena,  pyrite,  chalcopyrite,  and  sphaler- 
ite, with  some  argentiferous  tetrahedrite  and  stibnite.  Siderite 
is  the  most  abundant  gangue  mineral,  but  considerable  quartz 
and  a  little  barite  are  present.  In  d.epth  pyrrhotite  and  mag- 
netite appear,  indicating  conditions  of  the  deep  vein  zone.  En- 
richment has  probably  been  subordinate. 

The  deposits  are  cut  by  the  lamprophyre  dikes,  showing  that 
they  were  formed  before  the  end  of  igneous  activity.  Moreover, 
the  .lead-silver  deposits  grade  structurally  and  mineralogically 
into  deposits  of  contact-metamorphic  origin  (Granite  mine). 
These  and  other  facts  lead  to  the  conclusion  that  the  deposits 
were  formed  by  ascending  magmatic  waters  derived  either  from 
monzonite  intrusions  or  from  deeper-seated  and  nearly  related 
igneous  rocks.1 

San  Francisco  Region,  Utah. — The  San  Francisco  region2  em- 
braces the  San  Francisco  and  neighboring  ranges  in  Beaver 
County,  southwestern  Utah.  On  account  of  the  Cactus  mine  it 
is  known  at  present  principally  as  a  copper  district,  but  its  past 
product  was  mainly  lead  and  silver. 

The  sedimentary  series  consists  of  Paleozoic  limestone,  shales, 
and  quartzite.  These  beds  were  covered  by  lava  flows,  chiefly 
latites,  probably  of  early  and  middle  Tertiary  age.  Both  the 
sedimentary  rocks  and  the  lava  flows  are  intruded  by  large  bodies 
of  quartz  monzonite  and  by  aplite  and  basic  dikes.  Contact- 

1  BUTLER,  B.  S. :  Geology  and  Ore  Deposits  of  the  San  Francisco  and 
Adjacent  Districts,  Utah.     U.  S.  Geol.  Survey  Prof.  Paper  80,  1913. 
2RANSOME,  F.  L.,  and  CALKINS,  F.  C.:  Op.  cit.,  pp.  134-140. 


ZINC  AND  LEAD  497 

MINE  PRODUCTION  IN  BEAVEK  COUNTY,  UTAH,  1860-1915" 


Year 

Ore 
(short 
tons; 

Gold 

Silver 
(fine 
ouuces) 

Copper 
(pounds) 

Lead 
(pounds) 

Zinc 
(pounds) 

Total 
value 

1914  

94,521 

$20,717 

288  821 

1  511  888 

1915  

53,814 

13,028 

279,694 

428  916 

Total,  1860- 
1915  

680,593 

20,481  211 

42  732  610 

V.  C.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  398,  1916. 

metamorphic  deposits,  with  garnet,  are  developed  near  the 
quartz  monzonite. 

As  stated  by  Butler,  the  ore  deposits  include  (1)  replacement 
deposits  in  fissures  in  the  quartz  monzonite;  (2)  replacement  de- 
posits in  the  limestone,  including  contact  deposits  and  replace- 
ment deposits  along  fissures;  and  (3)  replacement  fissure  deposits 
in. the  lavas. 

Of  the  replacement  deposits  in  fissures  in  the  quartz  monzonite 
the  ore  zone  in  the  Cactus  mine  is  the  most  valuable.  This 
zone  is  brecciated  quartz  monzonite  that  has  been  altered  and 
replaced  by  pyrite  and  chalcopyrite,  with  small  amounts  of  tetra- 
hedrite  and  galena  and  abundant  specularite  in  a  gangue  of 
quartz,  sericite,  tourmaline,  siderite,  anhydrite,  and  some  barite. 
Oxidation  has  not  extended  below  100  feet  and  for  the  most  part 
is  confined  to  a  space  within  50  feet  of  the  surface. 

The  typical  contact-metamorphic  deposits  are  composed  of 
pyrite,  chalcopyrite,  magnetite,  and  sphalerite  in  a  gangue  of 
garnet,  tremolite,  and  other  silicates.  In  general  oxidation  has 
not  extended  to  great  depth.  The  richer  ores,  however,  have 
resulted  from  concentration  in  the  oxidized  zone. 

The  replacement  deposits  along  fissures  in  the  limestone  are 
typically  lead-silver  ores  containing  minor  amounts  of  copper  and 
zinc.  The  ores  are  largely  oxidized  to  the  depth  of  present  de- 
velopments— 500  to  600  feet.  The  metal-bearing  minerals  are 
principally  carbonates  with  minor  amounts  of  sulphates. 

The  Horn  Silver  mine  is  on  the  largest  deposit  in  the  volcanic 
rocks.  This  deposit  occurs  along  a  fault  that  has  thrown  the 
lavas  down  against  the  Paleozoic  limestone.  The  lavas  are 
shattered  along  this  fault,  especially  in  the  vicinity  of  minor  cross 
faults.  The  ore  deposits  have  been  formed  largely  by  replace- 
ment of  the  brecciated  lava.  The  primary  ore  consists  of  pyrite, 
galena,  sphalerite,  and  minor  amounts  of  other  metallic  minerals 


498      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

in  a  gangue  of  quartz,  sericite,  and  partly  altered  lava.  In  the 
oxidized  zone  the  ores  are  characteristically  sulphates,  anglesite 
being  the  principal  mineral  in  much  of  the  lead  ore.  Complex 
sulphates,  such  as  beaverite,  plumboj  arosite,  and  jarosite,  are 
rather  abundant,  and  the  oxidized  copper  ore  carries  much  bro- 
chantite.  Zinc  is  not  abundant  in  the  oxidized  ores.  In  the 
secondary  sulphide  zone  the  copper  ore  consists  of  covellite  and 
chalcocite,  partly  or  wholly  replacing  sphalerite,  wurtzite,  pyrite, 
and,  to  a  slight  extent,  galena.  Rich  copper  ores  were  mined  to 
a  depth  of  about  750  feet,  and  enrichment  along  favorable  chan- 
nels has  extended  deeper.  The  rich  zinc  ores  of  this  mine  are 
composed  of  sphalerite  and  wurtzite,  together  with  other  sul- 
phides. The  wurtzite  is  secondary,  forming  around  cores  of 
sphalerite,  and  the  richer  ores  have  resulted  from  the  addition  of 
the  zinc  in  the  wurtzite.  Normally  the  zinc  enrichment  extends 
to  greater  depth  than  the  copper  enrichment,  and  secondary  zinc 
sulphide  has  been  replaced  by  secondary  copper  sulphides. 


CHAPTER  XXVII 

MISCELLANEOUS  METALLIFEROUS  DEPOSITS 
MERCURY • 


Mineral 

Percentage  of 
mercury 

Composition 

Native  mercury, 

quicksilver  

100  0 

He 

Calomel  

84  9 

HeCl 

Cinnabar  

86  2 

HgS 

Mercury  Minerals  and  Deposits. — Cinnabar  is  almost  invari- 
ably primary;  calomel  and  native  mercury  are  generally  confined 
to  the  superficial  zones,  where  they  are  decomposition  products. 
There  are  many  other  mercury  minerals  found  almost  exclusively 
in  precious-metal  deposits.  Among  them  are  coloradoite  (HgTe) 
and  schwatzite  (mercury-bearing  tetrahedrite).  These  minerals 
by  weathering  break  down,  yielding  native  mercury. 

Calomel  forms  where  chloride  waters  have  access  to  quick- 
silver deposits  that  are  undergoing  oxidation.  Its  genesis  is  com- 
parable to  that  of  cerargyrite  (AgCl),  but  as  it  is  more  soluble  in 
water  than  silver  chloride  it  is  less  stable.  There  is  little  evidence 
of  extensive  enrichment  of  quicksilver  deposits.  Decrease  of 
tenor  with  increasing  depth  is  probably  a  feature  of  the  primary 
ore  shoots.  Some  solution  and  redeposition  are  indicated,  how- 
ever, in  a  few  deposits. 

Cinnabar  is  the  only  important  primary  ore  of  quicksilver. 
It  is  almost  invariably  associated  with  calcite,  chalcedony,  and 
quartz.  Barite,  marcasite,  and  pyrite  are  commonly  present, 
as  are  also  realgar  and  stibnite.  Chalcopyrite,  galena,  and  pre- 
cious metals  are  practically  absent  in  some  deposits.  Bitumin- 
ous matter  is  commonly  associated  with  the  deposits  in  the  United 
States.  Cinnabar  is  unknown  in  igneous  rocks,  in  pegmatites, 
and  in  contact-metamorphic  deposits.  It  is  present  in  a  few 
veins  of  the  deep  zone  and  in  a  larger  number  of  veins  formed  at 


500      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

moderate  depth,  but  practically  all  mercury  deposits  of  economic 
value  are  veins  or  nearly  related  deposits  formed  in  the  shallow 
vein  zone.  Many  of  them  fill  openings,  and  some  of  them  proba- 
bly replace  the  country  rock.  The  common  association  with 
igneous  rocks  and  hot  springs  points  to  the  agency  of  ascending 
hot  waters.  Many  mercury  deposits  are  of  late  Tertiary  age. 

The  quicksilver  deposits  of  California  are  in  the  Coast  Range, 
extending  northwest  from  a  point  near  Santa  Barbara  about  400 
miles  to  a  point  near  Colusa.  There  are  a  few  scattered  deposits 
also  in  the  north  end  of  the  State.  This  belt  contains  more  than 
a  score  of  districts  that  have  yielded  large  amounts  of  quicksilver. 
The  deposits  are  in  rocks  ranging  in  age  from  Mesozoic  to  Quater- 
nary. Igneous  rocks  of  Tertiary  and  Quaternary  age  are  found 
at  many  places  in  this  region:  andesites,  rhy elites,  and  basalts  are 
present  in  many  but  not  all  districts.  The  deposits  are  veins, 
fractured  zones,  stockworks,  or  chambered  breccia  veins.  They 
are  found  in  metamorphic  rocks,  sandstones,  tuffs,  slates,  serpen- 
tine, and  gravel.  In  some  deposits  the  ore  impregnates  and  proba- 
bly replaces  sandstone.1  In  general  the  deposits  decrease 
in  size  or  give  out  in  depth.  In  many  of  them  operations  ceased 
300  or  400  feet  below  the  surface.  At  the  New  Almaden  mine,  in 
Santa  Clara  County,  however,  the  ore  extended  downward  to  the 
1,600-foot  level.  The  deposits  were  formed  in  late  geologic  time, 
near  the  surface  at  the  time  of  deposition.  Many  of  them  are 
associated  with  hot  springs.  At  the  deposits  of  Sulphur  Bank, 
Cal.,  and  at  Steamboat  Springs,  Nev.,  hot  waters,  depositing 
minerals,  now  issue  (see  page  263).  These  deposits  were  ex- 
ploited until  the  heat  became  too  great  for  the  comfort  of  the 
miners. 

The  Terlingua  district  is  in  Brewster  County,  western  Texas, 
near  the  Mexican  border.  Here  Cretaceous  limestone,  shale,  and 
marl  are  cut  by  Tertiary  intrusives  and  in  places  are  overlain  by 
tuffs  and  flows.  Rhyolites,  basalts,  and  phonolite  are  present. 
The  country  is  extensively  faulted  and  shows  a  conjugate  system 
of  fractures.  The  ore  is  found  in  fractured  zones  and  brecciated 
veins  in  limestone.  A  figure  by  Turner2  shows  two  nearly  ver- 
tical ore  shoots  in  Lower  Cretaceous  limestone  which  appear  to 

1  Becker's  argument  opposes  the  view  that  wall  rocks  of  these  deposits 
have  been  replaced. 

z  TURNER,  H.  W.:  The  Terlingua  Quicksilver  Deposits.  Econ.  Geol., 
vol.  1,  pp.  265-281,  1906. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  501 

terminate  upward  at  or  near  shale.  Calcite  is  the  chief  gangue 
mineral.  Montroydite  (HgO),  terlinguaite  (Hg2C10),  and  egle- 
stonite  (Hg4Cl2O),  which  have  been  identified  only  in  this  area, 
are  probably  decomposition  products  of  cinnabar. 

Deposits  of  cinnabar  occur  also  in  New  Mexico,  Nevada,  Utah, 
and  Oregon. 

Recovery  and  Uses. — Mercury  is  recovered  from  its  ores  by 
distillation.  The  ore  is  charged  in  huge  retorts  or  in  shaft  fur- 
naces, where  it  is  heated  to  a  high  temperature.  The  mercury 
vapors  are  condensed  in  brick  chambers.  Mercury  is  used  for 
making  explosives,  alloys,  drugs,  paints,  electric  and  other  appa- 
ratus, for  silvering  mirrors,  and  for  floating  lights  in  lighthouses. 
Formerly  large  amounts  of  mercury  were  used  in  amalgamating 
silver  ores  by  the  Washoe  and  Reece  River  processes,  but  this 
use  has  now  decreased  nearly  to  the  vanishing  point,  the  practice 
being  superseded  by  cyaniding,  smelting,  and  other  methods  of 
beneficiation.  Quicksilver  is  still  used  for  amalgamation  in  many 
gold  mills,  and  many  cyaniding  gold  plants  pass  the  pulp  over 
amalgamation  plates  before  it  goes  to  cyanide  tanks. 

Production.— In  1875  the  United  States  produced  mercury 
valued  at  $4,228,538.  The  total  production  since  mining  began 
is  valued  at  over  $100,000,000.  The  production  has  declined  in 
recent  years,  owing  to  the  decreasing  demand  and  lower  prices, 
and  also  to  the  exhaustion  of  richer  parts  of  the  deposits.  Stimu- 
lated by  the  demand  from  manufacturers  of  explosives,  however, 
the  output  was  greatly  increased  in  1915. 


QUICKSILVER  ORE  TREATED  IN  THE  UNITED  STATES  AND  AVERAGE 
RECOVERIES,  1915" 


State 

Ore  treated 
(short  tons) 

Metal  recovered 
per  ton  (pounds) 

Percentage  of  ore 
recovered  as  metal 

Arizona  
Oregon 

7,321 

45.3 

2.27 

Texas 

California  .....'...  
Nevada             

129,521 
21,975 

8.3 
7.9 

0.41 
0.40 

158,817 

9.9 

0.497 

'.McCASKEY,  H.  D.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  1,  p.  262,  191C. 


502       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

References 

Summaries  of  the  geology  of  the  quicksilver  deposits  of  the  United  States 
with  bibliographies,  are  given  in  the  following  publications: 

McCASKEY,  H.  D.:  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part  1, 
p.  683,  1909;  also  subsequent  volumes. 

BECKER,  G.  F. :  Geology  of  the  Quicksilver  Deposits  of  the  Pacific  Slope. 
U.  S.  Geol.  Survey  Mon.  13,  1888. 

TURNER,  H.  W.:  The  Terlingua  Quicksilver  Deposits.  Econ.  Geol.,  vol. 
1,  pp.  265-281,  1906. 

PHILLIPS,  W.  B.:  The  Quicksilver  Deposits  of  Brewster  County,  Texas. 
Econ.  Geol,  vol.  1,  pp.  155-162,  1906;  Texas  Univ.  Geol.  Survey  Bull.  4, 
1902. 

ALUMINUM  AND  BAUXITE 


Minerals 

Percentage  of 
aluminum 

Composition 

Diaspore 

45   0 

A12O3  H2O 

Gibbsite 

34  6 

A12O3.3H2O 

Bauxite  
Alunite  
Kaolin  

39.1 
19.6 
20.9 

A12O3.2H2O 
K2O.3A12O3.4SO3.6H2O 
Al2O3.2SiO2.2H2O 

Sericite  

20.4 

K2O.3Al203.6Si02.2H2O 

Corundum                    .    . 

52  9 

Al2Oj 

Feldspars     

Variable 

Nepheline 

17  6 

NaAlSiOi 

Cryolite 

12  8 

Na3AlF« 

Aluminum  Ores. — The  principal  ore  of  aluminum  is  bauxite. 
Diaspore  and  gibbsite  are  commonly  present  in  bauxite  ores. 
Alunite  is  utilized  for  the  production  of  potash  salts,  and  alumi- 
num compounds  are  recovered  as  by-products.  Patents  have 
been  issued  for  a  process  for  the  recovery  of  aluminum  from 
kaolin,  but  the  value  of  this  process  is  problematical.  Corun- 
dum is  used  as  an  abrasive.  Kaolin,  sericite,  nepheline,  and  feld- 
spars are  protores  of  bauxite  deposits.  Aluminum  is  a  constitu- 
ent of  many  rock-making  minerals.  The  metal  constitutes  7.96 
per  cent,  of  the  average  of  analyzed  igneous  rocks,1  and  it  is 
present  in  considerable  percentages  in  many  sedimentary  rocks, 
especially  in  shales  and  limestones.  A  list  of  all  the  minerals  that 
contain  aluminum  is  too  long  to  be  given  here,  but  the  more  im- 
portant ones  are  included  in  the  above  list. 


1  CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.    U.  S  .Geol.  Survey 
Bull.  616,  p.  27,  1916, 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  503 

The  superficial  concentration  of  bauxite  is  closely  analogous 
to  that  of  iron  oxide,  and  in  some  deposits  both  are  found  to- 
gether. Both  oxides  also  are  commonly  pisolitic  in  secondary 
deposits.  As  iron  is  concentrated  from  dunite,  peridotite,  or 
greenalite  and  sideritic  rocks,  so  bauxite  is  concentrated  from 
nepheline  syenite,  clayey  limestone,  and  other  rocks  rich  in 
aluminum,  especially  from  those  that  supply  abundant  alkalies 
to  solutions  removing  silica. 

Uses  and  Production. — Aluminum  is  used  in  the  manufacture 
of  many  articles  where  strength  with  lightness  is  required. 
Aluminum  competes  with  copper  as  a  conductor  of  electricity. 
It  is  used  in  the  manufacture  of  alloys,  chemicals,  explosives, 
paints,  etc. 

Bauxite  is  used  for  the  manufacture  of  aluminum  and  alumi- 
num salts,  including  alundum,  or  fused  alumina,  which  is  used 
for  an  abrasive.  Bauxite  bricks  are  used  for  furnace  linings. 

The  production  of  aluminum  in  the  United  States  in  1915  was 
99,806,000  pounds,  valued  at  $17,985,500.  The  production  of 
bauxite  in  the  United  States  in  1915  was  297,041  long  tons, 
valued  at  $1,514,834. 

Arkansas. — The  principal  bauxite-producing  areas  in  the 
United  States  are  in  Arkansas.  The  Bryant  district  is  about  18 
miles  southwest  of  Little  Rock;  the  Fourche  Mountain  district 
is  immediately  south  of  the  city  limits  of  Little  Rock.  These 
districts  together  produce  about  80  per  cent,  of  the  bauxite  of  the 
United  States.  The  earliest  description  of  the  region  is  that  by 
Branner,1  who  discovered  the  deposits.  The  rocks  of  the  region 
have  been  described  by  Williams2  and  the  ore  deposits  by  Hayes.3 
Recently  detailed  explorations  have  been  made  by  Mead.4  The 
region  is  one  of  folded  Paleozoic  sedimentary  rocks  intruded  by 
large  bodies  of  nepheline  syenite.  These  rocks  were  eroded  and 
locally  were  extensively  weathered.  Subsequently,  probably  in 
Tertiary  time,  they  were  covered  with  clays,  sands,  and  gravels 

1  BRANNER,  J.  C.:  Bauxite  in  Arkansas.     Am.  Geol,  vol.  12,  pp.  181-183, 
1891. — The  Bauxite  Deposits  of  Arkansas.     Jour.  Geol,  vol.  5,  pp.  263-289, 
1897. 

2  WILLIAMS,  J.  F. :  Igneous  Rocks  of  Arkansas.     Ark.  Geol.  Survey  Ann. 
Rept.  for  1891,  vol.  2,  1891. 

3  HAYES,  C.  W.:  The  Bauxite  Deposits  of  Arkansas.     U.  S.  Geol.  Sur- 
vey Twenty-first  Ann.  Rept.,  part  3,  pp.  441-472,  1900. 

4  MEAD,  W.  J. :  Occurrence  and  Origin  of  the  Bauxite  Deposits  of  Arkan- 
sas.    Econ.  Geol.,  vol.  10,  pp.  28-64,  1915. 


504       THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

(see  Fig.  201).  The  syenite,  according  to  Mead,  was  more  resist- 
ant to  erosion  than  the  surrounding  softer  rocks  and  stood  above 
them  in  Tertiary  time,  and  the  bauxite  deposits  were  eroded 
contemporaneously  with  the  deposition  of  the  lower  Tertiary 
sediments,  the  result  being  that  the  bauxite  was  transported  and 
interstratified  with  sands  and  gravels  around  the  border  of  the 
syenite  area  and  in  depressions  within  it.  Recent  erosion  has  cut 
through  the  Tertiary  sediments,  exposing  the  underlying  igneous 
rocks  and  the  bauxite  deposits. 

The  relations  of  the  bauxite  deposits  to  syenite  and  Tertiary 
rocks  are  shown  by  Fig.  201.  Hayes,  following  Branner  and 
Williams,  considered  the  bauxite  deposits  to  be  the  results  of  the 

.Paleozoic  Rods  .Syenite  Bauxite 


Tertiary  Sediments 

~Y  ~" 


FiC.  4.    Generalized  cross-sections  illustrating  the  geological  history  of  the   bauxite  occurrences. 

E3  Tertiary  Sediments    •  Bauxite    EI  Syenite    ca  Paleozoic 

FIG.  201. — Generalized  cross-sections  illustrating  geologic  history  of  Arkan- 
sas bauxite  deposits.     (After  Mead.) 

action  of  hot  springs  on  the  still  heated  syenite  rocks  or  chemical 
sediments  precipitated  from  solution  in  shallow  salt  water. 
Mead  regards  the  deposits  as  products  of  normal  rock  weathering. 
He  recognizes  two  classes  of  deposits:  residual  bauxite  in  place 
and  transported  detrital  bauxite.  The  bauxite  in  place,  which 
constitutes  the  most  valuable  deposits,  grades  downward  into 
kaolin,  which  in  turn  grades  into  nepheline  syenite.  The  bauxite 
deposits  have  very  irregular  outlines.  The  maximum  known 
thickness  of  bauxite  is  35  feet,  but  this  is  exceptional.  The 
average  thickness  of  merchantable  ore  is  about  11H  feet.  The 
surface  exposed  after  the  bauxite  is  removed  is  uneven  and  irreg- 
ular. In  places  the  underlying  clay  extends  through  the  bauxite 
mantle  to  the  surface,  and  throughout  the  bauxite  beds  there  are 
horses  and  stringers  of  clay,  so  that  about  40  per  cent,  of  the 
material  handled  in  mining  is  discarded. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  505 

The  most  common  type  of  ore  is  a  pisolitic  or  oolitic  ore,  made 
up  of  spherical  grains  of  bauxite,  some  having  concentric  struc- 
ture, in  a  bauxite  matrix.  These  grains  vary  in  size  from  micro- 
scopic bodies  to  those  an  inch  or  more  in  diameter.  Here  and 
there  on  an  erosion  surface  of  ore  the  pisolites  are  weathered  out, 
yielding  a  gravel  ore.  Second  in  abundance  is  ore  having  the 
texture  of  the  syenite,  commonly  called  "sponge  ore"  or  "granitic 
ore."  This  bauxite  preserves  in  varying  degrees  the  original 
granitic  textura  of  the  syenite  and  carries  about  38.5  per  cent, 
pore  space.  A  third  type  of  ore  is  amorphous  and  has  the  tex- 
ture and  appearance  of  clay.  In  much  of  the  pisolitic  ore  the 
pisolites  are  scattered  sparsely  through  an  amorphous  matrix, 
but  some  of  the  ore  exhibits  gibbsite  bodies  having  the  shapes  of 
original  feldspars. 

According  to  Mead,  syenite  changes  to  bauxite  by  surface 
weathering.  In  the  following  table  are  analyses  of  samples  from 
a  cut  near  the  Lantz  mine.  No.  1  is  fresh  syenite,  No.  2  is 
partly  kaolinized  syenite,  No.  3  is  completely  kaolinized  syenite 
containing  some  bauxite,  and  No.  4  is  bauxite. 


ANALYSES  OP  SAMPLES  SHOWING  GRADATION  FROM  UNALTERED  SYENITE 
TO  BAUXITE  ORE 


1 

2 

3 

4 

SiO2     .     .  .            .            

58.00 

52.64 

39.80 

10.64 

A12O3 

27  10 

29  56 

37  74 

57.48 

Fe2O3  

1.86 

1.06 

1.60 

2.56 

FeO 

3.30 

0.80 

0.10 

0.20 

MgO  
CaO..               

0.25 
1.62 

0.00 
0.00 

0.00 
0.00 

Na2O  

6.70 

4.46 

K2O 

0.25 

0.44 

TiO,  

0.40 

1.20 

3.30 

1.20 

H2O                        

1.22 

9.00 

17.00 

28.36 

In  Fig.  202  the  volume  compositions  of  the  four  samples  are 
shown  in  terms  of  minerals  and  pore  space. 

The  top  of  the  bauxite  deposits  is  higher  in  silica  than  the  baux- 
ite below.  In  many  places  it  is  necessary  to  remove  the  upper  18 
to  24  inches  of  silicious  material  before  mining. 

This  increase  in  silica  toward  the  surface,  according  to  Mead, 


506      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

may  be  due  to  downward  concentration  of  alumina.  Those 
portions  of  kaolin  and  halloysite  which  persist  after  the  main 
portion  of  the  kaolinized  syenite  has  been  altered  to  bauxite  are 
the  dense,  impervious  parts.  Bauxite  is  soluble  in  surface  solu- 
tions to  a  certain  extent  and  on  being  dissolved  and  carried  down- 
ward leaves  the  kaolin  and  hence  the  silica  in  relatively  higher 
percentage  at  the  surface.  The  alumina  carried  down  is  de- 
posited below. 


Feldspars  and 
FeldspathoidB 

Ferromagneiian 

Ktc.X 

i 

.  V 

/ 

/ 
/ 
/ 
/ 
/ 
/ 

Pore  Space 

\ 

/ 
/ 

/ 
/ 
/ 
( 

Pore  Space; 

\ 

\    \ 
\     \ 

\ 
\ 

\ 
\ 

\ 
\ 
\ 
\ 
\ 

Pore  Space 

Feldspars 

Feldspars 

Kaolin,  Etc. 

Iron  and 
Titanium 
^Minerals 

Bauxite 
A120S3H20 

Hydrous 
Aluminum 
Silicates, 
Kaolin, 
Halloysite,  Etc. 

Kaolin,   Etc. 

Unaltered 
Syenite 

Partially 
Kaolinized 

Completely 
Kaolinized 

Bauxite 

Syenite  Syenite 

FIG.  202. — Diagram  showing  in  terms  of  volume  the  mineral  gradation 
from  syenite  to  bauxite.  Columns  represent  series  of  samples  from  a 
single  locality  near  Lantz  mine,  Arkansas.  (After  Mead.) 

Appalachian  Districts. — In  the  southern  Appalachian  region, 
in  a  narrow  belt1  about  60  miles  long,  extending  southwest  from 
Adairsville,  through  Rome,  Ga.,  and  Rock  River,  Ala.,  baux- 
ite deposits  are  found  here  and  there  in  a  residual  mantle,  100 
feet  or  more  thick,  that  rests  on  sedimentary  rocks,  mainly  on  the 
Knox  dolomite.  Thrust  faults  are  numerous;  no  igneous  rocks 
are  present.  It  is  not  certain  that  the  alumina  has  been  concen- 
trated by  simple  processes  of  weathering  from  materials  in  the 
dolomite,  or  in  sandy  shale  that  was  deposited  above  the  dolo- 
mite. Below  the  dolomite  is  a  heavy  bed  of  aluminous  (Cona- 

1  HAYES,  C.  W.:  The  Geological  Relations  of  the  Southern  Appalachian 
Bauxite  Deposits.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  24,  p.  243,  1894  — 
Geology  of  the  Bauxite  Region  of  Georgia  and  Alabama.  U.  S.  Geol. 
Survey  Sixteenth  Ann.  Rept.,  part  3,  p.  547,  1895. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  507 

sauga)  shale,  and  Hayes  suggested  that  waters  ascending  along 
faults  have  dissolved  alumina  from  the  shales  and  precipitated  it 
near  the  surface.  The  origin  of  these  deposits  and  their  connec- 
tion with  faulting  are,  however,  in  doubt.  Near  Keenburg, 
Carter  County,  Tenn.,  bauxite  is  found  as  a  large,  irregular,  deep 
pocket  deposit  in  residual  material  resulting  from  the  decomposi- 
tion of  the  Knox  dolomite.1  Other  deposits  in  Tennessee  are 
probably  residual  also,  and  these  appear  to  be  closely  similar  to 
those  in  the  Alabama  and  Georgia  belt. 

In  Georgia,  about  30  miles  east  of  Macon,  bauxite  is  mined2 
near  the  contact  between  the  Tuscaloosa  (Lower  Cretaceous)  and 
Claiborne  (Tertiary)  formations,  which  are  made  up  chiefly  of 
flat-lying  unconsolidated  clays  and  sands.  The  bauxite  de- 
posits rest  directly  on  the  Cretaceous  clays  or  occur  as  nodules  dis- 
seminated through  them.  Some  beds  are  10  feet  thick. 

CHROMIUM 

The  principal  ore  of  chromium3  is  chromite,  FeCr204  (Cr203  = 
68  per  cent.).  This  mineral  is  a  common  constituent  of  basic 
igneous  rocks,  such  as  olivine  gabbro,  peridotite,  and  pyroxenite, 
in  which  it  occurs  as  disseminated  grains,  as  ill-defined  streaks, 
and  segregated  in  irregular  masses.  Peridotite  and  pyroxenite 
alter  readily  to  serpentine,  and  much  chromic  iron  ore  is  derived 
from  serpentine.  Chromite  alters  very  slowly,  and  when  serpen- 
tine bodies  are  weathered  it  may  collect  in  gravel  deposits  or 
placers. 

The  principal  use  of  chromite  is  in  the  manufacture  of  refrac- 

1  PHALEN,  W.  C.:  Aluminum.  U.  S.  Geol.  Survey  Mineral  Resources, 
1912,  part  1,  p.  951,  1913. 

1  VEATCH,  OTTO  :  The  Bauxite  Deposits  of  Wilkinson  County,  Ga.  Geol. 
Survey  Bull.  18,  p.  430,  1909. 

3DiLLER,  J.  S.:  Production  of  Chromic  Iron  Ore  in  1913.  U.  S.  Geol. 
Survey  Mineral  Resources  1914,  part  1,  pp.  1-13,  1915. 

VOGT,  J.  H.  L. :  Beitrage  zur  genetischen  Classification  der  durch  magma- 
tische  Differentiationsprocesse  und  der  durch  Pneumatolyse  entstandenen 
Erzvorkommen.  Zeitschr,  prakt.  Geologic,  1894;  Chromeisenerz,  pp.  384- 
393. 

HARDER,  E.  C.:  Some  Chromite  Deposits  in  Western  and  Central  Cali- 
fornia. U.  S.  Geol.  Survey  Bull.  430,  p.  180,  1910. 

GLENN,  W.:  Chrome  in  the  Southern  Appalachian  Region.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  25,  pp.  481-499,  1896. 

GLASSER,  E. :  Les  richesses  minerales  de  la  Nouvelle  Caledonie.  Annales 
des  mines,  vol.  4,  p.  299,  1903. 


508      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


tory  brick.  Chromium  is  used  for  hardening  steel  and  for  the 
manufacture  of  chemicals  and  paints.  Chromium  salts  are  used 
in  printing,  dyeing,  tanning  leather,  etc.  The  marketable  ore 
generally  carries  40  per  cent.  Cr2O3  or  more.  The  production 
of  chromic  iron  ore  in  the  United  States  in  1915  was  3,281  long 
tons,  valued  at  $36,744. 

The  largest  deposits  of  chromic  iron  ore  are  in  Rhodesia,  Tur- 
key, New  Caledonia,  and  Greece.  Chromite  is  widely  distributed 
in  areas  of  serpentine  and  other  basic  rocks  in  various  parts  of 
the  United  States.  Such  rocks  are  found  at  a  few  localities  in 
the  crystalline  region  east  of  the  Appalachian  Mountains  and  at 
many  places  in  the  Sierra  Nevada  and  Coast  Range  in  California. 
In  the  San  Luis  Obispo  region,  California,  chromite  ores  are 
widely  scattered  in  extensive  serpentine  areas.  These  deposits 
are  said  to  have  produced  about  11,000  tons.  Many  of  the  de- 
posits, according  to  Harder,1  are  in  the  serpentine  near  the  con- 
tacts of  older  rocks  into  which  the  serpentine  has  been  intruded. 
Such  rocks  may  surround  serpentine  areas  or  may  occur  as  masses 
within  the  serpentine.  Hence  deposits  of  chromite  may  extend 
around  the  borders  of  serpentine  areas  or  may  occur  near  included 
older  rock  masses  within  serpentine  areas.  Other  deposits  ap- 
parently have  no  such  connection  with  older  rocks.  According 
to  Pratt,2  the  chrome  iron  deposits  of  North  Carolina  have  con- 
centrated in  the  magmas  through  the  action  of  gravity. 
MANGANESE 


Mineral 

Percentage  of 
manganese 

Composition 

Pyrolusite  

60  to  63.0 

MnO2 

Polianite. 

63  1 

MnO2 

Psilomelane  
Wad  

45  to  60.0 

Mn2O3XH2O 
Impure  oxides 

Manganite  

62.4 

Mn2O3.H2O 

Hausmanite  
Mallardite  
Alabandite  
Rhodochrosite  
Rhodonite  
Tephroite  (manganese  olivine)  .  .  . 
Spessartite  (manganese  garnet).... 

72.5 
19.9 
63.1 
47.6 
41.9 
54.3 
33.3 

.  Mn3O4 
MnSO4.7H2O 
MnS 
MnCOj 
MnSiO2 
2MnO.SiO2 
3MnO.Al2O3.3SiO2 

1  HARDER,  E.  C.:  Op.  cit.,  p.  170. 

2  PRATT,  J.  H.:  The  Occurrence,  Origin,  and  Chemical  Composition  of 
Chromite.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  29,  p.  17,  1899. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  509 

Origin  of  Deposits.— Manganese  constitutes  about  0.078  per 
cent,  of  the  average  igneous  rocks.1  In  its  chemical  relations 
and  in  its  geologic  occurrence  manganeses  is  closely  related  to 
iron.  Its  principal  deposits  are  formed  by  concentration  in 
superficial  zones.  Oxides  and  subordinately  carbonates  of  man- 
ganese are  deposited  in  bogs  in  much  the  same  way  as  bog  iron 
ores  are  formed.  An  example  is  afforded  at  Wicks,  Mont.,  where 
manganese  minerals  have  been  dissolved  in  the  weathering  of 
granite  and  precipitated  near  the  bottom  of  the  gulch.  Harder2 
states  that  the  manganese  was  carried  in  solution  at  the  surface 
and  in  underground  waters  that  issue  in  springs  near  the  bottom 
of  the  gulch.  Parts  of  many  sedimentary  beds,  as  well  as  igneous 
rocks,  are  rich  enough  to  yield  manganiferous  ores  by  super- 
ficial alteration  and  concentration.  In  general,  contact-meta- 
morphic  deposits  in  the  United  States  are  comparatively  free 
from  manganese.  The  zinc  deposits  of  Franklin  Furnace,  N.  J., 
which  have  contact-metamorphic  affiliations,  are,  however, 
highly  manganiferous.  Under  conditions  of  weathering  manga- 
nese oxides,  like  those  of  iron,  tend  to  remain  in  the  outcrop,  and 
when  other  material  is  removed  enrichment  is  accomplished. 
The  gossans  of  some  fissure  veins  have  been  worked  for  manga- 
nese. Some  manganese  is  dissolved,  however,  and  precipitated 
in  depth  forming  deposits  of  secondary  oxide.  Manganese  is  dis- 
solved and  reprecipitated  during  weathering,  somewhat  more 
readily  than  iron. 

Uses  and  Production. — The  principal  uses3  of  manganese  in 
the  United  States  are  for  making  ferromanganese  (Mn  =  75  to 
80  per  cent.)  and  spiegeleisen  (Mn  =  12  to  20  per  cent.).  These 
compounds,  with  carbon,  are  added  to  molten  iron  to  improve 
the  quality  of  steel.  Manganese  is  used  also  for  making  other 
alloys,  for  making  dry  batteries,  disinfectants,  glass,  colored 
brick,  paints,  chemicals,  etc.  It  is  used  for  flux  in  melting  silver 
and  gold  ores.  Manganese  ores  carrying  less  than  40  per  cent, 
of  manganese  normally  do  not  command  high  prices. 

Manganiferous  zinc  residuum  is  a  furnace  product  consisting 

CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.  U.  S.  Geol.  Sur- 
vey Bull.  616,  p.  27,  1916. 

2  HARDER,  E.  C.:  Manganese  Deposits  of  the  United  States.  U.  S. 
Geol.  Survey  Bull.  427,  pp.  135-137,  1910. 

"HEWETT,  D.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1912,  part  1, 
p.  215,  1913. 


510      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

of  manganese,  iron  oxides,  and  silicon,  obtained  from  furnaces 
using  New  Jersey  zinc  ores.  Small  quantities  of  zinc  residuum 
are  used  in  the  manufacture  of  spiegeleisen. 

The  marketed  production  of  manganese  ore  in  the  United 
States  in  1915  was  9,709  long  tons,  valued  at  $113,309.  The 
imports  of  manganese  ore  for  consumption  were  313,985  long  tons, 
valued  at  $2,633,286.  The  production  of  manganiferous  ore 
(exclusive  of  those  Lake  Superior  ores  running  so  low  in  manga- 
nese as  to  be  classed  with  iron  ore)  was  801,290  long  tons.  Large 
amounts  of  manganese  ores  are  imported  into  the  United  States 
from  Brazil,  Russia,  and  India.1 

Deposits. — Manganese  deposits  of  the  United  States  are  found 
in  rocks  ranging  in  age  from  pre-Cambrian  to  Recent.  In  the 
eastern  United  States,  on  the  Piedmont  Plateau,  the  crystalline 
schists  contain  manganese,  which,  according  to  Harder,2  is  pres- 
ent as  protoxide  in  amphiboles,  pyroxenes,  and  other  minerals 
and  by  weathering  is  concentrated  to  peroxide.  With  it  are  con- 
centrated clay,  iron  oxide,  and  silica.  The  manganese  is  dis- 
solved and  reprecipitated  by  ground  waters  and  is  concentrated 
in  nodules  or  cements,  crevices  and  rock  fragments. 

In  the  Blue  Ridge  region,  extending  from  Virginia  to  Georgia, 
the  residual  material  from  some  of  the  sedimentary  rocks  con- 
tains concentrated  deposits  of  manganese.  These  are  superficial 
deposits  and  generally  are  mixed  with  clay  and  sand.  As  in  the 
crystalline  schists,  the  manganese  in  these  deposits  tends  to  seg- 
regate as  nodules.  It  also  fills  cavities  and  seams  and  replaces 
sandstone.  In  west-central  Arkansas  Paleozoic  rocks  carry 
manganese.  At  Batesville3  the  manganese  is  found  mainly  in 
the  Cason  shale  above  the  Polk  Bayou  limestone  and  below  the 
Boone  chert.  Locally  it  is  concentrated  with  clay  by  weather- 
ing, and  on  hillsides  it  has  accumulated  as  eluvial  deposits. 
Some  deposits  are  20  feet  thick  or  more. 

In  the  Cuyuna  iron  range,  in  Minnesota  (page  311)  irregular 
deposits  of  manganiferous  iron  ore  are  associated  with  the  iron 
ore.  The  manganese  content  of  the  ore  bodies  varies  greatly 

1  HEWETT,  D.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1913,  part  1, 
p.  62,  1914. 

'HARDER,  E.  C.:  Manganese  Deposits  of  the  United  States.  U.  S.  Geol. 
Survey  Bull.  427,  pp.  46-47,  1910. 

3PENROSE,  R.  A.  F.,  JR.:  Manganese,  Its  Uses  and  Deposits.  Ark. 
Geol.  Survey  Ann.  Kept,  for  1890,  vol.  1,  pp.  145-147, 1891. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  511 


from  place  to  place,  small  masses  consisting  of  almost  pure  man- 
ganese oxide,  and  other  portions  containing  only  a  few  per  cent, 
of  manganese.  However,  deposits  of  considerable  size  average 
as  high  in  manganese  as  20  or  30  per  cent. 

In  California  beds  and  thin  bodies  ^f  manganese  ores  are  found 
in  the  Franciscan  (Jurassic?)  formation.  These  are  generally 
only  a  few  feet  thick  and  are  not  extensive.  At  Golconda,  Nev., 
a  manganiferous  bed  lies  between  beds  of  calcareous  tufa.  The 
manganiferous  oxidized  ores  of  Leadville,  Colo.,  and  of  Philips- 
burg  and  Butte,  Mont.,  have  been  mined  for  manganese.  The 
deposits  of  Franklin  Furnace,  N.  J.,  which  yield  manganiferous 
zinc  residuum,  are  mentioned  on  page  487. 

NICKEL 


Mineral 

Percentage 
of  nickel 

Composition 

Garnierite 

H2(NiMg)SiO2  +  H,O 

Annabergite  
Millerite  
Niccolite  

29.4 
64.7 
43.9 

Ni3As2O8  +  8H2O 

NiS 

NiAs 

Chloanthite 

28.1 

NiAs2 

Gersdorffite  
Pentlandite  

35.4 
22.0 

NiAsS 
(FeNi)S 

Nickel  Minerals  and  Ores. — Nickel  is  a  common  constituent 
of  igneous  rocks,  in  which  it  averages  0.02  per  cent.1  It  is  gener- 
ally present  in  basic  rocks,  especially  in  those  containing  olivine. 
The  principal  nickel  deposits  have  been  formed  by  magmatic 
segregation  or  by  superficial  concentration  from  basic  rocks. 
Nickel  minerals  occur  also  in  vein  deposits.  Ferric  salts  attack 
nickel  sulphide,  and  both  the  chloride  and  sulphate  of  nickel 
are  soluble.  From  solutions  the  sulphide  is  precipitated  by 
sulphides  of  iron.  Nickel  does  not  hydrolyze  like  iron  to  form 
the  trivalent  oxide;  iron  and  nickel  will  separate  by  weathering, 
much  of  the  iron  remaining  behind  in  the  gossan,  while  the  nickel 
is  carried  away  in  solution.  The  gossan  of  nickeliferous  pyrrho- 
tite  deposits  is  essentially  limonite.  If  arsenic  is  present  nickel 
forms  with  it  annabergite,  or  "nickel  bloom,"  a  moderately  stable 
salt  which  is  frequently  found  at  the  very  surface  and  may  indi- 
cate the  presence  of  a  nickeliferous  deposit  below.  Hydrous 

1  CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.  U.  S.  Geol.  Survey 
Bull.  616,  p.  27,  1916, 


512      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

nickel-magnesium  silicates  also  are  stable,  and  garnierite  forms 
valuable  deposits  where  nickeliferous  basic  rocks  are  weathering. 
It  is  a  common  alteration  product  of  nickeliferous  olivine  (Fig. 
203).  The  sulphate,  morenosite,  and  the  carbonate,  zaratite, 
are  soluble  and  do  not  accumulate  except  as  incrustations  of 
other  minerals.  Nickel  sulphide  is  precipitated  by  hydrogen 
sulphide  in  neutral  or  alkaline  but  not  in  acid  solutions.  There 
is  good  evidence  of  the  deposition  of  secondary  nickel  sulphides 
at  least  in  small  amounts.  According  to  Kemp,  secondary  miller- 
ite  was  of  economic  value  in  the  Lancaster  Gap  mine,  Pennsyl- 
vania. Gersdorffite  is  secondary  in  some  occurrences. 


FIG.  203. — Ideal  section  of  nickel-bearing  serpentinized  peridotite  alter- 
ing to  red  clay  and  garnierite.  Nickel  silicate  ore  accumulates  below  the 
clay  and  fills  cracks  extending  into  serpentine. 

Pentlandite,  which  is  probably  primary  in  all  occurrences,  is 
the  chief  ore  of  nickel.  It  is  found  at  Sudbury,  Ontario,  in  de- 
posits of  sulphide  ore  formed  by  magmatic  segregation,  in  which 
it  is  intergrown  with  pyrrhotite.1  The  crystals  being  invisible 
to  the  naked  eye,  this  ore  was  long  termed  nickeliferous  pyrrho- 
tite. Nickeliferous  pyrrhotite  is  found  at  Lancaster  Gap,  Pa. 

Gersdorffite  is  present  in  the  nickel  deposits  of  Annaberg, 
Schneeberg,  and  other  districts  in  the  Erzgebirge,  Saxony.  It 
is  probably  the  principal  unoxidized  constituent  of  the  ore  of  the 
Nickel  mine,  Cotton  wood  Canyon,  Nev.2  At  some  places  it 
is  without  doubt  a  secondary  sulphide. 

Uses  and  Production. — Nickel  is  used  in  the  manufacture  of 
many  alloys — german  silver,  monel  metal,  etc.  A  small  percent- 

1  CAMPBELL,  WILLIAM,  and  KNIGHT,  C.  W. :  Microscopic  Examination  of 
Nickeliferous  Pyrrhotites.  Eng.  and  Min.  Jour.,  vol.  82,  p.  909,  1906. 

2RANSOME,  F.  L.:  A  Reconnaissance  of  Some  Mining  Districts  in  Hum- 
boldt  County,  Nevada.  U.  S.  Geol.  Survey  Bull.  414,  p.  67,  1911. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  513 

age  greatly  increases  the  strength  and  hardness  of  steel,  and  the 
larger  part  of  the  nickel  recovered  is  used  for  making  nickel  steel 
and  nickel-chromium  steel.  Nearly  all  the  nickel  used  in  the 
United  States  is  obtained  from  the  Sudbury  district,  Ontario. 
The  ore  of  this  district  is  smelted  to  nickel  matte,  which  is  shipped 
to  refineries  in  the  United  States  and  Europe.  The  matte  pro- 
duced in  Canada  in  1913  was  valued  at  $7,076,945.1 

Nickel  and  nickel  sulphate  are  saved  from  the  electrolytes  used 
in  refining  blister  copper.  Most  blister  copper  carries  a  little 
nickel  which  goes  into  solution  and  accumulates  in  the  electrolyte. 

In  1915  in  the  United  States  822  tons  of  nickel  was  recovered, 
valued  at  $538,222. 

Sudbury,  Ont— The  Sudbury  nickel  region,  Ontario,2  is  in  a 
hilly,  glaciated  country  of  moderate  relief.  The  nickeliferous 
rocks  are  included  in  an  elliptical  area  some  40  miles  long  and  20 
miles  wide,  whose  longer  axis  strikes  north  of  east.  The  central 
portion  of  the  ellipse,  occupied  by  Upper  Huronian  or  post- 
Huronian  rocks,  has  been  eroded  to  a  peneplain,  which  is  sur- 
rounded by  a  hilly  belt  of  eruptive  rock.  The  oldest  series  in 
the  region  consists  of  Huronian  graywacke,  slate,  quartzite,  and 
conglomerate,  which  are  intruded  by  acidic  and  basic  rocks. 
The  Upper  Huronian  rocks  (Animikie  group)  include  conglom 
erate,  tuffs,  slates,  and  sandstones.  Intruded  between  the  Lower 
Huronian  rocks  or  their  igneous  intrusives  and  the  Upper  Huron- 
ian sedimentary  rocks  is  the  great  laccolithic  mass,  probably  of 
Keeweenawan  age,  which  contains  the  Sudbury  nickel  deposits. 
This  great  sheet  dips  toward  its  center,  forming  a  canoe-shaped 
body  which  crops  out  in  a  rudely  elliptical  belt  having  a  nearly 
plane  surface.  As  a  result  of  magmatic  differentiation  the  lower 
portion  of  the  laccolith  is  norite  and  the  upper  portion  is  micro- 
pegmatite,  the  two  rocks  grading  into  each  other. 

The  ore  deposits  include  (1)  those  formed  by  magmatic  segre- 
gation, which  occur  between  the  norite  and  the  underlying  rocks, 
especially  in  depressions  in  the  Huronian  or  in  rocks  intruded  in 

1  McLEiSH,  JOHN:  Preliminary  Report  on  the  Mineral  Production  of 
Canada  in  1913.     Canada  Dept.  Mines,  Mines  Branch,  1914. 

2  BARLOW,  A.  E. :  Report  on  the  Origin,  Geological  Relations,  and  Com- 
position of  the  Nickel  and  Copper  Deposits  of  the  Sudbury  Mining  District, 
Ontario.     Canada  Geol.  Survey  Ann.  Rept.,  vol.  14,  part  H,  1904. 

COLEMAN,  A.  P.:  The  Sudbury  Nickel  Field.    Ontario  Bur.  Mines  Rept., 
vol.  14,  part  3,  p.  14,  1905. 
33 


514      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

the  Huronian;  (2)  deposits  of  nearly  related  genesis  in  or  near 
dikes  of  norite  that  extend  outward  from  the  lower  contact  of  the 
main  laccolithic  body;  and  (3)  deposits  outside  the  laccolith, 
associated  with  norite  intrusions,  which  possibly  are  connected 
with  the  principal  body  of  the  nickeliferous  igneous  rock  beneath 
the  surface.  The  ore  consists  chiefly  of  pyrrhotite,  which  con- 
tains small  amounts  of  pentlandite  and  chalcopyrite.  At  many 
places  it  grades  imperceptibly  into  pyrrhotitic  norite.  Fissures 
in  the  laccolithic  rock  are  rilled  with  quartz  and  sulphides,  and 
along  the  contact  with  the  older  rocks  sulphides  have  been  de- 
posited by  contact-metamorphic  processes.  Pyrite  is  intimately 
associated  with  pyrrhotite.  Other  minerals  are  magnetite,  nic- 
colite,  cassiterite,  gersdorffite,  polydymite,  danite,  galena,  sperry- 
lite,  and  gold.  The  gangue  includes  the  rock-making  minerals  of 
norite,  with  some  quartz,  calcite,  and  other  carbonates.  Some 
of  the  deposits,  as  shown  by  Knight,  exhibit  evidence  of  the  pres- 
ence of  aqueous  solutions  at  the  time  of  their  deposition. 

Alteration  products  include  limonite,  chalcocite,  bornite, 
morenosite,  annabergite,  millerite,  and  probably  several  other 
species.  Rounded  hills  of  gossan,  indicating  the  presence  of  sul- 
phide ore  beneath,  extend  almost  unbroken  for  miles  along  the 
contact  of  norite  with  underlying  rocks.  The  offsets  and  isolated 
masses  of  norite  with  which  some  of  the  ore  bodies  are  associated 
are  generally  made  brownish  by  the  decomposition  of  disseminated 
sulphides.  Locally  the  covering  of  gossan  is  as  much  as  6  feet 
deep,  although  its  ordinary  depth  is  2  or  3  feet,  and  it  merges  into 
sulphide  ore  beneath.  Chalcocite  ores  are  not  conspicuously 
developed.  In  the  Vermilion  mine,  where  the  gossan  is  deepest, 
chalcocite  and  copper  carbonate  occur,  and  there  is  a  concentra- 
tion of  platinum  or  sperrylite  in  the  gossan. 

Alexo,  Ont. — The  Alexo  nickel  deposit,  in  Dundonald  Town- 
ship, northern  Ontario,1  is  nickeliferous  pyrrhotite  that  occurs 
along  a  contact  of  serpentinized  peridotite  and  rhyolite,  the 
latter  probably  of  Keewatin  age.  The  solid  pyrrhotite  ore, 
about  5  feet  thick,  rests  with  sharp  contact  on  the  rhyolite.  It 
passes  gradually  into  ore  mixed  with  serpentine.  The  ore 
minerals  are  pyrrhotite,  pentlandite,  magnetite,  and  chalcopy- 

IUGLOW,  W.  L.:  The  Alexo  Nickel  Deposit,  Ontario.  Ontario  Bur. 
Mines,  Twentieth  Ann.  Rept.,  part  2,  p.  34,  1911. 

COLEMAN,  A.  P.:  The  Alexo  Nickel  Deposit.  Econ.  Geol.,  vol.  5,  pp. 
373-376,  1910. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  515 


rite.  Well-formed  crystals  of  olivine  are  surrounded  by  a  matrix 
of  pyrrhotite.  Pyrrhotite  veinlets  cut  the  serpentine,  and  pent- 
landite  occurs  as  stringers  in  pyrrhotite.  Uglow  believes  that 
deposit  was  formed  by  replacement  from  aqueous  solution,  but 
Coleman  considers  it  due  to  magmatic  segregation. 

Lancaster  Gap,  Pa.— At  Lancaster  Gap,  Pa.,1  pyrrhotite  ores 
occur  in  amphibolite  which  is  inclosed  in  mica  schist.  The  de- 
posits were  worked  for  nickel  before  the  Sudbury  ores  were  devel- 
oped. The  amphibolite,  which  is  probably  an  altered  norite, 
carries  pyrrhotite  and  chalcopyrite.  Kemp  considers  these  de- 
posits as  formed  by  magmatic  segregation  from  norite. 

New  Caledonia. — New  Caledonia2  is  the  world's  most  produc- 
tive nickel-bearing  region  except  the  Sudbury  district.  The 
deposits  cap  serpentine  and  peridotite  and  are  covered  by  fer- 
ruginous clay.  The  ores  are  segregated  in  flat-lying  deposits, 
veinlets,  and  stockworks.  They  have  evidently  been  concen- 
trated by  weathering  from  nickeliferous  serpentine  and  peridotite. 
The  principal  minerals  are  garnierite  and  other  nickel  silicates. 

Riddle,  Oreg. — At  Riddle,  Oreg.,3  nickel  silicate  ores  are  formed 
from  weathering  peridotite. 

COBALT 


Mineral 

Percentage 
of  cobalt 

Composition 

Erythrite  (cobalt  bloom)  

29.4 

Co3As208.8H2O 

Asbolite  

32.0  + 

Oxides    of    cobalt    and 

Jaspurite                   

64.7 

manganese 
CoS 

Smaltite     

28.2 

CoAs2 

Cobaltite  

35.5 

CoAsS 

Cobalt  ores  are  found  principally  in  veins  and  in  deposits 
formed  by  magmatic  segregation.  Cobalt  oxidizes  readily  under 
conditions  of  weathering.  Cobaltite  and  smaltite  are  primary; 
asbolite  and  erythrite  are  secondary.  Jaspurite  is  a  rare  sul- 

1KEMP,  J.  F.:  The  Lancaster  Gap  Nickel  Mine.  Am.  Inst.  Min.  Eng. 
Trans.,  vol.  24,  p.  620,  1894. 

2  GLASSER,  E. :  Rapport  sur  les  richesses  minerales  de  la  Nouvelle  Cale- 
donie.     Annales  des  mines,  10th  ser.,  vol.  5,  pp.  503-701,  1904. 

3  KAY,  G.  F.:  Nickel  Deposits  of  Nickel  Mountain,  Oregon.     U.  S.  Geol. 
Survey  Bull.  315,  p.  120,  1907. 


516      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

phide;  of  its  genesis  little  is  known.  Asbolite  is  a  hydrated  oxide 
of  uncertain  composition  in  which  are  oxides  of  manganese  and 
cobalt,  the  latter  in  some  specimens  amounting  to  32  per  cent. 
In  deposits  in  New  Caledonia  it  is  a  decomposition  product  of 
serpentinized  peridotite.1  It  was  common  in  the  deposits  of  the 
Mine  La  Motte  area,  Missouri.2 

Cobalt  is  obtained  as  a  by-product  from  refining  silver,  copper, 
or  nickel  ores.  The  silver  ores  of  Cobalt,  Ontario,3  are  rich  in 
cobalt,  but  comparatively  little  is  recovered  from  them.  Some 
cobalt  is  recovered  from  the  Sudbury  nickel  ores.  Nearly  all 
the  cobalt  used  in  the  United  States  is  imported  from  Europe, 
where  it  is  obtained  principally  from  copper  ores  shipped  from 
the  Belgian  Kongo,4  some  of  which  carry  about  3.0  per  cent,  of 
cobalt.  Formerly  considerable  asbolite  was  obtained  from 
nickel  mines  in  New  Caledonia.  Cobalt  is  used  principally  as  a 
pigment.  Refined  cobalt  sells  at  $1  to  $2  a  pound. 

PLATINUM 

The  platinum  of  commerce  generally  contains  small  amounts 
of  other  metals  of  the  platinum  group — iridium,  osmium,  pallad- 
ium, rhodium,  and  ruthenium.  These  metals  are  closely  asso- 
ciated in  nature,  almost  invariably  as  native  alloys.  Two  other 
minerals  of  the  platinum  group  are  sperrylite  (PtAs2)  and  laurite 
(RuS2).  Platinum  is  found  in  basic  igneous  rocks,  such  as  perid- 
otites  and  dunites,  and  in  serpentine  derived  by  weathering  of 
such  rocks.  By  disintegration  of  these  rocks  and  concentration 
in  stream  gravels,  platinum  accumulates  in  placers.  Nearly  all 
of  the  world's  supply  comes  from  the  placers  in  the  Ural  Moun- 
tains.5 The  United  States  produced  in  1915  about  8,665  ounces 

1  GLASSER,  E. :  Rapport  sur  les  richesses  minerales  de  la  Nouvelle  Cal6- 
donie.     Annales  des  mines,  10th  ser.,  vol.  5,  pp.  503-701,  1904. 

2  KEYES,  C.  R. :  A  Report  on  the  Mine  La  Motte  Sheet.     Mo.  Geol. 
Survey  Rept.  4,  vol.  9,  p.  82,  1895. 

3  MILLER,  W.  G. :  The  Cobalt-Nickel  Arsenides  and  Silver  Deposits  of 
Temiskaming.     Ontario  Bur.  Mines  Rept.,  vol.  19,  part  2,  pp.  12,  17,  1913. 

4  BALL,  S.  H.,  and  SHALER,  M.  K:  Mining  in  the  Belgian  Congo  in  1913. 
Min.  and  Sci.  Press,  vol.  108,  pp.  320-325,  1914. 

6  PXJRINGTON,  C.  W.:  The  Platinum  Deposits  of  the  Jura  River  System, 
Ural  Mountains,  Russia.  Am.  Inst.  Min.  Eng.  Trans.,  vol.  29,  pp.  3-16, 
1899. 

LINDGREN,  WALDEMAR:  Platinum  and  Allied  Metals.  U.  S.  Geol.  Sur- 
vey Mineral  Resources,  1911,  part  1,  p.  987,  1912. 


MISCELLANEO  US  ME TALLIFERO  US  DEPOSI TS  517 

of  platinum  and  allied  metals,  valued  at  $478,688.  Practically 
all  the  platinum'  produced  in  the  United  States  is  derived  from 
black  sand  in  California  and  Oregon  and  by  refining  the  mud  that 
forms  in  vats  where  blister  copper  is  purified  electrolytically. 
Some  of  the  platinum  reported  as  produced  in  the  United  States 
should  be  credited  to  Sudbury,  Ontario,  as  copper  and  nickel 
mattes  from  that  region  are  refined  in  the  United  States,  and  the 
muds  are  treated  for  recovery  of  platinum.  Sperrylite  is  found 
in  the  superficial  zone  of  some  of  the  Sudbury  mines, l  also  in  the 
Rambler  mine,  Wyoming,2  and  near  Moapa,  in  eastern  Nevada.3 
It  is  not  an  important  source  of  platinum.  Platinum  and  palla- 
dium are  found  with  gold  in  ores  of  the  Boss  mine,  in  the  Yellow 
Pine  district,  Clark  County,  Nevada.4  The  ore  minerals  are 
assopiated  with  quartz  that  replaces  dolomite  along  vertical  frac- 
tures. Granite  porphyry  intrudes  the  dolomite,  and  no  basic 
igneous  rocks  are  known  in  this  region.  The  ore  is  oxidized  and 
contains  a  bismuth-bearing  variety  of  plumbojarosite.  The 
platinum  appears  to  have  been  reconcentrated  by  surface 
agencies. 


ANTIMONY 


Mineral 

Percentage 
of  antimony 

Composition 

Native  antimony                       

100.0 

Sb 

Cervantite         .        

78.9 

Sb204 

Senarmontite      

83.3 

Sb2O, 

83.3 

Sb2O3 

Bindheimite                                 ...... 

Pb3Sb208  +  zH20 

Tetrahedrite                       

24.8 

4Cu2S.Sb2S3 

29.5 

Pb2Sb2S5 

Bournonite  
Stibnite  

24.7 
71.4 

PbCuSbSa 
Sb2S3 

1  BARLOW,  A.  E. :  Report  on  Origin,  Geologic  Relations,  and  Composition 
of  the  Nickel  and  Copper  Deposits  of  the  Sudbury  Mining  District,  Ontario. 
Canada  Geol.  Survey  Ann.  Kept.,  vol.  14,  part  1,  p.  97,  1904. 

2EMMONS,  S.  F.:  Platinum  in  Copper  Ores  in  Wyoming.  U.  S.  Geol. 
Survey  Butt.  213,  pp.  94-97,  1903. 

3  BANCROFT,  HOWLAND:  Platinum  in  Southeastern  Nevada.     U.  S.  Geol. 
Survey  Bull.  430,  p.  192,  1910. 

4  KNOPF,  ADOLPH:  A  Gold-Platinum-Palladium  Lode  in  Southern  Ne- 
vada.    U.  S.  Geol.  Survey  Bull.  620,  pp.  1-18,  1915. 


518      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Stibnite,  bournonite,  and  jamesonite  are  primary;  the  anti- 
mony oxides  and  native  antimony  are  secondary.  In  weather- 
ing activities  antimony  minerals  resemble  those  of  lead;  the 
metal  tends  to  remain  or  accumulate  in  zones  of  oxidation.  Stib- 
nite is  by  far  the  most  abundant  ore  of  antimony.  It  occurs  in 
quartz  veins  and  related  deposits  formed  chiefly  at  moderate  and 
shallow  depths.  Many  of  them  replace  limestone. 

Antimony  is  recovered  from  its  ores  by  roasting  or  by  smelt- 
ing. In  antimony-lead  ores  it  is  recovered  as  antimonial  lead, 
which  is  used  for  making  type  metal.  Antimony  is  also  used 
for  making  "hard"  lead,  metal  for  bearings,  and  many  other 
alloys,  drugs,  paints,  mordants  for  dyeing,  and  fireworks,  for 
vulcanizing  rubber,  and  for  many  other  purposes.  Wet  methods 
are  utilized  for  making  antimonial  salts. 

The  production  of  antimony  in  the  United  States  is  small. 
Nearly  all  of  the  domestic  supply  comes  from  China,  France, 
Algiers,  Austria,  and  Mexico.  A  little  is  obtained  from  slimes 
as  a  by-product  of  refining  copper  and  precious-metal  ores.  In 
1915,  stimulated  by  high  prices  resulting  from  the  war,  the  domes- 
tic production  reached  5,000  tons  of  ore  containing  2,000  tons  of 
antimony,  worth  about  $325,000. 

Deposits  of  antimony  40  miles  southwest  of  Marysvale,  in 
southern  Utah,1  have  produced  antimony  valued  at  over  $100,000. 
The  ore  consists  of  stibnite  and  its  oxidation  products,  which 
occur  in  small  flat-lying  deposits,  in  the  sandstone  and  conglom- 
erate. 

Near  Gilham,  Sevier  County,  Ark.,2  deposits  of  antimony 
occur  in  Paleozoic  sandstones  and  shales.  These  rocks  are 
thrown  into  regular  parallel  folds,  and  in  the  country  northeast 
of  Gilham  the  Paleozoic  beds  and  later  rocks  are  cut  by  many 
small  igneous  intrusives.  The  ore  deposits  are  thin  tabular 
masses  which  generally  follow  the  bedding  planes;  some  are 
as  much  as  100  feet  long,  40  feet  wide,  and  2  feet  thick.  The  orig- 
inal minerals  are  quartz,  stibnite,  jamesonite,  zinkenite,  galena, 
sphalerite,  pyrite,  chalcopyrite,  siderite,  and  calcite.  Comb 
quartz  is  developed.  Traces  of  gold  and  silver  are  present. 
Oxides  and  the  sulphide  of  antimony  or  lead  ores  prevail  for  40  to 

1  RICHARDSON,  G.  B. :  Antimony  in  Southern  Utah.     U.  S.  Geol.  Survey 
Bull.  340,  pp.  253-256,  1908. 

2  HESS,  F.  L. :  The  Arkansas  Antimony  Deposits.     U.  S.  Geol.  Survey 
Butt.  340,  pp.  241-252,  1908. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  519 

115  feet  from  the  surface,  below  which  sphalerite  and  other  sul- 
phides appear.  According  to  Hess,  the  deposits  have  probably 
formed  through  the  agency  of  hot  waters. 

In  1915  the  largest  production  of  antimony  was  obtained  from 
deposits  near  Wild  Rose  Spring,  in  the  Panamint  Range,  Cali- 
fornia. These  deposits  contain  stibnite  and  antimony  ocher. 
Deposits  30  miles  northeast  of  Mojave  and  at  many  other  points 
in  Kern  County,  in  the  east  end  of  San  Benito  County,  and  near 
Grass  Valley,  Calif.,  were  also  mined,  and  considerable  quantities 
were  produced  at  several  places  in  Nevada.  The  Fairbanks 
district,  Alaska,  produced  about  625  tons  of  stibnite  ore  carry- 
ing 58  per  cent,  of  antimony. 

ARSENIC 


Mineral 

Percentage 
of  arsenic 

Composition 

Native  arsenic 

100   0 

As 

Realgar  

70   1 

AsS 

Orpiment  

61    0 

As2S3 

Tennantite  

17.0 

4Cu2S.As2S3 

Enargite 

19  1 

CuaAsSi 

Arsenopyrite  .... 

46  0 

FeAsS 

Enargite  and  arsenopyrite  are  the  principal  sources  of  arsenic. 
Both  are  primary.  Realgar  and  orpiment  are  both  primary  and 
secondary;  native  arsenic  is  secondary.  The  commercially 
valuable  deposits  of  arsenic  are  principally  lodes  formed  at  mod- 
erate depths.  Arsenic  minerals  are  common  in  many  gold  and 
copper  ores,  and  arsenopyrite  has  been  found  at  many  places  in 
the  Appalachian  region.  At  Brinton,  Floyd  County,  Virginia, 
arsenopyrite  deposits  in  mica  schist  are  mined  and  the  ore  is 
calcined  for  white  arsenic.1  Near  Carmel,  N.  Y.,  stringers  of 
arsenopyrite  occur  in  gneiss.  Arsenopyrite  is  mined  for  arsenic 
at  Monte  Cristo,  Wash.2 

In  the  recovery  of  arsenic  from  its  ores  they  are  crushed  and 
charged  into  earthenware  retorts.  The  arsenic  volatilizes  on  the 

1  HESS,  F.  L. :  The  Arsenopyrite  Deposits  of  Brinton,  Va. .   U.  S.  Geol. 
Survey  Bull.  470,  p.  209,  1912. 

WATSON,  T.  L.:  "Mineral  Resources  of  Virginia,"  p.  210,  1907. 

2  SPURR,  J.  E. :  Geology  and  Ore.  Deposits  of  Monte  Cristo,  Wash.    U.  S. 
Geol.  Survey  Twenty-Second  Ann.  Rept.,  part  2,  p.  803,  1901. 


520      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

application  of  heat  and  condenses  in  a  vessel  at  the  mouth  of  the 
retort.  Redistillation  in  the  presence  of  air  will  yield  the  oxide. 
Arsenic  is  produced  also  in  the  electric  furnace.  The  greater 
part  of  the  arsenic  produced  in  the  United  States  is  a  by-product 
from  the  gases  of  copper  furnaces,  particularly  from  plants  smelt- 
ing enargite  ores  of  Butte,  Mont.,  and  Tintic,  Utah.  The  fumes 
from  the  furnaces  are  conducted  through  a  labyrinth  of  chambers, 
on  the  walls  of  which  arsenic  oxide  is  deposited.  This  is  subse- 
quently refined  by  roasting  in  cylindrical  revolving  furnaces, 
from  which  the  fumes  are  again  conducted  through  chambers. 
The  United  States  produced  5,498  tons  of  arsenious  oxide  in  1915, 
valued  at  $302,116.  Arsenic  and  its  compounds  are  used  for 
making  Paris  green,  drugs,  alloys,  poisons,  dyes,  and  glass. 
Much  arsenic  is  used  with  lead  for  hardening  shot. 

BISMUTH 


Mineral 

Percentage 
of  bismuth 

Composition 

Native  bismuth  
Bismite,  bismuth  ocher  

100.0 
°89.6 

Bi 
Bi2O3  +  aq 

Bismutite  

79.1 

Bi2O3.CO2.H2O 

Bismuthinite 

81  2 

Bi2S3 

Tetradymite     

51.9 

Bi2(Te,  S)« 

0  Percentage  of  bismuth  in  BUO».    The  water  present  is  variable. 

Native  bismuth  and  bismuth  sulphides  are  primary;  the  oxides 
and  carbonates  are  secondary.  Bismuth  compounds  are  rela- 
tively insoluble  and  alter  very  slowly.  In  its  alteration  activities 
bismuth  resembles  antimony  and  lead. 

Bismuth  minerals  are  found  in  pegmatite  veins  and  in  some 
contact-metamorphic  deposits,  but  the  metal  is  derived  mainly 
from  lode  ores  of  gold,  silver,  and  copper.  It  is  recovered  princi- 
pally from  the  muds  obtained  from  refining  blister  copper. 
Although  bismutn  is  present  in  small  amounts  in  the  ores  of 
several  western  districts,  the  United  States  produces  only  a  few 
thousand  dollars'  worth  annually.  The  imports  come  chiefly 
from  Germany  and  normally  amount  to  about  $300,000  a  year. 
Bismuth  is  used  for  making  plugs  for  automatic  fire  sprinklers, 
other  fusible  alloys,  electrical  fuses,  solders,  and  glass  and  for 
toilet  and  medicinal  preparations.  The  price  is  about  $2  a 
pound. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  521 

Bismuth  carbonate  ore  is  mined  at  Engle,  N.  Mex.,1  where  it  is 
associated  with  copper  carbonates  and  scheelite.  It  has  been 
mined  also  at  Leadville,  Colo.,2  where  it  is  associated  with  sul- 
phides.3 A  small  vein  of  bismuth  carbonate  was  found  in  the 
Jelm  Mountains,  Albany  County,  Wyoming.4  On  the  Mole 
Tableland,  in  northern  New  South  Wales,  bismuth  ore  is  found  in 
pegmatite  veins.5 

TIN 


Mineral 

Percentage 
of  tin 

Composition 

Cassiterite  

78.6 

SnO2 

Startnite   

27  5 

Cu2FeSnS4 

Occurrence. — Cassiterite  is  found  sparingly  in  some  igneous 
rocks  and  is  a  constituent  of  some  pegmatites  and  of  a  few  con- 
tact-metamorphic  deposits.  It  occurs  also  in  many  veins, 
nearly  all  of  which  are  of  deep-seated  origin  (see  page  49). 
In  veins  formed  at  moderate  and  shallow  depths  it  is  exceedingly 
rare.  Stannite  is  found  in  many  deposits  in  Bolivia,  particu- 
larly at  Potosi.6  It  is  known,  though  rare,  in  some  of  the 
mines  of  Cornwall,  England  (page  234);  also  at  Zinnwald,  in 
the  Erzgebirge,  Saxony.  Both  stannite  and  Cassiterite  are  almost 
insoluble  in  ground  water;  consequently  tin  deposits  are  enriched 
near  the  surface  when  other  minerals  are  removed  by  solution. 
Stannite  probably  alters  to  "wood  tin,"  or  amorphous  cassiter- 
ite.  In  some  deposits  the  tin  minerals  have  probably  been  dis- 
solved and  reprecipitated  to  a  moderate  extent,  causing  some 

1  HESS,  F.  L.:  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part  1,  p.  714, 
1909. 

2  HESS,  F.  L.t  Idem,  1912,  part  1,  p.  1043,  1913. 

3  EMMONS,  S.  F. :  Geology  and  Mining  Industry  of  Leadville,  Colo.  U.  S. 
Geol.  Survey  Mon.  12,  p.  377,  1886. 

4  DARTON,  N.  H.,  BLACKWELDER,  ELIOT,  and  SIEBENTHAL,  C.  E. :  U.  S. 
Geol.  Survey  Geol.  Atlas,  Laramie-Sherman  folio,  No.  173,  p.  15,  1910. 

5  CARNE,  J.  E.:     The  Tungsten  Mining  Industry  in  New  South  Wales. 
New  South  Wales  Geol.  Survey  Mineral  Resources,  No.  15,  pp.  68-71,  1912. 

6WENDT,  A.  F.:  The  Potosi,  Bolivia,  Silver  Districts.  Am.  Inst.  Min. 
Eng.  Trans.,  vol.  19,  p.  90,  1891. 

RUMBOLD,  W.  R.:  The  Origin  of  the  Bolivia  Tin  Deposits.  Econ.  Geol, 
vol.  4,  p.  321,  1909. 


522      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

enrichment,  but  the  amount  of  such  enrichment  is  small  com- 
pared with  that  in  deposits  of  many  other  metals.1 

Uses  and  Production. — Tin  is  used  for  plating  steel,  iron,  and 
other  metals,  and  for  making  tin  ware,  tinfoil,  and  bronze  and 
other  alloys.  The  production  of  tin  in  the  United  States  in 
1915  was  204,000  pounds,  valued  at  $78,846.  Imports,  excluding 
tin  ore,  tinfoil,  and  other  manufactures,  in  1913  amounted  to 


Pegmatite 

FIG.  204.  —  Sketch  map  showing  distribution  of  pegmatite  and  tin  in   the 
Carolina  tin  belt.     (After  Graton,  U.  S.  Geol.  Survey.) 

53,315  short  tons,  valued  at  $46,946,756.2  The  chief  sources  of 
tin  are  the  Malay  Peninsula  and  the  islands  of  Banka  and  Billi- 
ton,  near  by,  where  the  principal  deposits  are  placers,  and  the 
lode  deposits  of  Bolivia  and  Cornwall  (see  page  233). 


,  W.  H.:  The  Enrichment  of  Ore  Deposits.     U.  S.  Geol.  Sur- 
vey Bull.  625,  p.  399,  1917. 

2  HESS,  F.  L.:  U.  S.  Geol.  Survey  Mineral  Resources,  1913,  part  1,  pp. 
347-349,  1914. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  523 

Tin  Deposits  of  the  United  States.— In  the  Carolina  tin  belt, 
which  extends  from  Gaffney,  S.  C.,  nearly  to  Lincolntown,  N.  C. 
(Fig.  204), x  the  tin  occurs  in  pegmatite,  which  is  character- 
ized by  abundant  muscovite,  quartz,  and  a  little  plagioclase  feld- 
spar. The  grade  of  the  ore  ranges  from  less  than  1  per  cent,  to 
more  than  20  per  cent.  The  cassiterite  is  unevenly  distributed 
throughout  the  rock.  In  general  it  is  concentrated  along  certain 
lines,  in  the  main  steeply  pitching  in  the  dikes.  These  ore  shoots 
are  irregular  and  pinch  and  swell  in  an  erratic  manner.  Some  of 
them,  however,  have  a  considerable  extent  in  two  dimensions. 
The  cassiterite  occurs  as  minute  grains  and  as  bodies  weighing  2 
pounds  or  less. 


SO  feet 

FIG.  205.— Section  of  tin  deposit  at  Silver  Hill,  near  Spokane,  Washington. 
(After  Collier,  U.  S.  Geol.  Survey.) 

At  Silver  Hill,  about  8  miles  southeast  of  Spokane,  Wash.,2 
cassiterite  deposits  are  found  in  pegmatite  veins.  The  area  con- 
tains Ordovician  schists  cut  by  granite.  The  pegmatites  are 
in  the  schists  not  far  from  the  granite  and  are  rudely  parallel  to 
the  schistosity  (see  Fig.  205).  The  tin-bearing  rock  consists 
essentially  of  quartz,  orthoclase  feldspar,  sillimanite,  and  andalu- 
site.  The  quartz  contains  minute  fluid  and  gaseous  inclusions 
arranged  in  parallel  lines,  along  many  of  which  fractures  have 
been  developed.  The  cassiterite  is  apparently  an  original  con- 
stituent of  the  pegmatite.  In  the  principal  deposit  a  dike 

i  GRATON,  L.  C.:  The  Carolina  Tin  Belt.  U.  S.  Geol.  Survey  Bull.  260, 
p.  191,  1905. 

*  COLLIER,  A.  J.:  Tin  Ores  at  Spokane,  Wash.  U.  S.  Geol.  Survey  Bull. 
340,  pp.  295-305^  1908. 


524      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

between  schist  and  granite  is  followed  for  125  feet  down  the  dip 
of  45°.  The  lower  part  of  the  body  is  tin-bearing  pegmatite; 
the  upper  part  is  quartz.  Tin  is  most  abundant  in  the  central 
part  of  the  pegmatite  mass.  The  ore  body  locally  is  about  10 
feet  wide. 

At  the  Etta  mine,  South  Dakota,1  cassiterite  occurs  in  pegma- 
tite. The  pegmatite  body  is  about  200  feet  long  and  150  feet 
wide.  It  carries  huge  crystals  of  spodumene  that  are  mined  for 
lithium.  Associated  minerals  include  orthoclase,  muscovite, 
biotite,  tourmaline,  beryl,  quartz,  zircon,  arsenopyrite,  bismuth, 
and  stannite. 

Tin  veins  are  found  in  the  Franklin  Mountains,  Texas,2  about 
12  miles  north  of  El  Paso.  In  the  vein  quartz  the  oxide  of  tin 
occurs,  in  bunches  and  irregularly  disseminated,  intergrown  with 
the  quartz.  More  concentrated  deposits  of  cassiterite  occur  in 
intimate  association  with  the  quartz  and  feldspar  of  the  granite 
adjacent  to  the  veins.  Mineralization  apparently  extended  only 
a  few  inches  from  the  veins.  Associated  minerals  are  wolframite, 
tourmaline,  fluorspar,  and  pyrite. 

On  Lost  River,  Seward  Peninsula,  Alaska,  where  granite  in- 
trudes limestone,  a  contact  zone  is  developed,  in  which  are  found 
axinite,  tourmaline,  ludwigite,  vesuvianite,  fluorite,  scapolite, 
galena,  sphalerite,  arsenopyrite,  pyrrhotite,  scheelite,  cassiterite, 
and  the  ferromagnesian  stannoborates  hulsite  and  paigeite.3 
Cassiterite  is  found  also  in  this  region  in  granite  and  in  quartz 


TUNGSTEN 


Mineral 

Percentage 
of  WOa 

Composition 

Tungstite  
Ferberite 

100.0 
76  3 

WO3 
FeWO4 

Wolframite  
Hiibnerite  
Scheelite  

76.4 
76.6 
80.6 

(Fe,  Mn)WO4 
MnWO4 
CaWO4 

1  HESS,  F.  L. :  Tin,  Tungsten,  and  Tantalum  Deposits  of  South  Dakota. 
U.  S.  Geol.  Survey  Bull.  380,  pp.  131-163,  1909. 

2  RICHARDSON,  G.  B.:  Tin  in  the  Franklin  Mountains,  Texas.     TJ.  S. 
Geol.  Survey  Butt.  285,  pp.  146-149,  1906. 

3  KNOPF,   ADOLPH:  Geology  of   the   Seward   Peninsula   Tin   Deposits, 
Alaska,     U,  S.  Geol.  Survey  Bull.  358,  p.  23,  1908. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  525 

Ferberite,  wolframite,  and  hiibnerite  are  probably  everywhere 
primary.  Tungstite  and  tungstic  ocher  are  alteration  products. 
Scheelite  is  primary  in  the  main,  but  some  is  secondary.  The 
tungsten  minerals,  like  those  of  tin,  are  not  very  soluble  in  ground 
water  and  at  many  places  accumulate  in  placers,  along  with  cas- 
siterite  and  other  minerals  that  are  relatively  stable  in  the  oxidiz- 
ing zone.  The  world's  largest  production  comes  from  placers  in 
Burma. 

Although  the  tungsten  minerals  are  very  common  in  pegma- 
tites and  in  lodes  formed  at  considerable  depths,  the  most  valu- 
able deposits  in  the  United  States  are  lodes  formed  by  ascending 
hot  waters  at  moderate  or  shallow  depths.  At  Atolia,  San 
Bernardino  County,  California,  in  an  area  of  schists  cut  by 
granite  and  granite  porphyry,  scheelite  is  found  in  gold-bearing 
quartz  veins.1  Locally  in  that  region  sands  and  residual  surface 
material  have  been  worked  as  dry  placers.  Hiibnerite  and  wol- 
framite placers  are  worked  in  the  Little  Dragoon  Mountains, 
Arizona.2 

In  Boulder  County,  Colorado,3  the  principal  tungsten  ore  is 
ferberite,  which  occurs  in  small  veins  in  granite.  Associated 
minerals  are  quartz,  calcite,  adularia,  chalcopyrite,  and  a  little 
galena.  Precious  metals,  molybdenite,  and  tellurides  are  locally 
present.  The  ferberite  resists  weathering  and  forms  placers. 
In  the  Black  Hills4  bedding-plane  deposits  of  wolframite  replace 
flat-lying  calcareous  beds  where  the  latter  are  crossed  by  thin 
fissures.  Tungsten  deposits  are  found  also  in  the  Snake  Range, 
eastern  Nevada,5  and  in  the  Dillon  quadrangle,  Montana.6 

1  HESS,  F.  L.:  U.  S.  Geol.  Survey  Mineral  Resources,  1909,  part  1,  p.  734, 
1910. 

2RiCKARD,  FORBES:  Notes  on  Tungsten  Deposits  of  Arizona.  Eng.  and 
Min.  Jour.,  vol.  77,  p.  268,  1904. 

3  GEORGE,  R.  D.:   The  Main  Tungsten  Area  of  Boulder,  Colo.     Colo. 
Geol.  Survey  Ann.  Kept,  for  1908,  pp.  7-103,  1909. 

LINDGREN,  WALDEMAR:  Some  Gold  and  Tungsten  Deposits  of  Southern 
Colorado.  Econ.  Geol.,  vol.  2,  pp.  453-463,  1907. 

HESS,  F.  L.,  and  SCHALLER,  W.  T:  Colorado  Ferberite  and  the  Wolframite 
Series.  U.  S.  Geol.  Survey  Bull.  583,  1914. 

4  IRVING,  J.  D. :  Economic  Resources  of  the  Northern  Black  Hills.     U.  S. 
Geol.  Survey  Prof.  Paper  26,  p.  158,  1904. 

6  WEEKS,  F.  B.:  Tungsten  Deposits  in  the  Snake  Range,  White  Pine 
County,  Eastern  Nevada.  U.  S.  Geol.  Survey  Bull.  340,  pp.  263-270,  1908. 

6WiNCHELL,  A.  N.:  The  Mining  Districts  of  the  Dillon  Quadrangle, 
Montana,  and  Adjacent  Areas.  U.  S.  Geol.  Survey  Butt.  574,  p.  123,  1914. 


526      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Tungsten  is  used  for  making  high-speed  tool  steel  and  other 
alloys  and  electric-light  filaments,  for  coloring  glass,  and  for  fire- 
proofing  textiles. 

Tungsten  minerals  are  recovered  from  tungsten  ores  by  me- 
chanical concentration.  In  1914  the  United  States  produced 
990  tons  of  concentrates  (contents  equivalent  to  60  per  cent. 
WO3)  valued  at  $435,000.  The  production  in  1915  was  2,332 
tons  of  concentrates,  valued  at  $4,100,000. 

URANIUM  AND  RADIUM 

Mineral 

Carnotite. K2O.2UO3.V2O5.8(?)H2O 

Tyuyamunite CaO.2UO3.V2O6.8(?)H2O 

Torbernite CuO.2UO3.P2O5.8H2O 

Autunite CaO.2UO3.P2O6.8H.O 

Uvanite 2UO3.3V2O6.15H2O 

Pitchblende,   an   amorphous   mineral   containing   uranium,    rare 

earths,  etc. 

Gummite,  hydrous  uranium  oxide  with  other  bases. 
Samarskite,   of  uncertain  composition;   contains  uranium,   iron, 

lime,  and  several  rare  earths. 
Uraninite,  crystalline  variety  of  pitchblende. 

Uranium  minerals  are  found  in  veins  associated  with  igneous 
rocks  and  disseminated  in  sandstone  in  regions  where  igneous 
activity  is  not  prominent.  Some  of  the  uranium  ores  appear  to 
have  been  deposited  by  cold  water  and  enriched  by  superficial 
alteration.  The  uranium  ores  are  valuable  for  the  radium  they 
contain.  Deposits  are  found  in  Colorado  and  Utah.  In  1915 
the  production  of  uranium,  vanadium,  and  radium  ores  in  the 
United  States  amounted  to  $693,750. 

The  best-known  deposits  of  uraninite  or  pitchblende  are  in  the 
Erzgebirge,  Bohemia  and  Saxony,1  and  in  Gilpin  County,  Colo- 
rado. In  the  Erzgebirge  sedimentary  and  metamorphic  rocks  are 
intruded  by  granitic  rocks.  At  Joachimsthal,  Bohemia,  the 
pitchblende  ores  are  associated  with  quartz,  dolomite,  pyrite, 
and  chalcopyrite. 

At  Quartz  Hill,  Gilpin  County,  Colorado,  schists  and  granites 
are  cut  by  Tertiary  intrusive  monzonite  and  bostonite  porphyries. 
Mineralization  occurred  at  two  periods;  in  the  first  pyrite,  quartz, 
tetrahedrite,  rhodochrosite,  and  other  minerals  were  formed;  in 

1  MULLER,  HERMANN:  Die  Erzgange  des  Annaberger  Bergrevieres. 
Erlauterung  zur  Specialkarte  des  Konigreichs  Sachsen,  p.  66,  Leipzig,  1894. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  527 

the  later  period  the  minerals  were  quartz,  calcite,  galena,  sphaler- 
ite, pyrite,  and  chalcopyrite.  Bastin1  states  that  the  pitchblende 
was  deposited  during  the  earlier  pyrite  mineralization  and  that 
it  was  subsequently  fractured  and  veined  with  ores  of  the  lead- 
zinc  mineralization. 

Uraninite,  the  crystallized  form  of  the  mineral  which  in  its 
amorphous  form  is  known  as  pitchblende,  is  found  in  pegmatites. 
It  occurs  in  North  and  South  Carolina,  where  it  is  largely  altered 
to  gummite  and  other  minerals.  Uraninite  is  comparatively 
easily  attacked  by  weathering  processes  and  so  is  almost  unknown 
in  placer  deposits. 

Carnotite  is  found  at  Radium  Hill,  near  Olary,  South  Aus- 
tralia.2 The  country  is  an  area  of  metamorphic  gneisses  and 
schists  cut  by  dikes  of  granite  and  diorite.  The  carnotite  occurs 
in  a  lode  associated  with  quartz,  biotite,  magnetite,  ilmenite,  and 
rutile.  As  a  yellow  powder  and  small  platy  crystals  it  coats 
cracks  and  fills  cavities.  Crook  and  Blake  believe  that  the 
gangue  minerals  are  related  to  eruptive  activity. 

A  vanadiferous  vein  which  carries  also  a  little  uranium  as 
autunite  occurs  l^j  miles  northeast  of  Placerville,  Colo.  The 
district  contains  nearly  flat  sedimentary  rocks,  cut  by  intru- 
sive diorite  porphyry  and  by  basic  dikes.  The  deposit  is  in 
a  fault  fissure  in  the  Dolores  formation.3  Near  the  vein  the 
limestone  wall  rock  is  recrystallized  to  calcite  and  coated  with  a 
chromium  mica.  The  minerals  include  chalcopyrite,  chalcocite, 
autunite,  asphaltum,  malachite,  azurite,  and  molybdenite.  Gold 
and  silver  are  also  present.  Near  this  vein  are  valuable  bedding- 
plane  deposits  of  rare  metals  in  the  overlying  La  Plata  sandstone, 
which  is  nearly  pure  quartz  cemented  by  calcite.  Near  the 
deposits  the  sandstone  is  indurated,  and  the  calcite  cement  is 
apparently  replaced  by  quartz.  A  chromium  mica  is  deposited 
over  extensive  areas  at  the  horizon  of  the  vanadium  ore.  Sec- 
ondary carnotite  deposited  on  fractures  has  evidently  been 
leached  out  of  the  vanadium  ore. 


1  BASTIN,  E.  S. :  Geology  of  the  Pitchblende  Ores  of  Colorado.     U.  S. 
Geol.  Survey  Prof.  Paper  90,  p.  1,  1914. 

2  CROOK,  T.,  and  BLAKE,  G.  S.:  On  Carnotite  and  an  Associated  Mineral 
Complex  from  South  Australia.     Mineralog.  Mag.,  vol.  15,  p.  271,  1910. 

3  HESS,  F.  L. :  Notes  on  the  Vanadium  Deposits  near  Placerville,  Colo. 
U.  S.  Geol.  Survey  Bull.  530,  p.  151,  1913. 


528      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Ransome1  considers  the  Placerville  deposits  to  have  replaced 
calcite  in  the  sandstone.  Many  of  them  are  found  to  give  out  in 
depth  or  when  followed  into  the  hills  by  tunnels,  and,  because 
they  appear  to  be  related  to  the  present  surface,  they  are  supposed 
to  have  been  formed  or  enriched  by  superficial  agencies.  Some 
of  the  carnotite  deposits  in  La  Sal  Creek  are  pockets  in  sandstone 
above  a  shale,  and  some  occupy  fissures  or  planes  of  movement 
such  as  might  have  been  formed  by  slipping  of  the  sandstone  on 
the  shale  since  the  region  acquired  the  present  topography. 

Since  the  Placerville  deposits  were  discovered  numerous 
occurrences  of  carnotite  and  other  nearly  related  minerals  have 
been  developed  in  rocks  of  the  same  or  approximately  the  same 
age,  extending  over  a  wide  area  in  southwestern  Colorado  and  into 
Utah.  All  the  deposits  appear  to  be  in  the  La  Plata  (Jurassic?) 
or  the  McElmo  (Jurassic)  formation.  Fossil  wood  and  bones 
are  nearly  everywhere  associated  with  the  deposits,  and  the  com- 
mon mineral  associates  are  copper  carbonates,  vanadium  and 
chromium  minerals,  and  some  pyrite.  Hess2  mentions  a  deposit 
in  the  La  Sal  Mountains  where  a  petrified  tree  was  mined  for 
ore,  the  uranium  being  richest  around  the  edge,  or  the  part  that 
was  probably  most  decayed  before  burial.  To  account  for  these 
deposits  Hess  proposes  a  hypothesis  that  assumes  older  uranium 
and  vanadium  veins  in  the  drainage  basins  that  supplied  the 
La  Plata  and  McElmo  sediments  and  cites  several  deposits  as 
possible  examples.  Uranium  and  vanadium  from  these  veins 
would  be  dissolved  by  sulphuric  acid  generated  by  pyrite  and 
carried  to  the  sea,  where  they  might  be  precipitated  by  decaying 
reeds  and  trees. 

VANADIUM 

Vanadium  in  small  amounts  is  commonly  present  in  igneous 
rocks.  It  occurs  in  alteration  products  of  many  veins  of  copper, 
lead,  and  other  metals,  but  its  source  is  not  known.  It  forms 
rather  soluble  salts  and  migrates  readily  in  cold  solutions. 
The  principal  ores  of  vanadium  in  the  United  States  are  carnotite, 
(2U203.V205.K20.3H20),  and  roscoelite,  (AlV2.KH2.Si9024). 

1  HILLEBRAND,  W.  F.,  and  RANSOME,  F.  L. :  On  Carnotite  and  Associated 
Vanadiferous  Minerals  in  Western  Colorado.     U.  S.  Geol.  Survey  Bull. 
262,  p.  14,  1905. 

2  HESS,  F.  L. :  A  Hypothesis  for  the  Origin  of  the  Carnotites  of  Colorado 
and  Utah.     Econ.  Geol.,  vol.  9,  p.  681,    1914. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  529 

These  ores  occur  in  sandstone  in  southwestern  Colorado  and 
southeastern  Utah.  The  deposits  are  mentioned  above  as  uran- 
ium and  radium  ores. 

Near  Placerville,  Colo.,1  roscoelite,  together  with  a  little  carno- 
tite,  cements  grains  of  quartz  sand  of  the  La  Plata.  These 
deposits  are  worked  for  vanadium..  In  this  district  a  vein  which 
occupies  a  fault2  carries  vanadium  and  some  uranium  (see 
page  527). 

Deposits  of  vanadium  at  Minasragra,  Peru,3  are  in  Mesozoic 
sediments  that  are  intruded  by  numerous  dikes  of  eruptive  rock. 
The  principal  vanadium  deposit  occupies  a  fault  fissure,  and  the 
sulphide,  patronite,  is  associated  with  coke  and  asphaltum. 
The  carbon  compounds  occupy  the  walls  and  the  sulphide  the 
center  of  the  vein.  Oxidation  yields  green  and  brown  oxides, 
which  have  been  mined. 

The  principal  use  of  vanadium  is  for  making  a  special  steel, 
to  which  it  gives  increased  hardness,  toughness,  and  power 
to  resist  shock.  It  is  used  also  in  making  copper  alloys.  Vana- 
dium salts  are  used  as  mordants  in  dyeing,  for  drugs,  and  in 
many  chemical  preparations.  The  annual  production  in  the 
United  States  is  included  with  that  of  uranium. 

CADMIUM 

Cadmium  is  not  rare,  but  it  is  a  comparatively  unimportant 
metal  in  the  arts.  In  the  United  States  91,415  pounds,  valued  at 
$108,  443,  was  produced  in  1915.  Cadmium  sulphide,  greenock- 
ite,  is  found  in  the  Joplin  region,  Missouri,  as  a  yellow  powder 
coating  crevices,4  and  the  sphalerite  of  this  region  carries  as 
much  as  0.4  per  cent,  of  cadmium.  Some  other  zinc  ores  and 
some  lead  ores  contain  small  percentages  of  cadmium.  When 
zinc  sulphide  is  heated  with  carbon  in  a  retort,  the  cadmium 
comes  off  at  a  lower  temperature  than  the  zinc,  and  by  fractional 

1  HILLEBRAND,  W.  FM  and  RANSOMB,  F.  L. :  On  Carnotite  and  Associated 
Vanadiferous  Minerals  in  Western  Colorado.     U.  S.  Geol.  Survey  Bull.  262, 
p.  14,  1905. 

2  HESS,   F.  L.:  Notes  on  the  Vanadium  Deposits  near  Placerville,  Colo. 
U.  S.  Geol.  Survey  Bull.  530,  p.  142,  1913. 

3  HEWETT,  D.  F.:  Vanadium  Deposits  in  Peru.     Am.  Inst.  Min.  Eng. 
Trans.,  vol.  40,  p.  274,  1909. 

<  SIEBENTHAL,  C.  E.  I  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part 
1,  p.  793,  1909. 

34 


530      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

distillation  a  concentration  is  effected.  This  method  is  used  for 
recovering  cadmium  in  Germany.  The  cadmium  produced  in 
the  United  States  is  obtained  partly  from  treatment  of  bag-house 
products  of  smelters  and  as  a  by-product  in  the  manufacture  of 
zinc  chloride. 

Metallic  cadmium  is  used  for  making  amalgams  and  alloys, 
and  salts  of  cadmium  are  used  in  photography,  in  medicine,  and 
in  electroplating.  The  sulphide  forms  the  basis  of  a  high-grade 
yellow  paint. 

MOLYBDENUM 

The  principal  ore  of  molybdenum  is  molybdenite  (MoS2). 
Other  molybdenum  minerals  are  wulfenite  (PbMo04),  molybdite 
(MoO3),  and  a  yellowish  oxidation  product,  molybdic  ocher1  (a 
hydrous  ferric  molybdate).  Molybdenite  is  primary;  the  other 
minerals  named  are  probably  everywhere  alteration  products. 

Molybdenite,  though  not  abundant,  is  widespread.  It  is  a 
constituent  of  some  igneous  rocks,  especially  of  granites,  and  of 
pegmatite  veins.  It  occurs  in  both  these  forms  at  Catherine 
Hill2  and  near  Cooper,3  Maine.  In  the  O  K  mine,  Utah,4 
it  is  found  in  aplite  dikes,  and  veins  3  or  4  inches  thick  are  reported 
to  occur  in  pegmatitic  quartz.  In  the  Santa  Rita  and  Patagonia 
mountains,  Arizona,5  molybdenite  is  found  at  many  places  as 
quartz  veins  and  impregnations  in  granite.  Many  other  occur- 
rences are  described  by  Hess.6 

Molybdenum  is  used  in  making  hard  steel  and  other  alloys, 
permanent  magnets  and  other  electric  apparatus,  and  chemicals, 
also  for  coloring  porcelain  green.  A  little  molybdenum  is" 

1  SCHALLER,   W.   T. :  The   Chemical   Composition  of   Molybdic   Ocher. 
Am.  Jour.  Sci.,  4th  ser.,  vol.  23,  p.  297,  1907. 

2  EMMONS,  W.  H. :  Some  Ore  Deposits  in  Maine  and  the  Milan  Mine, 
New  Hampshire.     U.  S.  Geol.  Survey  Bull.  432,  p.  42,  1910. 

3  SMITH,  G.  O. :  Molybdenum  Deposits  in  Eastern  Maine.     U.  S.  Geol. 
Survey  Bull.  260,  p.  197,  1905. 

4  BUTLER,  B.  S. :  Geology  and  Ore  Deposits  of  the  San  Francisco  and 
Adjacent  Districts,  Utah.     U.  S.  Geol.  Survey  Prof.  Paper  80,  p.  110,  1913. 

6ScHRADER,  F.  C.,  and  HILL,  J.  M.:  Some  Occurrences  of  Molybdenite 
in  the  Santa  Rita  and  Patagonia  Mountains,  Arizona.  U.  S.  Geol.  Survey 
Bull.  430,  p.  162,  1910. 

6  HESS,  F.  L.:  Molybdenum.  U.  S.  Geol.  Survey  Mineral  Resources, 
1908,  part  1,  p.  745,  1909.— Some  Molybdenum  Deposits  of  Maine,  Utah, 
and  California.  U.  S.  Geol.  Survey  Bull.  340,  p.  231,  1908. 


MISCELLANEOUS  METALLIFEROUS  DEPOSITS  531 

mined  in  the  United  States;  some  is  imported.  In  1912,  accord- 
ing to  Hess,1  the  imports  of  molybdenum  and  ferromolybdenum 
were  3.5  tons,  valued  at  $4,670. 

SELENIUM 

Selenium  in  small  amounts  is  found  in  some  gold,  silver,  lead, 
and  copper  ores.  It  is  commonly  associated  with  tellurium  in 
ores  of  precious  metals.  Appreciable  quantities  are  present  in 
ores  of  Tonopah,  Nev.,  Republic,  Wash.,  and  Waihi,  New  Zea- 
land. Selenium  is  used2  for  making  red  gloss  enamels,  and 
glazes.  Because  its  electric  conductivity  varies  with  the  inten- 
sity of  light  it  has  many  unique  applications  is  making  electric 
apparatus.  Some  selenium  is  obtained  from  the  anode  muds 
resulting  from  the  electrolytic  refining  of  copper.  Selenium  sells 
for  about  $2.50  a  pound. 

TELLURIUM 

Tellurium,  which  is  in  the  sulphur  group,  is  closely  allied  also 
with  some  of  the  metals.  It  is  found  with  native  sulphur  in 
Japan  and  is  combined  with  gold,  bismuth,  and  other  metals  in 
many  vein  deposits,  especially  in  those  of  the  late  Tertiary  group. 
In  the  United  States  it  is  most  abundant  in  the  gold  deposits  of 
Cripple  Creek,  Colo.  It  is  generally  present  in  small  amounts 
in  muds  obtained  in  the  electrolytic  refining  of  copper.  Tel- 
lurium is  at  present  unimportant  in  the  arts. 

TITANIUM 

The  principal  titanium  minerals  are  ilmenite  (FeTi03)  and 
rutile  (TiO2).  Ilmenite  is  a  rock-making  mineral  and  is  a  com- 
mon constituent  of  titaniferous  magnetite3  which  has  been  formed 
at  many  places  by  magmatic  segregation  (see  page  343).  Be- 
cause of  metallurgical  difficulties  titaniferous  ores  are  not  much 
used,  notwithstanding  the  fact  that  titanium  increases  the 
strength  of  steel. 

1  HESS,  F.  L.:  U.  S.  Geol.  Survey  Mineral  Resources,  1912,  part  1,  p.  969, 
1913. 

2  HESS,  F.  L.:  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part  1,  pp. 
715-717,  1909. 

3SiNGEWALD,  J.  T.:  The  Titaniferous  Iron  Ores  of  the  United  States; 
Their  Composition  and  Economic  Value.  U.  S.  Bur.  Mines  Bull.  64,  1913. 


532      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Rutile  is  found  at  many  places  in  the  United  States  and  has 
been  exploited  in  the  Magnet  Cove  region  of  Arkansas  and  in 
Nelson  County,  Virginia.1  In  the  Virginia  region  a  biotite  gneiss, 
probably  pre-Cambrian,  is  cut  by  great  pegmatite  dikes  composed 
of  feldspar,  quartz,  apatite,  hornblende,  and  rutile.  The  pegma- 
tites are  cut  by  smaller  dikes  composed  mainly  of  rutile  and 
apatite.  One  of  these,  from  a  few  inches  to  5  feet  thick,  may  be 
followed  for  half  a  mile  or  more.  Much  of  the  rutile  carries  iron. 

Titanium  and  its  compounds  are  used  for  coloring  various  prod- 
ucts, for  hardening  steel,  and  for  electric  and  other  purposes.  In 
1915  the  United  States  produced  250  tons  of  rutile,  valued  at 
$27,500. 

TANTALUM 

The  principal  minerals  of  tantalum  are  tantalite  (FeTa206) 
and  samarskite,  a  complex  tantalate  of  several  rare  metals. 
Tantalum  minerals  are  found  in  pegmatite  veins  in  the  Black 
Hills,  South  Dakota,2  and  at  a  few  places  in  the  Appalachian 
region  of  the  United  States.  Many  foreign  occurrences  are 
known. 

Tantalum  has  been  used  for  filaments  of  incandescent  lights, 
but  tungsten  has  almost  completely  superseded  it,  and  in  1912 
no  tantalum  was  mined.  Like  platinum,  it  is  little  affected  by 
many  chemical  reagents,  and  in  the  future  it  may  be  used' instead 
of  platinum  to  some  extent  in  chemical  laboratories.  The  price 
was  59.4  cents  per  gram  in  1912. 

1  WATSON,  T.  L.:  The  Occurrence  of  Rutile  in  Virginia.     Econ.  Geol.,  vol. 
2,  p.  493,  1907. 

2  HESS,  F.  L.:  Tin,  Tungsten,  and  Tantalum  Deposits  of  South  Dakota. 
U.  S.  Geol.  Survey  Butt.  380,  pp.  131-163,  1909;  U.  S.  Geol.  Survey  Mineral 
Resources,  1908  and  later  years. 


CHAPTER  XXVIII 

DEPOSITS  OF  THE  NONMETALS 

BUILDING  STONES 

Granite,  sandstone,  limestone,  marble,  slate,  and  many  other 
rocks  are  used  for  building  and  for  ornamental  purposes.  To  be 
desirable  for  such  uses  a  stone  should  have  a  pleasing  color, 
satisfactory  structure,  strength,  durability,  and  uniform  texture, 
and  for  use  in  a  moist  country  it  should  have  low  porosity. 
The  color  is  of  course  a  matter  of  taste  and  fashion.  At  present 
the  lighter  shades  are  more  desirable  than  the  darker  ones.  Most 
stone  used  in  the  main  courses  of  buildings  has  strength  far 
beyond  that  required  of  it,  but  if  the  stone  is  to  be  used  for  window 
caps  or  near  other  openings  where  stresses  are  unequal,  its  strength 
must  be  ascertained  more  carefully.  Blocks  near  doors  and 
windows,  sill  blocks,  and  caps  are  frequently  broken  in  buildings. 
Strength  tests  are  made  by  measuring  the  force  necessary  to 
crush  a  block  of  stone  in  a  testing  machine.  Tests  of  transverse 
strength — that  is,  strength  to  withstand  pressure  applied  un- 
equally at  different  places — are  made  by  supporting  the  two  ends 
of  a  bar  of  stone  and  applying  a  force  between  the  two  supports. 

Durability  depends  on  several  qualities.  In  general,  fine- 
textured  rocks,  especially  sandstone  and  marble,  are  more  durable 
than  coarse-textured  rocks,  although  this  is  not  universally  true. 
Certain  minerals  are  undesirable  for  building  stone — particularly 
pyrite  and  other  iron  sulphides,  because  they  oxidize,  staining 
the  rock  yellow,  and  as  they  are  soluble  the  surf  ace  becomes  pitted. 
Moreover,  sulphuric  acid  formed  by  oxidation  dissolves  the  stone, 
especially  limestone.  Nearly  all  igneous  rocks  and  many  sedi- 
mentary rocks  contain  some  pyrite.  A  little,  say  about  0.3  per 
cent.,  if  disseminated  in  the  rock  is  not  objectionable,  but  if  it  is 
concentrated  in  seams  and  veinlets  it  oxidizes  arid  weakens  the 
structure. 

Mica  is  considered  undesirable  when  it  forms  nodules  and 
533 


534      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

bunches  in  granite,  because  it  renders  the  stone  unsightly.  In 
some  quarries  mica  "knots"  cause  considerable  waste.  Small 
amounts  of  mica  disseminated  in  the  rocks  are  not  injurious  nor 
undesirable.  Mica  schists  have  been  used  very  effectively  in 
certain  cities  to  produce  the  rough  surfaces  that  are  preferred  by 
some.  In  marble,  chert  is  injurious  because  it  is  cut  and  polished 
with  more  difficulty  than  the  calcite,  and  moreover  its  presence 
causes  the  limestone  to  weather  unevenly. 

Freezing  and  thawing  tests  are  sometimes  made.  The  stone 
is  soaked  in  water  and  the  effects  of  freezing  noted.  Instead  of 
water,  a  saturated  sodium  sulphate  solution  is  sometimes  used  to 
simulate  the  effect  of  freezing  by  crystallization  of  the  sulphate 
in  the  rock  pores.  The  value  of  this  test,  however,  has  been 
questioned.  Absorption  tests  also  have  value  for  rocks  to  be 
used  in  structures  exposed  to  water,  and  heat  tests  for  rocks  ex- 
posed to  fire.1 

Aside  from  the  laboratory  tests,  valuable  information  may  be 
obtained  by  studying  a  stone  in  the  quarry  and  noting  the  effects 
of  weathering  on  it,  and  also  by  studying  the  effects  of  weathering 
on  buildings  of  known  age.  There  is  a  great  difference  in  the 
durability  of  stones.  Some  will  weather  badly  in  less  than  20 
years;  others  endure  for  centuries. 

Chemical  analyses  are  not  especially  valuable  for  showing  the 
desirability  of  a  stone  for  building.  Microscopic  examinations 
are  sometimes  useful,  as  they  disclose  the  minerals  contained  in 
the  rock.  It  is  more  important  to  ascertain  the  larger  struc- 
tural features,  particularly  the  bedding,  jointing,  and  sheeting. 
Many  quarries  are  profitable  because  the  joint  systems  are  fa- 
vorably spaced.  For  monoliths  there  should  be  but  two  systems 
of  joints  spaced  fairly  far  apart.  For  paving  blocks  closely  spaced 
sheeting  is  desirable.  Joint  systems  that  make  angles  of  45° 
or  less  with  other  systems  are  highly  undesirable,  for  they  break 
the  rocks  into  sharp  wedges  that  must  be  trimmed. 

A  plane  along  which  a  granite  may  be  easily  broken  is  called 
the  rift.  Some  rocks  that  appear  to  be  perfectly  homogeneous 
are  found  to  be  more  readily  broken  in  one  direction  than  in 
.others.  When  such  planes  lie  in  two  directions,  one  is  frequently 
called  the  rift  and  the  other  the  run  or  grain  of  the  rock.  Such 

1  McCouRT,  W.  E. :  Fire  Tests  of  Some  New  York  Building  Stones. 
N.  Y.  State  Mus.  Bull  100,  pp.  1-36,  1906. 


DEPOSITS  OF  THE  NONMETALS  535 

planes  are  probably  incipient  fracture  planes,  but  they  are  as  yet 
little  understood. 

In  considering  the  opening  of  a  quarry,  aside  from  the  quality 
of  the  stone,  cost  of  operation  is  an  important  factor.  It  is 
determined  in  part  by  the  jointing,  bedding,  and  other  features 
mentioned  above.  The  demand  for  the  stone,  the  character  of 
competing  quarries,  transportation  facilities,  and  freight  rates 
also  are  obviously  to  be  considered.  The  quarry  should  have 
sufficient  material  available  so  that  its  product  once  established  in 
the  market  may  be  supplied  through  a  period  of  years.  Among 
undesirable  features  in  granite  are  knots,  inclusions,  dikes,  hair 
lines  (small  dikes  of  dark  rocks),  quartz  veins,  pegmatite  areas, 
pyrite  lumps,  and  sheeting  too  closely  spaced.  Objectionable 
features  in  sandstone  and  limestone  are  clay  seams  too  closely 
spaced,  too  much  clay  in  the  rock,  shattered  zones,  pyrite  areas, 
and  mica  bands.  The  building  stone  produced  in  the  United 
States  in  1915  was  valued  at  $74,595,352. 

An  adequate  presentation  of  the  distribution  of  building  stone 
in  the  United  States  would  involve  a  discussion  of  the  regional 
geology  of  the  whole  country,  a  subject  too  extensive  to  be  in- 
cluded in  this  volume.  General  papers  with  bibliographies  are 
mentioned  below.  1 

SLATE 

When  mud  and  clay  are  deeply  buried  and  subjected  to  pres- 
sure, they  generally  assume  a  slaty  cleavage  by  virtue  of  which 
they  may  be  separated  into  thin,  tough  plates.  These  plates  are 
commercially  termed  slates  and  are  utilized  for  roofing,  stair 
treads,  wainscoting,  laboratory  tables,  etc.  When  a  clay  is 
changed  to  slate  by  pressure  or  dynamic  metamorphism,  the 
stable  minerals  like  quartz  are  rearranged  so  that  the  long  dimen- 

1  BURCHARD,  E.  F. :  U.  S.  Geol.  Survey  Mineral  Resources,  1909-1912. 

RIES,  HEINRICH,  and  WATSON,  T.  L.:  "Engineering  Geology,"  pp.  490- 
492,  New  York,  1913. 

MERRILL,  G.  P.:  "Stones  for  Building  and  Decoration,"  New  York, 
1903. 

ECKEL,  E.  C.:  "Building  Stones  and  Clays,"  New  York,  1912. 

RIES,  HEINRICH:  "Building  Stones  and  Clay  Products,"  New  York,  1912. 

BUCKLEY,  E.  R.,  and  BUEHLER,  H.  A.:  The  Quarrying  Industry  of  Mis- 
souri. Mo.  Bur.  Geol.  and  Mines,  2d  ser.,  vol.  2,  1904. 

BUCKLEY,  E.  R.:  On  the  Building  and  Ornamental  Stones  of  Wisconsin. 
Wis,  Geol.  and  Nat.  Hist.  Survey  Bull  4,  1898. 


536      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

sions  of  the  particles  lie  in  one  plane,  and  new  minerals  such  as 
actinolite,  tremolite,  mica,  and  chlorite,  developed  from  kaolin 
and  other  minerals  in  the  clay,  will  form  with  their  long  axes  in 
the  same  plane  as  the  long  dimensions  of  the  mineral  fragments 
(see  page  116).  The  slate  will  split  most  readily  along  such  a 
plane.  Igneous  tuffs  and  other  material  of  igneous  origin  may 
also  be  changed  by  dynamic  metamorphism  into  slaty  rocks,  but 
nearly  all  the  commercial  slates  are  of  sedimentary  origin. 

Slates  are  black,  green,  gray,  or  red,  their  color  depending  on 
that  of  the  constituent  minerals.  Some  slates  change  color  on 
long  exposure,  particularly  many  green  slates,  which  become 
bleached,  and  many  red  slates,  which  tend  to  become  brown. 
Black  slates  and  gray  slates  are  generally  more  nearly  permanent 
in  color.  Moderate  bleaching  is  not  very  objectionable  unless 
the  slate  becomes  spotted.  For  roofing,  a  slate  should  be  of 
nearly  permanent  color  and  should  not  break  on  being  punched. 
Lime  carbonate,  iron  carbonate,  and  pyrite  are  objectionable 
constituents. 

Slates  are  formed  in  areas  where  the  rocks  have  been  subjected 
to  dynamic  metamorphism.  They  are  found  in  the  Appalachian 
region  from  Maine  to  Alabama.  Beds  that  are  but  little  worked 
occur  in  Michigan,  in  Minnesota,  and  in  the  western  Cordillera. 
The  value  of  the  slate  produced  in  the  United  States  in  1915  was 
$4,958,915. 

References  to  general  papers  that  contain  bibliographies  are 
given  below.1 

CLAY 

Clay  is  a  term  used  without  any  very  definite  mineralogic 
significance  to  define  material  that  is  plastic  when  wet  and  that 
when  shaped  and  dried  will  retain  its  shape  and  when  burned  will 
become  hard.  The  principal  constituent  of  most  clays  is  kaolin- 
ite,  H4Al2Si209,  or  a  colloid  of  approximately  similar  composition, 
but  fragments  of  quartz,  iron  oxides,  silicates,  and  other  minerals 
are  generally  present.  Kaolin  is  the  commonest  product  of 

1  DALE,  T.  N. :  Slate  Deposits  and  Slate  Industry  of  the  United  States. 
U.  S.  Geol.  Survey  Bull.  275,  1906. 

Dale,  T.  N.,  and  others:  Slate  in  the  United  States.  U.  S.  Geol.  Survey 
Bull.  586,  1914. 

ECKEL,  E.  C.:  "Building  Stones  and  Clays."  pp.  95-126,  New  York,  1912. 

RIES,  HEINRICH:  "Building  Stones  and  Clay  Products,"  New  York,  1912. 


DEPOSITS  OF  THE  NON METALS  537 

weathering  of  aluminum  minerals  such  as  feldspars  and  micas. 
Nearly  all  igneous  rocks  contain  aluminum  minerals,  and  kaolin 
is  almost  invariably  formed  as  a  result  of  their  weathering. 
Thorough  weathering  of  feldspar-bearing  pegmatites  may  yield 
high-grade  kaolin  deposits.  Limestones  also  generally  contain 
aluminum  as  kaolinite  and  by  weathering  form  residual  clays. 
Shales  are  clays  that  have  been  consolidated  by  pressure.  On 
weathering  they  break  down  and  again  form  clays.  If  a  surface 
has  long  been  exposed  to  weathering  with  little  erosion,  as  hap- 
pens when  a  region  approaches  base-level,  soluble  substances  are 
almost  completely  leached  out,  and  the  residual  clay  remains. 
Ordinarily  the  parent  rock  may  be  found  a  few  feet  or  rarely  more 
than  50  or  100  feet  below  the  surface.  Such  clayey  substance  or 
mantle  rock  commonly  constitutes  the  soil  of  old  unglaciated 
surfaces.  Even  sandstone  and  quartzite  contain  some  clay,  and 
their  residual  soils  may  be  more  aluminous  than  the  parent  sandy 
rocks. 

In  normal  erosion  much  of  the  weathered  clayey  material  is 
carried  by  streams  and  deposited  along  rivers  or  in  deltas. 
Through  uplift  or  rejuvenation  clay  terraces  may  form.  Where 
a  weathered  surface  is  rejuvenated,  erosion  of  the  upland  is  more 
rapid,  and  more  clayey  material  is  deposited. 

Glacial  till  generally  contains  much  clay,  and  where  the  till  is 
worked  over  and  sorted  by  glacial  waters,  clay  deposits  of  con- 
siderable magnitude  are  commonly  formed.  Such  deposits  usu- 
ally differ  from  residua  clays  in  that  they  contain  "rock  flour," 
or  material  derived  from  the  mechanical  abrasion  of  rocks  and  not 
by  weathering,  which  removes  the  more  soluble  constituents  of 
rocks.  Many  glacial  clays  contain  considerable  lime  carbonate 
and  effervesce  freely  with  acid. 

The  plasticity  of  clays  is  probably  due  to  colloidal  substances 
or  "gels"  that  they  contain.  Halloysite  and  pholerite,  colloidal 
aluminum  silicates  more  highly  hydrated  than  kaolin,  are  prob- 
ably present  in  most  clays  and  doubtless  are  important  in  con- 
nection with  plasticity.1  Grout2  believes  that  molecular  attrac- 
tion also  plays  a  part  in  rendering  clays  plastic. 

Analyses  of  clays  do  not  go  very  far  to  show  their  availability. 

1  CUSHMAN,  A.  S. :  On  the  Cause  of  the  Cementing  Value  of  Rock  Powders 
and  the  Plasticity  of  Clays.     Am.  Chem.  Soc.  Jour.,  vol.  25,  pp.  451-468, 
1903. 

2  GROUT,  F.  F.:  The  Plasticity  of  Clays.     Idem,  vol.  27,  p.  1037,  1905. 


538      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Kaolin  contains  silica,  46.5  per  cent.;  aluminum,  39.5  per  cent.; 
and  water,  14  per  cent. ;  but  few  clays  approach  kaolin  closely  in 
composition.  The  china  clays  and  fire  clays  are  nearest  to  kao- 
lin; as  a  rule  they  result  from  weathering  of  feldspathic  igneous 
rocks  or  of  sedimentary  rocks.  Alkalies,  lime,  magnesium,  and 
iron  lower  the  fusing  points  of  clays  and  render  them  useless  as 
refractory  material. 

Burning  tests  are  the  most  satisfactory  tests  of  clays.  Iron 
in  clay  on  burning  colors  the  brick  red.  It  is  counteracted  by 
lime  carbonate.  Many  glacial  clays — for  example,  those  of 
Milwaukee,  Wis. — burn  buff,  owing  to  excess  of  lime.  Too 
much  lime,  however,  is  undesirable,  and  limestone  pebbles  will 
cause  the  brick  to  crumble.  Too  much  carbonaceous  matter  is 
also  undesirable,  as  it  causes  bloating.  Silica  lowers  shrinkage 
and  plasticity.  Sand  may  be  added  to  clay  where  less  fire  shrink- 
age is  desirable. 

Fire  clays  are  formed  where  the  physiographic  conditions  are 
favorable  for  the  leaching  out  of  all  materials  other  than  kaolin- 
ite.  Such  conditions  may  exist  where  coal  beds  are  formed. 
The  ground  waters,  aided  by  vegetation,  remove  the  more  soluble 
substances.  Thus  fire  clay  is  commonly  found  below  a  bed  of 
coal.  At  some  places  in  the  Missouri  and  Illinois  coal  fields  the 
bed  of  fire  clay  is  removed  and  the  coal  is  left  as  a  support  for  the 
overlying  material. 

Aside  from  the  residual  clays  and  the  sedimentary  and  glacial 
clays,  loess,  a  wind-blown  material,  is  used  at  many  places  for 
brickmaking. 

In  making  brick,  the  following  changes  take  place.  The 
molded  material  is  dried,  with  some  shrinkage.  On  firing  there 
is  further  shrinkage,  the  total  decrease  in  volume  commonly 
amounting  to  10  per  cent,  or  more.  Combined  water'is  driven  off 
at  about  450°  C.,  and  any  organic  matter  will  undergo  combustion 
at  temperatures  somewhat  higher.  As  the  temperature  is  raised 
iron  is  oxidized,  and  if  enough  iron  is  present  the  brick  becomes 
red.  If  carbonates  are  present  carbon  dioxide  escapes.  Incipi- 
ent fusion  takes  place,  and  this  hardens  the  mass.  If  the  tem- 
perature is  raised  high  enough  the  brick  is  vitrified,  particularly 
the  outer  surface,  which  is  hottest. 

In  the  United  States  nearly  every  State  has  large  supplies  of 
clays.  In  the  Coastal  Plain  region  from  New  York  south  and 
along  the  Atlantic  and  Gulf  coast  to  Texas  transported  clays  are 


DEPOSITS  OF  THE  NON METALS  539 

abundant.  They  are  coast  deposits  later  elevated  above  the 
water  level.  In  the  glaciated  regions  glacial  clays  abound. 
In  the  southern  Appalachians  and  at  some  other  places  in  the 
unglaciated  regions  residual  clays  are  found.  Shales  and  slates 
that  represent  clay  beds  of  past  geologic  ages  are  also  used  at 
many  places.  The  raw  clay  produced  in  the  United  States  in 
1915  amounted  to  2,362,954  tons,  valued  at  $3,971,941.  The 
clay  products  of  1915  were  valued  at  $163,120,232. 

The  uses  of  clays  and  clay  products  are  well  known. 

Discussions  of  the  technology  of  clays  and  descriptions  of 
occurrences  are  given  in  the  papers  cited  below.1 


FULLER'S  EARTH 

Fuller's  earth  is  a  clayey  material  used  for  cleaning  grease  from 
cloth  and  for  refining  petroleum  oils  and  other  fluids  by  filtra- 
tion. Like  clay,  it  is  a  product  of  weathering  or  of  weathering 
and  sedimentation.  It  differs  from  clay,  however,  in  that  it  has 
low  plasticity.  Its  water  content  is  high.  Chemical  analyses  are 
of  little  aid  in  determining  whether  a  clay  will  serve  as  fuller's 
earth.  Tests  for  the  purposes  for  which  it  is  intended  to  be  used 
give  the  only  trustworthy  evidence.  In  1915,  the  deposits  worked 
in  the  United  States  yielded  47,901  short  tons  of  fuller's  earth, 
valued  at  $489,219,  and  the  imports  amounted  to  $143,594.2 
The  demand  for  fuller's  earth  is  good.  The  largest  quantity  is  ob- 
tained in  Florida  and  Georgia,  where  it  is  quarried  from  Oligo- 
cene  beds.  In  Arkansas  it  is  derived  from  weathered  basic 

1  RIES,  HEINRICH  :  The  Clays  of  the  United  States  East  of  the  Missis- 
sippi River.     U.  S.  Geol.  Survey  Prof.  Paper  11,  1903.     "Clays:  Occurrence, 
Properties,  and  Uses,"  New  York,  1908. 

MERRILL,  G.  P.:   "Rocks,  Rock  Weathering,  and  Soils,"  New  York,  1906. 

ECKEL,  E.  C.:  "Building  Stones  and  Clays,"  New  York,  1912. 

The  technology  and  uses  of  clays,  especially  those  of  glacial  origin,  are 
summarized  in  Bull.  11  of  the  Minnesota  Geological  Survey,  by  F.  F.  GROUT 
and  E.  K.  SOPER.  A  complete  list  of  papers  on  clays  would  be  too  long  for 
this  volume.  Bibliographies  will  be  found  in  the  volume's  by  RIES,  MERRILL, 
and  ECKEL,  noted  above;  also  in  one  by  J.  C.  BRANNER  (Bibliography  of 
Clays  and  the  Ceramic  Arts.  U.  S.  Geol.  Survey  Bull.  143, 1896).  ECKEL 
(op.  cit.,  p.  236)  gives  also  a  list  of  references  on  origin. 

2  MIDDLETON,  JEFFERSON:  U.  S.  Geol.  Survey  Mineral  Resources,  1915 
part  2,  pp.  10,  12,  1916. 


540      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

rocks.    Papers  treating  deposits  of  fuller's  earth  in  the  United 
States  are  cited  below.1 

FELDSPAR 

Occurrences  of  feldspar  are  mentioned  on  pages  18-28.  The 
feldspar  utilized  in  the  arts  is  obtained  exclusively  from  peg- 
matite veins  and  deposits  formed  by  magmatic  segregation.  The 
principal  use  is  for  making  pottery,  glass,  porcelain,  enamel  brick, 
and  other  enamel  ware.  Potash,  soda,  and  lime  feldspars  are 
utilized.  Much  of  the  feldspar  marketed  contains  10  to  20  per 
cent,  of  quartz.  Because  the  hardness  of  feldspar  is  less  than 
that  of  quartz,  it  will  not  so  readily  scratch  glass  and  consequently 
there  is  a  demand  for  quartz-free  feldspar  for  window  wash  and 
polishing  powder.  The  production  of  feldspar  in  the  United 
States  in  1915  was  113,769  short  tons,  valued  at  $629,356.  Nearly 
all  of  this  was  obtained  in  the  New  England  States  and  the 
southern  Appalachian  region.  California  also  produces  consid- 
erable feldspar.2 

MICA 

Although  mica  is  a  common  constituent  of  igneous  rocks,  of 
crystalline  schists,  of  contact-metamorphic  deposits  and  of  some 

1Au>EN,  W.  C.:  Fuller's  Earth  and  Brick  Clays  near  Clinton,  Mass. 
U.  S.  Geol.  Survey  Bull.  430,  pp.  402-404,  1910. 

DAY,  D.  T.:  The  Occurrence  of  Fuller's  Earth  in  the  United  States. 
Franklin  Inst.  Jour.,  pp.  214-223, 1900. 

MERRILL,  G.  P.:  "The  Nonmetallic  Minerals,"  pp.  248-250,  New  York, 
1904. 

MISER,  H.  D.:  Developed  Deposits  of  Fuller's  Earth  in  Arkansas.  U.  S. 
Geol.  Survey  Bull  530,  pp.  207-219,  1912. 

PARSONS,  C.  L.:  Fuller's  Earth.  U.  S.  Bur.  Mines  Bull.  71,  pp.  1-38, 
1913. 

PORTER,  J.  T.:  Properties  and  Tests  of  Fuller's  Earth.  U.  S.  Geol. 
Survey  Bull  315,  pp.  268-290,  1907. 

RIES,  HEINRICH:  "Clays,  Their  Occurrence,  Properties,  and  Uses," 
pp.  460-467,  New  York,  1906. 

VAUQHAN,  T.  W.:  Fuller's  Earth  of  Florida  and  Georgia.  U.  S.  Geol. 
Survey  Bull  213,  pp.  392-399,  1903. 

2  The  bibliography  of  feldspar  deposits  of  the  United  States  is  extensive. 
Citations  under  "Quartz"  (page  551)  refer  also  to  feldspar.  The  principal 
deposits  are  discussed  by  BASTIN,  E.  S. :  Economic  Geology  of  the  Feldspar 
Deposits  of  the  United  States.  U.  S.  Geol.  Survey  Bull.  420,  pp.  1-85, 
1910. 


DEPOSITS  OF  THE  NONMETALS  541 

veins,  the  mica  of  commerce  is  derived  from  pegmatites.  Most 
pegmatites  are  composed  chiefly  of  quartz,  feldspar  and  mica. 
In  the  commonest  type  feldspar  predominates,  but  in  some  peg- 
matites quartz  is  the  principal  constituent,  and  in  a  few  mica 
is  abundant.  The  micas1  include  muscovite  (H2KAl3Si3Oi2), 
phlogopite  (H2KAlMg3Si3Oi2),  and  biotite  (HKA^MgsSisO^). 
Iron  also  is  commonly  present  in  biotite,  taking  the  place  of 
aluminum  or  magnesium.  Lepidolite,  or  lithium  mica,  is  a  source 
of  lithium  salts.  The  important  commercial  micas  are  muscovite 
and  phlogopite. 

The  larger  pieces  of  mica  are  used  for  glazing  stoves  and  lamp 
chimneys  and  in  making  electric  apparatus.  Smaller  pieces  are 
used  in  making  "micanite"  or  built-up  mica  board,  used  chiefly  for 
electric  appliances.  In  making  dynamos  mica  is  used  as  an  insu- 
lator; phlogopite  is  preferred  for  such  use,  as  it  wears  more  uni- 
formly than  muscovite  and  does  not  cause  the  machine  to  spark. 
Sterrett  states  that  black  specks  or  magnetite  dendrites  in  mica 
do  not  impair  its  insulating  quality.  Ground  mica  is  used  for 
making  paints,  wall  paper,  lubricants,  building  paper,  etc.  The 
United  States  in  1915  produced  553,821  pounds  of  trimmed  mica, 
valued  at  $378,259,  and  3,959  tons  of  scrap  mica,  valued  at  $50,510. 
Large  clear  pieces  of  mica  cut  to  dimensions  of  several  inches 
square  sell  for  $1  a  pound  or  more,  but  low-grade  material  is 
cheap. 

The  mica  produced  in  the  United2  States  comes  from  New 
England,3  the  southern  Appalachian  region,4  the  Black  Hills, 
South  Dakota,5  and  Colorado.  The  imports  are  mainly  from 
Canada6  and  from  Ceylon. 

1  STERRETT,  D.  B.:  Mica.     U.  S.  Geol.  Survey  Mineral  Resources,  1911, 
part  2,  p.  1130,  1912  (includes  bibliography). 

2  HOLMES,  J.  A. :  Mica  Deposits  in  the  United  States.     U.  S.  Geol.  Sur- 
vey Twentieth  Ann.  Rept.,  part  6  (continued),  pp.  691-707,  1899. 

3  RICE,  C.  F. :  Description  of  Mica-Mining  Company  in  Grafton  County, 
New  Hampshire.     Min.  and  Sci.  Press,  Feb.  23,  1901. 

4  STERRETT,    D.   B.:  Mica   Deposits  of   North  Carolina.     U.   S.   Geol. 
Survey  Bull.  430,  pp.  593-638,  1910. 

PRATT,  J.  H.:  The  Mining  Industry  in  North  Carolina  (an  annual  publi- 
cation of  the  North  Carolina  Geol.  Survey),  1900-1910. 

6  STERRETT,  D.  B. :  Mica  Deposits  of  South  Dakota.  U.  S.  Geol.  Sur- 
vey Bull.  380,  pp.  382-397,  1906. 

6  ELLS,  R.  W.:  Mica  Deposits  of  Canada.  Canada  Geol.  Survey  Pub. 
869,  1904. 


542      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 
LITHIUM  MINERALS 

Lithium  is  obtained  principally  from  lepidolite  (lithium  mica), 
spodumene,  and  amblygonite.  Lithium  minerals  are  exploited 
in  San  Diego  County,  California,  and  in  the  Etta  mine,  in  the 
southern  Black  Hills.1  Smaller  amounts  are  found  at  Paris, 
Maine,  and  at  many  other  places  in  New  England.  Bodies  of 
these  minerals  of  commercial  size  are  practically  confined  to 
pegmatite  veins  (see  page  25).  Only  a  few  hundred  tons  of 
lithium  minerals  are  produced  annually  in  the  United  States. 
The  prices  range  from  $10  to  $40  a  ton.  The  material  is  normally 
exported  to  Germany,  where  soluble  lithium  salts  are  made  from 
the  silicates.  Lithium  salts  are  used  in  medicine  and  in  medicinal 
waters. 

CEMENTS  AND  LIMES 

Cement. — Cement2  is  made  by  fusing  mixtures  of  oxides  of 
calcium,  aluminum,  and  silicon.  When  the  clinker  is  ground  and 
mixed  with  water  it  sets  to  a  strong,  hard,  solid  mass.  The  three 
oxides  need  not  be  present  in  exact  proportions,  but  within  certain 
limits  the  mixture  may  vary.  Some  limestones  containing  con- 
siderable clay  are  of  suitable  composition  for  cement  without  the 
addition  of  other  material.  The  cements  made  of  such  limestones 
are  termed  natural  cements.  If  the  mixture  is  made  artificially 
it  is  called  Portland  cement.  The  hardening  is  not  brought  about 
by  the  addition  of  material  from  the  atmosphere,  as  when  mortar 
hardens  by  the  addition  of  CC>2,  but  is  due  to  the  formation  of 
hydrates  of  calcium  and  aluminum.  Portland  cement  and  some 
natural  cements  will  harden  under  water  as  well  as  in  air. 

1  HESS,  F.  L.:  Tin,  Tungsten,  and  Tantalum  Deposits  of  South  Dakota. 
U.  S.  Geol.  Survey  Bull.  380,  pp.  159-161,  1909;  U.  S.  Geol.  Survey  Mineral 
Resources,  1909,  part  2,  p.  649,  1910. 

2  ECKEL,  E.  C.:  Portland  Cement  Materials  and  Industry  in  the  United 
States.     U.  S.  Geol.    Survey   Bull.  522,  1913.— "Limes,  Mortars,  and  Ce- 
ments," New  York,  1907. 

BURCHARD,  E.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2, 
pp.  485-515,  1912. 

RIES,  HEINKICH,  and  ECKEL,  E.  C. :  Lime  and  Cement  Industries  of  New 
York.  N.  Y.  State  Mus.  Bull.  44,  1901. 

BLEININGER,  A.  V.:  The  Manufacture  of  Hydraulic  Cements.  Ohio 
Geol.  Survey  Bull.  3,  1905. 

RANKIN,  G.  A.,  and  WRIGHT,  F.  E.:,The  Ternary  System  CaO-Al2Os- 
SiO2.  Am.  Jour.  Sci.,  4th  ser.,  vol.  39,  pp.  1-79,  1915. 


DEPOSITS  OF  THE  NONMETALS  543 

Analyses  of  calcareous  rock  to  be  used  for  natural  cements 
should  show  between  15  and  40  per  cent,  of  clay  and  silica  and 
not  more  than  a  small  percentage  of  magnesia.  After  burning 
only  a  small  percentage  of  free  lime  should  be  present. 

Portland  cement  is  made  by  mixing  a  calcareous  material, 
such  as  limestone,  chalk,  or  marl,  with  a  siliceous  and  aluminous 
material,  such  as  clay,  shale,  slag,  or  mud.  As  it  is  possible  to 
vary  the  proportions  of  the  substances  used  the  mixture  is  made 
to  approach  closely  a  desirable  standard  composition  that  has 
been  found  satisfactory  by  previous  experimentation.  Portland 
cement  is  generally  superior  to  most  natural  cements,  because 
the  mixture  is  more  easily  controlled  and  is  nearly  uniform. 
The  mixture  before  burning  generally  consists  of  about  75  per 
cent.  CaCOs,  not  more  than  5  per  cent.  MgCOs,  12  to  15  per  cent. 
SiO2,  and  4  to  7  per  cent.  A12O3.  Between  1  and  2  per  cent, 
of  Fe2O3,  K2O,  and  organic  matter  are  commonly  present. 
None  of  these  substances  are  detrimental  except  MgO,  which,  if 
running  above  6  per  cent.,  at  least  under  some  conditions,  will 
weaken  the  product.  Iron  oxide  and  potassium  oxide,  on  the 
other  hand,  are  desirable,  for  they  lower  the  temperature  neces- 
sary for  sintering.  On  burning  CO2  and  H2O  are  driven  off,  and 
the  percentages  of  other  substances  in  the  clinker  are  thereby 
increased  about  one-half. 

Cement  making  consists  of  three  steps — (1)  grinding  and  mix- 
ing, (2)  clinkering,  (3)  grinding  the  clinker. 

In  grinding,  chert  and  other  siliceous  matter  in  the  limestone 
are  generally  considered  undesirable,  as  they  increase  the  cost. 
Recently  improvements  in  grinding  machines  have  reduced  the 
cost  of  grinding,  and  there  is  an  increasing  tendency  to  utilize 
the  purer  limestone  and  the  slags,  instead  of  softer  marl  and 
argillaceous  limestone.  In  burning  the  cement  mixtures,  the 
temperatures  employed  are  generally  above  1500°F.,  depend- 
ing on  the  composition  of  the  mixture  and  the  nature  of  the 
burning  apparatus.  The  mixture  contains  considerable  CO2  in 
carbonates,  and  this  is  driven  off  at  about  750°F.  to  1400°F., 
increasing  porosity  and  decreasing  volume  of  material.  In  burn- 
ing complete  fusion  is  avoided,  for  on  fusion  the  constituents 
might  segregate.  Incineration  is  accomplished,  therefore,  not 
in  a  shaft  like  those  employed  in  smelting  the  metals,  but,  in 
many  plants,  in  long  revolving  cylinders,  tilted  a  little  so  that 
the  material  moves  automatically  from  the  intake  at  the  upper 


544      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

end  to  the  point  of  discharge  at  the  lower  end.  Flame,  generally 
kindled  by  powdered  coal  or  oil,  is  introduced  at  the  lower  end 
and  escapes  at  the  upper  end  of  the  cylinder  through  a  flue 
provided  there.  The  burner  resembles  the  Howell-White  dryer 
formerly  used  extensively  in  the  West  in  silver  mills,  but  in 
general  the  temperatures  employed  are  higher  and  the  cement 
kilns  are  much  larger,  some  of  them  being  240  feet  long. 

The  clinker,  which  comes  out  as  smah1  masses,  is  ground  in 
ball  mills.  Fine  grinding  is  necessary,  as  cement  that  is  not  finely 
ground  will  not  set. 

Natural  cement  is  made  by  burning  impure  limestone  contain- 
ing 15  to  40  per  cent,  silica,  alumina,  and  iron  oxide.  The  tem- 
perature required  is  not  much  above  that  at  which  lime  is  burned, 
and  kilns  like  the  ordinary  limekilns  may  be  used.  Carbon 
dioxide  is  driven  off,  and  lime  silicates,  aluminates,  and  ferrates 
are  formed.  The  ground  clinker  does  not  slake  and  sets  slowly 
under  water. 

The  production  of  cement  in  the  United  States  in  1915  was 
87,685,222  barrels,  valued  at  $75,155,102. 

Mortar. — The  function  of  a  mortar  is  to  act  as  a  binding 
material,  converting  the  blocks  of  a  wall  or  other  structure  into 
a  coherent  mass.  It  is  made  by  mixing  from  1  to  5  parts  of  clean 
sand  with  1  part  of  slaked  lime  or  cement.  Generally  cement  is 
considered  more  durable  for  buildings,  and  in  some  that  are 
centuries  old  it  still  endures. 

Concrete. — Concrete  is  a  mixture  of  sand,  cement,  and  gravel 
or  crushed  stone,  in  which  the  cement  acts  as  a  mortar.  Be- 
cause the  material  can  be  poured  into  forms  and  thus  handled 
cheaply,  its  use  is  rapidly  increasing.  The  principle  that  the 
total  pore  space  in  a  rock  depends  not  on  the  size  of  particles 
but  upon  uniformity  of  size,  being  greatest  when  particles  are 
of  equal  size  (see  page  172),  is  important  in  this  connection. 
So  long  as  the  cement  makes  into  a  coherent  mass  the  various 
particles,  the  less  required  the  better,  provided  the  other  materials 
used  in  the  concrete  mixture  are  sufficiently  strong.  In  the 
larger  concrete  structures  it  is  considered  desirable  to  introduce 
large  irregular  blocks  of  stone,  which  generally  decrease  cost  and 
increase  strength.  Even  in  neat  cement  not  all  of  the  mixture 
becomes  hydrated  to  form  new  compounds,  but  the  small  part 
that  does  is  sufficient  to  bind  the  other  parts  together  firmly. 
Sand  is  sometimes  ground  fine  and  added  to  cement.  It  should 


DEPOSITS  OF  THE  NONMETALS  545 

not  be  regarded  as  an  adulterant,  because  a  small  amount  may  even 
add  to  the  strength  of  the  product,  especially  if  the  sand  grains 
are  angular  and  thus  interlock  and  present  large  surfaces  of 
contact.' 

Lime.— Lime  is  made  by  heating  limestone  to  about  800°. 
Impure  limestones  are  heated  to  a  higher  temperature.  At 
such  temperatures  the  limestone  loses  CO2  and  becomes  "quick- 
lime," CaO.  When  water  is  poured  on  quicklime  chemical  action 
takes  place:  CaO  +  H2O  =  Ca(OH)2.  This  reaction  increases 
volume:  about  32  parts  of  water  is  added  to  100  parts  CaO. 
As  calcium  hydroxide  is  somewhat  soluble  it  is  used  extensively 
in  the  arts  to  neutralize  acids  and  to  make  solutions  alkaline. 
Large  amounts  are  used  in  sugar  refining,  in  cyaniding  gold  ores, 
and  in  tanning.  Quicklime  is  used  in  the  manufacture  of  alkalies 
and  bleaching  powders,  etc.  It  is  the  cheapest  alkali,  just  as 
sulphuric  acid  is  the  cheapest  acid,  and  it  has  a  correspondingly 
important  position  in  the  arts. 

Common  lime  mortar  is  made  by  mixing  sand,  quicklime,  and 
water.  It  is  hardened  by  C02  of  the  air: 

Ca(OH)2  +  C02  =  CaC03  +  H20 

A  crust  first  forms,  enough  to  hold  the  bricks  or  stones,  and 
very  slowly  the  carbonation  extends  inward.  The  calcite  crystals 
form  only  a  matrix  or  "groundmass"  for  the  particles  of  sand, 
just  as  hydrates  form  the  matrix  in  Portland  cement  mixtures. 
The  sand  adds  strength  and  decreases  shrinkage;  without  sand, 
cracks  will  form  on  drying.  Lime  mortar  does  not  harden  in 
damp  places,  and  its  use  is  not  recommended  for  cellars  and  deep 
foundations.  Calcium  carbonate,  if  nearly  pure  makes  a  lime 
that  slakes  readily  ("fat  lime").  If  magnesia  is  present  it  slakes 
more  slowly  (meager  or  cold  lime).  This  is  not  always  unde- 
sirable, and  more  magnesium  is  allowable  than  in  limestone  to 
be  used  for  making  Portland  cement.  For  many  structural 
purposes  magnesian  lime  is  preferable.  Some  lime  is  hydrated, 
dried,  and  powdered  before  being  marketed. 

With  increasing  alumina  and  silica  lime  grades  into  natural 
cement.  Some  argillaceous  limestones,  heated  somewhat  above 
the  point  of  decarbonation,  yield  bricklaying  cement  that  is  said 
to  be  superior  to  common  lime  for  making  mortar. 

Puzzolan  Cement. — Puzzolan  cement  is  made  by  mixing  slaked 
lime  with  finely  ground  clayey  material,  finely  ground  furnace 

35 


546      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

slag,  or  volcanic  ash.  It  is  not'  calcined,  and  the  mixture  is 
little  more  than  a  lime  mortar.  The  material  is  cheap  but  is 
not  recommended  for  structures  intended  to  endure,  although 
in  Italy  some  houses  made  of  such  .material  have  had  a  long  life. 
Puzzolan  cement  should  not  be  confused  with  Portland  cement 
made  of  slags  from  iron  blast  furnaces.  In  making  Portland 
cement  from  slags,  the  slags  are  ground  and  other  material  is 
added  to  give  a  suitable  mixture.  After  sintering  the  clinker  is 
ground  as  in  making  Portland  cement  from  other  materials. 

Distribution  of  Materials. — In  1911,  according  to  Burchard, 
mixtures  of  argillaceous  limestone  and  pure  limestone  supplied 
material  for  34.1  per  cent,  of  the  Portland  cement  product  of 
the  United  States;  limestone  and  clay  or  shale,  51.8  per  cent.; 
marl  and  clay,  4.2  per  cent.;  and  slag  and  limestone,  9.9  per  cent. 

It  is  desirable  that  limestone  to  be  used  for  making  Portland 
cement  should  be  nearly  homogeneous  in  composition,  for  then 
the  composition  of  mixtures  may  be  very  easily  controlled.  At 
some  places,  however,  the  beds  that  are  used  show  considerable 
variation  in  composition,  both  along  and  across  the  strike. 
Magnesian  beds,  or  those  containing  too  much  clay,  are  avoided, 
or  when  they  are  used  purer  limestone  is  added.  A  limestone 
with  some  clay  is  preferable  to  a  pure  limestone  if  the  clay  is 
evenly  distributed,  because  it  simplifies  mixing. 

Limestones  suitable  for  making  Portland  cement  are  widely 
distributed  in  the  United  States;'  detailed  accounts  of  their 
distribution  are  given  in  papers  by  Burchard1  and  Eckel.2  The 
Lehigh  Valley  district  of  Pennsylvania3  is  the  principal  source. 
It  supplied  in  1910  about  one-third  of  the  product  of  the  United 
States.  The  rotfks  are  of  Paleozoic  age  and  are  closely  folded. 
The  Trenton  limestone  and  Hudson  River  shale  supply  much 
of  the  cement  material.  The  lower  part  of  the  Trenton  is 
nearly  pure  limestone  but  contains  some  dolomite  beds  that  are 
avoided  as  far  as  practicable  in  quarrying.  The  upper  part  of 
the  Trenton  is  argillaceous  limestone,  some  of  which  has  the 
composition  of  natural  cement  rock.  Mixtures  of  the  lower  and 
upper  beds  provide  a  material  of  suitable  composition.  The 

^TJRCHABD,  E.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1910,  part  2, 
pp.  489-526,  1911. 

2  ECKEL,  E.  C.:  "Limes,  Mortars,  and  Cements,"  New  York,  1907. 

3  PECK,  F.  B.:  Geology  of  the  Cement  Belt  in  Lehigh  and  Northampton 
Counties,  Pennsylvania.    Econ.  Geol,  vol.  3,  pp.  37-76,  1909. 


DEPOSITS  OF  THE  NONMETALS 


547 


Lehigh  belt  of  Trenton  limestone  extends  into  New  Jersey,  where 
also  this  formation  supplies  much  material  for  cement. 

In  eastern  New  York  Portland  cement  is  made  of  Ordovician 
and  Silurian  limestones  mixed  with  surface  clay.  In  Kansas 
Mississippian,  Pennsylvanian,  Permian,  and  Cretaceous  lime- 
stones are  used.  In  Texas  and  Arkansas  Cretaceous  shales  and 
limestones  supply  material  for  Portland  cement.  Devonian 
beds  are  used  in  Kentucky,  Ohio,  and  Wisconsin.  In  the  glacial 
belt,  especially  in  Michigan,  Indiana,  and  Ohio,  fresh-water 
marls  are  used  with  Pleistocene  clays.  The  marl  forms  in  the 
lakes  through  the  agency  of  small  plants,  especially  the  stone- 
wort,  Chara.1  It  is  commonly  incoherent  and  in  some  lakes  is 
pumped  out  and  dried  for  cement  making.  Some  of  the  lakes 
are  drained,  and  the  partly  dried  marl  is  excavated  with  steam 
shovels. 

Rocks  for  natural  cement  are  quarried  from  the  Silurian  beds 
in  Ulster,  Schoharie,  Erie,  and  Onondaga  counties,  New  York. 
Natural  cement  rocks  are  obtained  also  in  Maryland  and  Virginia, 
at  Utica,  111.,  near  Milwaukee,  Wis.,  and  Mankato,  Minn.,  and 
at  several  other  localities. 


LIME  SOLD  IN  THE  UNITED  STATES  IN  1915,  AND  USES" 


Quantity 
(short  tons) 

Value 

Average  per 
ton 

Building  lime  

1,136,696 

$4,812,710 

$4.23 

Chemical  works  
Paper  mills  
Sugar  factories  
Tanneries  

492,870 
216,819 
34,025 
47,104 

1,653,750 
782,396 
230,368 
190,864 

3.36 
3.61 
6.77 
4.06 

Fertilizer  
Dealers  —  uses  not  specified  
Other  uses*  

653,686 
595,128 
413,371 

2,163,874 
2,508,907 
1,993,887 

3.31 
4.22 
4.82 

3,589,699 

14,336,756 

3.99 

0  LODGHLIN,  G.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  2.  p.  245,  1916. 

6  Includes  lime  for  sand-lime  brick,  slag  cement,  alkali  works,  steelworks,  glassworks, 
smelters,  shep  dip,  disinfectant,  manufacture  of  soap,  cyanide  plants,  glue  factories, 
purification  of  water,  etc. 

1  DAVIS,  C.  E. :  A  Contribution  to  the  Natural  History  of  Marl.  Jour. 
Geol,  vol.  8,  pp.  485-497,  1900;  vol.  9,  pp.  491-506,  1901. 


548      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Limestones  suitable  for  lime  are  widely  distributed.  In  a 
search  for  deposits  of  limestone  to  supply  lime  or  cement  for  a 
certain  market  it  is  advisable  to  consult  geologic  maps  of  regions 
near  by1  and  ascertain  the  lithologic  character  of  the  formations. 
Analyses  will  show  what  beds  are  available.  Some  hundreds  of 
analyses  of  limestones  quarried  in  the  United  States  are  recorded 
by  Burchard.2 

NATURAL  ABRASIVES 

Many  minerals  and  rocks  are  used  in  their  natural  state  as 
abrasives.  These  include  sandstone,  grit,  chert,  garnet,  corun- 
dum, emery,  quartz,  feldspar,  infusorial  (diatomaceous)  earth, 
tripoli,  pumice,  etc. 

Millstones  are  shaped  from  grit  and  sandstone.  They  are 
manufactured  in  several  States,  especially  in  New  York  and 
Virginia.  Formerly  they  were  in  considerable  demand,  and  in 
18803  the  production  in  the  United  States  was  valued  at  $200,000. 
In  1915,  however,  the  total  was  only  $53,480.  In  recent  years 
the  use  of  steel  rolls  and  ball  mills  for  grinding  has  caused  a 
marked  decrease  in  the  demand  for  millstones. 

Grindstones  also  are  shaped  from  grit  or  sandstone.  They 
are  used  mainly  to  sharpen  steel  tools.  The  grain  should  be 
fairly  uniform  and  the  particles  cemented  firmly,  yet  the  pore 
space  should  not  be  entirely  filled  with  cement  so  that  cutting 
edges  are  buried.  Too  much  clay  in  the  sandstone  will  cause 
a  smooth  surf  ace.  to  form  on  grinding,  and  that  will  impair 
cutting  efficiency.  Certain  layers  of  the  Berea  grit  of  Ohio  and 
Michigan  are  extensively  used  for  grindstones. 

Pulpstones  are  large  grindstones  used  for  grinding  wood  to 
paper  pulp.  The  wood  is  softened  by  introducing  steam  in  a 
jacket  around  the  circular  pulpstone,  and  the  pulpstone  must 
have  a  cement  that  does  not  disintegrate  in  the  presence  of  steam. 
Most  of  the  pulpstones  used  in  the  United  States  come  from 
England.  Some  of  the  stones  are  5  feet  or  more  in  diameter 
and  weigh  over  2  tons.  Experiments  have  been  made  to  utilize 

1  WILLIS,  BAILEY:  Index  to  the  Stratigraphy  of  North  America.     U.  S. 
Geol.  Survey  Prof.  Paper  71,  1912  (contains  extensive  references). 

2  BURCHARD,  E.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2, 
pp.  658-680,   1912. 

'PHALEN,  W.  C.:  Abrasive  Materials.  U.  S.  Geol.  Survey  Mineral 
Resources,  1911,  part  2,  p.  837,  1912. 


DEPOSITS  OF  THE  NONMETALS  549 

Band  and  cement  to  make  artificial  pulpstones,  so  as  to  avoid  the 
expense  of  shaping. 

Scythestones,  whetstones,  and  oilstones  are  small  abrasive 
stones  ge'nerally  used  as  hand  tools.  They  are  made  from  rocks 
of  varied  character,  but  mainly  from  sandy  sedimentary  rocks 
and  schists.  The  novaculite  (hard)  and  Wachita  (soft)  stones 
of  southwestern  Arkansas  are  noteworthy.  Both  of  these  stones 
are  nearly  pure  silica.  They  are  found  as  thin  beds  in  a  folded 
series  of  sandstones  and  shale.  The  hard  Arkansas  stone  is  very 
dense  and  has  but  small  pore  space ;  it  is  much  prized  as  a  finisher 
of  razors  and  other  fine  instruments.  The  soft  Arkansas  stone 
has  much  greater  abrasive  power  on  account  of  its  greater  pore 
space,  but  it  does  not  produce  so  fine  an  edge. 

Volcanic  "ash"  is  volcanic  matter  in  a  very  finely  divided 
state.  It  is  used  for  polishing  and  for  making  scouring  soaps. 
It  is  common  in  many  Western  States  and  is  mined  in  Nebraska. 1 
Pumice  is  a  solidified  rock  froth  formed  of  rock  material  by 
gases  escaping  from  lavas  and  having  in  general  about  the  com- 
position of  rhyolite.  It  has  many  domestic  uses.  Ground 
pumice  resembles  volcanic  ash.  The  United  States  in  1915 
produced  27,708  tons  of  pumice  and  volcanic  ash,  valued  at 
$63,185,  principally  from  Nebraska  and  Kansas.  The  pumice 
blocks  of  commerce  come  mainly  from  Lipari,  an  island  north 
of  Sicily.  Deposits  are  known  in  the  western  part  of  the  United 
States,  but  apparently  they  can  not  compete  with  those  of  Italy, 
where  labor  is  cheaper. 

Tripoli  is  a  fine,  nearly  pure  silica,  that  is  found  in  siliceous 
limestone  in  Missouri  and  Illinois.  It  is  ground  and  sold  for 
abrasive  dust  and  is  used  also  for  wood  filler. 

Garnet  is  extensively  used  as  an  abrasive,  particularly  the 
varieties  almandine,  pyrope,  and  grossularite.  The  garnets 
used  for  this  purpose  are  obtained  from  schists  in  New  York, 
New  Hampshire,  and  North  Carolina.  Considerable  garnet 
is  used  in  the  manufacture  of  sandpaper,  and  for  certain  pur- 
poses it  is  superior  to  quartz. 

Corundum  (A1203)  is  one  of  the  hardest  minerals,  standing  at 
9  on  the  Mohs  scale.  Its  powder  is  used  for  grinding,  and  small 
crystals  are  used  for  watch  jewels.  It  is  found  in  certain  igneous 

HARBOUR,  E.  H.:  Pumice.  Nebr.  Geol.  Survey,  vol.  1,  pp.  214-220, 
1903. 


550      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

rocks  that  are  rich  in  aluminum  and  poor  in  silica.  Corundum 
occurs  also  in  nepheline  syenite,1  schist,  and  pegmatite  and  in 
gravel  formed  by  the  waste  of  corundum  rocks.  Corundum  is 
found  at  many  places  in  the  crystalline  schist  belts  of  the  Ap- 
palachian region.  Pratt2  has  shown  that  some  of  the  corundum 
ores  are  derived  from  di-nite  by  segregation. 

Emeiy  is  a  mixture  of  hematite  and  corundum,  and  some  of  it 
contains  other  minerals.  The  ground  powder  is  sized  and  used 
for  polishing  and  for  making  sandpaper.  It  is  mined  in  small 
quantities  in  the  Appalachian  region.  Ground  quartz  also  is 
used  for  making  sandpaper. 

Feldspar,  which  is  softer  than  quartz,  is  ground  and  used  for 
polishing  powder,  especially  to  polish  glass  and  other  materials 
where  scratching  is  to  be  avoided.  Diamonds  are  used  as  dust 
and  uncut  in  bits  of  core  drills.  Chromic  oxide  and  rouge 
(hematite  powder)  are  used  for  fine  polishing.  The  natural 
abrasives  must  compete  more  and  more  with  the  artificial 
abrasives,  especially  with  carborundum  (CSi)  and  alundum 
Both  of  these  are  made  in  electric  furnaces. 


VALUE  OF  NATURAL  ABRASIVES*  PRODUCED  AND  MARKETED  IN  THE  UNITED 
STATES,  191  5  4 

Millstones  ...............  $53,480  Garnet...  ..............  139,584 

Grindstones  and  pulpstones.  648,479  Diatomaceous  (infusorial) 

Oilstones  and  scythestones.  115,175  earth  and  tripoli  .......  611,021 

Emery  ...................  31,131  Pumice6  ................  63,185 


1,662,055 
INFUSORIAL  (DIATOMACEOUS)  EARTH 

.Minute  organisms  such  as  diatoms  and  radiolaria — extract 
silica  from  water,  and  their  tests  accumulate  in  great  abundance 
in  some  places  at  the  bottoms  of  ponds,  lakes,  and  in  the  sea. 
This  material  when  pure  is  very  light  and  fluffy,  and  as  it  contains 

1  ADAMS,  F.  D. :  On  the  Occurrence  of  a  Large  Area  of  Nepheline  Syenite 
in  the  Township  of  Dungannon,  Ontario.     Am.  Jour.  Sci.,  3d  ser.,  vol.  48, 
pp.  10-16,  1894. 

2  PRATT,  J.  H. :  The  Occurrence  and  Distribution  of  Corundum  in  the 
United  States.     I".  S.  Geol.  Survey  Bull.  180,  p.  12,  1901. 

J  Abrasive  quartz  and  feldspar  not  included. 

4  KATZ,  F.  J.:  U.  S.  Geol.  Survey  Mineral  Resources,  1915,  part  2,  p.  66, 
1916. 

6  Includes  volcanic  ash. 


DEPOSITS  OF  THE  NONMETALS  551 

much  air  space  it  is  an  excellent  nonconductor  of  heat.  Gener- 
ally some  clay  or  fine  sand  occurs  in  the  deposits,  but  the  pure 
material  is  nearly  pure  silica.  It  is  white,  cream-colored,  or 
light  gray  and  is  easily  recognized  under  the  microscope  by  the 
symmetrical  boundaries  of  the  constituent  tests.  Enormous 
deposits  are  found  at  Reno  and  Goldfield,  Nev.,  in  Santa  Barbara 
County,  California,  and  in  Oregon.  Deposits  are  known  also 
at  Richmond,  Va.,  and  in  Herkimer  County,  New  York,  Diato- 
maceous  earth  is  used  for  packing  steam  pipes,  for  polishing 
powder,  as  an  absorbent  for  nitroglycerine,  and  for  many  other 
purposes.  Its  great  abundance  and  various  uses  promise  an 
increasing  yield.1  In  1915  the  United  States  produced  infusorial 
earth  valued  at  $611,012. 

QUARTZ 

Quartz  is  a  persistent  mineral,  occurring  in  igneous  and  sedi- 
mentary rocks,  in  ore  veins,  in  pegmatites,  and  elsewhere. 
In  the  West  there  are  many  big  white  barren  mineral  veins  termed 
"bull  quartz"  by  prospectors.  Vein  quartz  is  mined  for  many 
purposes.  The  purer  quartzites  and  also  flint  and  chert  are 
mined  in  some  places.  Flint  nodules  are  in  demand,  especially 
for  tube  mills. 

Quartz  is  used2  for  flux,  for  filling  acid  towers,  and  as  a  re- 
fractory material.  Various  forms  of  silica  are  used  for  paint, 
for  wood  filler,  for  glazing,  for  making  glass,  and  for  mixing  with 
clay  to  decrease  shrinkage  of  pottery.  As  an  abrasive  it  is  used 
to  make  sandpaper,  soap,  and  polishing  powder  and  for  the  air 
blast.  Rose  quartz  and  some  other  varieties  of  silica  are  used 
for  gems.  The  United  States  in  1915  produced  112,575  tons  of 
silica  (quartz),  valued  at  $273,553. 

GLASS  SAND 

By  the  weathering  of  rocks  quartz  is  separated  and  accumulates 
along  rivers,  lakes,  and  seas.  Where  the  sea  bottoms  are  ele- 
vated, or  where  wind  blows  sand  in  dunes,  the  sands  become  more 


,  W.  C.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2, 
pp.  851-853,  1912. 

2  BASTIN,  E.  S.  :  Quartz  and  Feldspar.  U.  S.  Geol.  Survey  Mineral 
Resources,  1907,  part  2,  pp.  843-872,  1908;  Idem,  1910,  part  2,  pp.  963-975, 
1911. 

MIDDLE-TON,  JEFFERSON:  Idem,  1911,  part  2,  pp.  1027-1030,  1912. 


552      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

readily  available.  The  washing  by  water  and  blowing  about  by 
winds  are  generally  processes  of  purification,  and  some  sands  are 
nearly  pure  quartz,  with  about  99  per  cent,  silica  and  only  1  per 
cent,  or  less  total  alumina,  lime,  magnesium,  and  iron.  Such 
pure  sands  are  used  for  making  glass.  Sandstones  and  pure 
quart zites  are  likewise  used.  If  more  than  one  per  cent,  of  iron 
is  present  it  may  give  the  glass  an  undesirable  color;  magnesium 
renders  the  melt  more  difficultly  fusible.  Commonly  some  of  the 
impurities  are  removed  from  sand  by  washing.  Glass  sands  are 
widely  distributed  both  geologically  and  geographically.1 

GEMS  AND  PRECIOUS  STONES 

Gems  are  minerals  used  for  ornaments.  To  be  valuable,  they 
should  be  rare  and  hard  enough  to  be  durable;  they  should  also 
appeal  to  the  taste  for  the  beautiful.  Diamond,  emerald,  ruby, 
and  sapphire  are  the  more  valuable  precious  stones.  The  less 
valuable  or  "semiprecious"  stones  include  garnet,  amethyst, 
topaz,  tourmaline,  spodumene,  beryl,  peridot,  turquoise,  agate, 
quartz,  chrysocolla — in  fact,  crystallized  specimens  of  any  of  the 
harder  minerals  may  be  used  for  ornaments.2  The  gems  and 
precious  stones  produced  in  the  United  States  in  1915  were  valued 
at  $170,431. 

Diamond  is  crystallized  carbon.  Its  largest  use  is  as  a  gem, 
but  its  hardness  is  10  on  the  Mohs  scale  and  considerable 
quantities  are  used  as  an  abrasive,  particularly  as  diamond  dust 
and  for  bits  of  core  drills.  For  core  drills  the  black  diamond, 
carbonado,  is  in  great  demand.  It  has  no  prominent  cleavage 
and  is  therefore  more  durable.  Black  diamonds  sell  for  $50 
to  $100  a  carat.  Impure  white  diamonds  used  for  core  drills 
are  much  cheaper.  They  sell  at  $10  a  carat  or  more,  the  price 

IBUBCHARD,  E.  F.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2, 
p.  593,  1912.— Glass  Sand  of  the  Middle  Mississippi  Basin.  U.  S.  Geol. 
Survey  Bull.  285,  pp.  459-472,  1906.  Glass-Sand  Industry  of  Indiana, 
Kentucky,  and  Ohio.  U.  S.  Geol.  Survey  Bull.  315,  pp.  361-376,  1907. 
Notes  on  Various  Glass  Sands,  Mainly  Undeveloped.  Idem,  pp.  377-382. 

STOSE,  G.  W.:  The  Glass-Sand  Industry  in  Eastern  West  Virginia.  U.  S. 
Geol.  Survey  Bull.  285,  pp.  473-475,  1906. 

2  For  data  relating  to  the  gems  produced  in  the  United  States  see  KXTNZ, 
G.  F.:  Gems  and  Precious  Stones  of  North  America,  1892,  and  several 
papers  by  D.  B.  STERRETT  in  U.  S.  Geol.  Survey  Mineral  Resources,  espe- 
cially the  volumes  for  1909,  part  2,  pp.  739-808,  1910,  and  for  1911, 
part  2,  pp.  1037-1078,  1912. 


DEPOSITS  OF  THE  NONMETALS 


553 


depending  on  grade  and  durability.  Uncut  gems  are  valued  at 
$7  to  $12  a  carat  or  more,  according  to  size,  color,  and  appearance. 
White  diamonds  are  most  common.  Those  with  blue  tints  are 
highly  prized.  Yellow  diamonds  are  generally  lower  priced 
than  white  ones.  The  United  States  imports  diamonds  from 
Africa  and  from  Brazil.  The  domestic  production  is  small. 

Diamonds  are  mined  from  plugs  of  basic  igneous  rocks  (perido- 
tite)  in  the  Kimberly  field,  Cape  Colony;  at  the  Premier  mine, 
near  Pretoria,  in  the  Transvaal;  and  in  Pike  County,  Arkansas. 
They  are  found  in  placers  in  Brazil,  India,  and  at  many  other 
places. 


BLACK  SHALE- 


MELAPHYRE- 


QUARTZITE-      £ 


SLATE 


FIG.  206.  —  Cross-section  of  Kimberly  peridotite  plug.     (After  Williams.') 


The  Kimberly  peridotite  plug  is  shown  in  Fig.  206.  The 
diameter  decreases  in  depth  and  has  been  followed  downward 
over  2,000  feet.  The  peridotite  ("kimberlite")  is  extensively 
serpentinized.  At  the  surface  it  is  "yellow;"  in  depth,  "blue 
ground."  It  disintegrates  after  it  has  been  mined  and  exposed 
to  the  elements  for  a  few  months.  The  material  is  passed  over 
greased  tables  to  which  the  diamonds  adhere.  About  1  cara^ 
to  the  ton  is  recovered.  The  peridotite  plug  is  intruded  into  a 
series  of  beds  among  which  are  carbonaceous  shales.  Dunn  re- 
garded the  carbon  of  the  shales  as  the  source  of  the  diamonds, 
which,  according  to  his  view,  were  formed  by  the  recrystalliza- 
tion  of  carbon  from  the  shales  dissolved  by  the  intruding  magma.1 
More  recent  work  has  shown  that  the  diamonds  are  original  rock- 
making  minerals.  Many  of  the  crystals  are  broken,2  showing 
that  they  had  formed  (as  phenocrysts)  before  the  magma  came 
to  rest. 

1  DUNN,  E.  J.  :  Notes  of  the  Diamond  Fields  of  South  Africa.  London 
Geol.  Soc.  Quart.  Jour.,  vol.  37,  pp.  609-612,  1881. 

"WILLIAMS,  G.  F.:  "Diamond  Mines  of  South  Africa,"  pp.  490,  510, 
New  York,  1902. 


554      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

The  deposit  at  the  Premier  mine,  Transvaal,1  is  a  pipe  of 
serpentinized  peridotite  intruding  sedimentary  rocks.  In  Pike 
County,  Arkansas,2  diamonds  have  been  formed  in  several  small 
peridotite  bodies  that  break  through  the  Trinity  (Lower  Cre- 
taceous) formation,  which  consists  of  clay,  sand,  gravel,  and 
limestone.  The  production  of  Arkansas  diamonds  to  the 
end  of  1913  was  550  carats.  The  diamonds  of  Minas  Geraes, 
Brazil,  are  obtained  from  placers.  They  are  associated  with 
fragments  of  quartzites  and  schists.  Their  original  source  is 
uncertain.3 

A  discussion  with  bibliography  on  occurrences  and  syntheses 
of  diamonds  is  given  by  Clarke.4 

Emerald  (Be3Al2(Si03)6)  is  a  clear  green  variety  of  beryl, 
highly  prized  as  a  gem.  It  is  obtained  from  pegmatite  veins5 
and  from  placers.  Some  emeralds  equal  or  exceed  diamonds  in 
value.  Emeralds  are  imported  from  India,  Ceylon,  Siberia,  and 
Brazil.  In  the  United  States  they  are  found  in  pegmatites  at 
several  places  in  New  England  and  in  North  Carolina.  Beryls 
of  colors  other  than  green  are  also  used  for  gems. 

Corundum  Ruby  (red),  sapphire  (blue),  oriental  emerald 
(green),  oriental  topaz  (yellow),  and  oriental  amethyst  (purple) 
are  all  varieties  of  gem  corundum  (A1203).  The  colors  are  prob- 
ably due  to  small  amounts  of  various  metallic  oxides.  All  are 
found  in  placers,  where  they  have  probably  formed  from  the 
waste  of  igneous  rocks  and  pegmatite  dikes.  Most  of  these 
varieties  come  from  the  Orient  or  from  South  America.  Beau- 
tiful blue  sapphires  are  mined  from  a  decomposed  dike  of 

1  PENROSE,  R.  A.  F.,  JR.  :  The  Premier  Diamond  Mine,  Transvaal,  South 
Africa.     Econ.  Geol.,  vol.  2,  pp.  275,  1907. 

2  KUNZ,  G.  F.,  and  WASHINGTON,  H.  S.:  Diamonds  in  Arkansas.     Am. 
Inst.  Min.  Eng.  Trans.,  vol.  39,  pp.  169-176,  1908. 

MISER,  H.  D. :  New  Areas  of  Diamond-Bearing  Peridotite  in  Arkansas. 
U.  S.  Geol.  Survey  Bull.  540,  p.  534,  1914. 

3  DERBY,  O.  A. :  Brazilian  Evidences  on  the  Origin  of  the  Diamond.     Jour. 
Geol.,  vol.  6,  pp.  121-146,  1898. 

BRANNER,  J.  C.:  The  Minerals  Associated  with  Diamonds  and  Carbon- 
ates in  the  State  of  Bahia,  Brazil.  Am.  Jour.  Sri.,  4th  ser.,  vol.  31,  pp. 
480-490,  1911. 

4  CLARKE,  F.  W.:  The  Data  of  Geochemistry,  3d  ed.     U.  S.  Geol.  Survey 
Butt.  616,  pp.  322-328,  1916. 

5  STERRETT,  D.  B.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2,  pp. 
1051-105.8,  1912. 


DEPOSITS  OF  THE  NONMETALS  555 

lamprophyre  at  Yogo  Gulch,  Fergus  County,  Montana.1  In 
this  dike  the  sapphire  is  a  rock-making  mineral.  Near  Phil- 
ipsburg,  Mont.,  sapphires  are  extensively  mined  from  placers. 
In  the  original  state  they  are  apparently  phenocrysts  in  a  sur- 
face lava.2  Though  some  of  the  material  is  of  gem  quality,  most 
of  it  is  used  for  watch  jewels  and  other  bearings. 

Spinel  (MgO.Al203)  when  red  is  called  "spinel  ruby."  There 
are  also  other  shades.  Spinel  is  found  in  peridotite  and  in 
placers  which  are  in  part  derived  from  basic  rocks.  Its  hardness 
is  8. 

Tourmaline  is  a  complex  hydrous  borosilicate  which  may  con- 
tain also  magnesium  and  iron.  It  has  a  great  variety  of  colors. 
The  dark  brown  and  black  tourmalines,  which  contain  iron,  are 
little  used  as  gem  material.  Gem  tourmalines  are  green,  white, 
red,  yellow,  etc.  Gems  are  found  at  Paris,  Maine,3  and  at  a  few 
other  places  in  New  England,  and  pink  tourmaline  is  obtained 
from  San  Diego  County,  California.  Gem  tourmalines  are  found 
in  pegmatite  dikes,  where  they  commonly  occur  as  well- 
defined  prismatic  crystals  lining  small  cavities.  The  hardness 
of  tourmaline  ranges  from  7  to  7.5. 

Kunzite  (lilac-colored  spodumene)  occurs  with  rubellite  in 
pegmatites  in  southern  California.  It  is  a  gem  of  great  beauty 
but  is  not  yet  widely  used. 

Turquoise  (hydrated  copper-aluminum  phosphate;  hardness  6) 
forms  small  veinlets  in  rhyolite,  granite,  and  other  igneous  rocks. 
It  is  generally  associated  with  kaolin  and  other  secondary 
minerals  and  is  probably  a  product  of  surface  decomposition. 
It  commonly  fills  small  fractures  in  kaolinized  igneous  rock. 
Turquoise  is  obtained  in  several  localities  in  Arizona4  and  other 
States  in  the  arid  Southwest. 


,  W.  H.:  Geology  of  the  Little  Belt  Mountains,  Montana.  U.S. 
Geol.  Survey  Twentieth  Ann.  Rept.,  part  3,  pp.  454-460,  1900. 

PIRSSON,  L.  V.:  On  the  Corundum-Bearing  Rocks  from  Yogo  Gulch, 
Montana.  Am.  Jour.  Sci.,  4th  ser.,  vol.  4,  1897,  p.  421. 

STERRETT,  D.  B.:  U.  S.  Geol.  Survey  Mineral  Resources,  1907,  part  2, 
p.  291,  1908. 

2  1  have  had  an  opportunity  to  study  this  material  through  the  courtesy 
of  Mr.  O.  J.  Berry,  of  Philipsburg.—  W.  H.  E. 

3BASTiN,  E.  S.:  Geology  of  the  Pegmatites  and  Associated  Rocks  of 
Maine.  U.  S.  Geol.  Survey  Bull.  445,  pp.  79-93,  1911. 

4  PAIGE,  SIDNEY:  The  Origin  of  Turquoise  in  the  Burro  Mountains,  New 
Mexico.  Econ.  Geol,  vol.  7,  pp.  382-392,  1912, 


556      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Variscite  (A1PO4  +  2H20)  is  a  green  mineral  found  at  a  few 
places  in  Utah1  and  Nevada.  A  mixture  of  variscite  matrix 
with  chalcedony  and  other  minerals  is  called  "amatrice."  Varis- 
cite is  probably  formed  by  processes  of  superficial  alteration. 

Peridot,  a  clear  yellow  or  greenish-yellow  olivine,  is  found 
in  the  Apache  and  Navajo  reservations,  Arizona.2  It  is  asso- 
ciated with  basic  igneous  rocks,  from  which  it  has  separated 
by  weathering. 

Garnets  (pyrope,  almandine,  and  spessartite) ,  are  prized  as 
gems.  India  is  the  principal  source.  The  red  garnet  rhodolite 
of  Macon  County,  North  Carolina,  is  a  stone  of  remarkable 
beauty.  Many  red  garnets  of  fine  color  but  of  rather  small 
size  are  found  in  the  Navajo  Indian  Reservation,  Arizona. 

GRAPHITE 

Graphite  occurs  in  veins,  in  pegmatites,  in  igneous  rocks,  and 
in  metamorphosed  rocks.  The  most  productive  deposits  in  the 
United  States  are  in  the  region  of  Crown  Point  and  Ticonderoga, 
N.  Y.,3  where  there  are  quartz-graphite  schists  that  contain  from 
3  to  10  per  cent,  of  graphite.  The  principal  deposits,  according 
to  Bastin,  are  metamorphosed  sedimentary  rocks  that  con- 
tained organic  matter.  Minerals  associated  with  the  graphite 
include  mica,  feldspar,  pyrite,  and  zoisite. 

Near  La  Colorada,  Sonora,  Mexico,4  Triassic  sandstones  in- 
closing coal  beds  have  been  metamorphosed  by  intruding  granite 
and  the  coal  beds  are  changed  to  graphite.  One  bed  10  feet  or  more 
thick  supplies  high-grade  amorphous  graphite  that  is  greatly 
prized  for  making  lead  pencils.  Graphite  deposits  are  found 
also  in  Wisconsin,  in  the  "Upper  Peninsula"  of  Michigan,  in 
the  southern  Appalachian  region,  and  at  many  other  places. 

The  most  productive  graphite  deposits  of  the  world  are  those 

1  PEPPERBERG,  L.  J. :  Variscite  near  Lucin,  Utah.  Min.  and  Sci.  Press, 
Aug.  11,  1911,  p.  233. 

ZSTERRETT,  D.  B.:  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part  2, 
pp.  832-835,  1909. 

3  BASTIN,  E.  S.:  Origin  of  Certain  Adirondack  Graphite  Deposits.     Eccn. 
Geol,  vol.  5,  pp.  134HL57,  1910.— Graphite.     U.  S.  Geol.  Survey  Mineral 
Resources,  1908,  part  2,  pp.  717-738,  1909  (contains  bibliography). 

4  HESS,  F.  L. :  Graphite  Mining  near  La  Colorada,  Sonora,  Mexico.     Eng. 
Mag.,  vol.  38,  p.  36,  1909. 

BASTIN,  E.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part  2,  p.  734, 
1909. 


DEPOSITS  OF  THE  NONMETALS  557 

of  Ceylon,1  where  the  graphite  occurs  in  small  veins  in  gneisses 
that  are  intruded  by  granite  and  pegmatite.  Associated  minerals 
are  quart?,  feldspar,  mica,  garnet,  and  pyroxene.  There  are 
many  other  occurrences  of  graphite  associated  with  intruding 
rocks. 

Graphite  is  used  for  making  paints,  crucibles,  lubricants,  lead 
pencils,  polishing  powder,  etc.  Amorphous  graphite  is  preferred 
for  lead  pencils,  and  crystalline  graphite  for  crucibles.  Some 
of  the  graphitic  rock  mined  carries  only  a  small  percentage  of 
graphite  and  is  concentrated  mechanically.  Material  carrying 
as  low  as  30  per  cent,  graphite  is  ground  and  used  for  paint. 
Graphite  is  used  also  as  an  adulterant  for  fertilizer,  to  which  it 
gives  the  black  color  which  is  popularly  associated  with  fertility 
of  soil.  As  it  is  insoluble,  it  can  not  add  to  fertility,  and  its  use 
as  fertilizer  is  to  be  discouraged.  The  United  States  produced  in 
1915  about  4,718  short  tons  of  graphite,  valued  at  $429,631. 
Most  of  the  importations  came  from  Ceylon  and  from  Sonora, 
Mexico.  A  large  amount  of  graphite  is  manufactured  from  coal 
at  Niagara  Falls. 

A  bibliography  of  graphite  with  brief  abstracts  of  the  more 
important  papers  is  given  by  Bastin.2 

BARITE 

Barite  (BaSCX)  is  a  common  gangue  mineral  in  ore  veins,  espe- 
cially in  those  formed  at  intermediate  and  shallow  depths.  In 
deposits  of  the  deep  zone,  in  contact-metamorphic  deposits,  and 
in  pegmatites  barite  is  rarely  present.  Igneous  rocks  commonly 
contain  the  barium  silicate  molecule  in  feldspars.  The  sulphate 
is  present  in  appreciable  amounts  in  many  limestones.  Barite 
is  almost  insoluble,  and  in  the  weathering  of  limestone  it  collects 
with  clay  and  iron  oxide  in  the  mantle  rock.  Nearly  all  the  barite 
produced  in  the  United  States  is  associated  with  weathered  lime- 
stones. It  could  doubtless  be  recovered  profitably  from  many 
lode  deposits  of  the  West  if  markets  or  transportation  facilities 
were  more  favorable. 

The  production  of  barite  in  the  United  States  in  1915  was 

1  BASTIN,  E.  S. :  The  Graphite  Deposits  of  Ceylon,  a  Review  of  Present 
Knowledge,  with  a  Description  of  a  Similar  Graphite  Deposit  near  Dillon, 
Mont.     Econ.  Geol,  vol.  7,  pp.  419-443,  1912. 

2  BASTIN,  E.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1912,  part  2,  pp. 
1061-1069,  1913. 


558      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

108,547  short  tons,  valued  at  $381,032.  The  price,  less  than 
$4  a  ton,  is  for  the  natural  or  hand-picked  material  at  points  of 
production.  Ground  and  refined  barite  is  worth  about  $13  a  ton 
or  more.1 

Barite  is  used  for  making  paints,  especially  in  the  manufacture 
of  lithopone,  a  white  paint  consisting  of  barium  sulphate  and 
zinc  sulphide  made  by  a  complex  process  involving  the  reduction 
of  barite  at  high  temperature.  Barite  is  used  also  for  treating 
rubber,  for  filling  paper,  and  for  tanning.  Its  great  weight  makes 
it  desirable  from  the  packer's  view-point  for  painting  ham  sacks 
and  cheese. 

The  most  productive  barite  deposits  in  the  United  States  are 
in  Washington  County,  Missouri,  in  the  region  of  the  dissemi- 
nated lead  deposits  (page  491).  There  Ordovician  limestone  is 
capped  by  residual  clay.  In  the  lower  part  of  the  clay  are 
numerous  fragments  of  barite,  limestone  and  chert.  Small  veins 
and  disseminated  deposits  of  barite  are  found  in  the  limestone.2 
The  region  has  suffered  great  erosion,  and  the  relations  indicate 
that  the  clay  and  barite  are  residual. 

In  Virginia  barite  deposits  are  found  as  irregular  pockets  re- 
placing pre-Cambrian  limestone  and  as  fissure  fillings  in  schists, 
Paleozoic  limestones,  and  Triassic  limestones  and  shales.  The 
deposits  are  concentrated  by  weathering  and  some  lie  in 
residual  clay  with  iron  oxide.3  Small  barite  veins  are  found  also 
in  many  other  Eastern  States. 

CELESTITE 

Celestite  (SrSO-O  is  found  in  crevices  in  limestones  and  in 
beds  of  chemical  sediments,  where  it  is  generally  associated 
with  gypsum  and  sulphur.  Rarely  it  is  a  gangue  mineral  of 
metalliferous  ores.  In  Texas  it  is  found  in  cavities  of  Cretaceous 
limestone. 

1  PHALEN,  W.  C.:  U.  S.  Geol.  Survey  Mineral  Resources,  part  2,  pp.  965- 
970,  1912  (contains  bibliography). 

2  BUCKLEY,  E.  R.:  Geology  of  the  Disseminated  Lead  Deposits  of  St. 
Francois  and  Washington  Counties,   Mo.  Bur.  Geol.  and  Mines,  vol.  9, 
part  1,  pp.  23^-248,  1909. 

STEEL,  A.  A.:  Geology,  Mining,  and  Preparation  of  Barite.  Am.  Inst. 
Min.  Eng.  Trans.,  vol.  40,  pp.  711-743,  1910. 

3  WATSON,  T.  L. :  Geology  of  Virginia  Barite  Deposits.     Am.  Inst.  Min. 
Eng.  Trans.,  vol.  38,  pp.  710-733,  1907. 


DEPOSITS  OF  THE-  NONMETALS  559 

A  noteworthy  occurrence  is  in  the  Avawatz  Mountains,  San 
Bernardino  County,  California,1  where  celestite  beds  associated 
with  salt  and  gypsum  are  included  in  a  series  of  steeply  tilted 
sediments.  Locally  the  celestite  zone  is  75  feet  thick.  It  lies 
below  a  salt  bed.  Celestite  associated  with  gypsum  is  found 
about  15  miles  north  of  Gila  Bend,  Ariz.,2  in  a  series  of  tilted 
sedimentary  beds  and  lava  flows. 

Celestite  is  used  for  making  salts  that  are  used  in  refining  beet 
sugar,  for  making  fireworks,  and  in  medicines.  The  production 
is  small.3 

WITHERITE 

Witherite  (BaC03)  occurs  in  veins  and  is  found  as  a  gangue 
mineral  of  a  few  metalliferous  veins.  It  is  abundant  at  Fallow- 
field,  near  Hexham,  Northumberland,  England,  where  it  occurs 
with  barite  in  fissures  that  cut  Carboniferous  rocks.  It  is  found 
in  the  Rabbit  Mountains,  near  Thunder  Bay,  Lake  Superior. 
Barium  salts  are  used  for  making  glass,  for  making  fireworks, 
and  in  the  refining  of  beet  sugar.  They  are  not  mined  in  the 
United  States. 

FLUORITE  AND  CRYOLITE 

Fluorite,  or  fluorspar  (CaF2),  is  a  common  mineral  in  vein 
deposits.4  It  is  found  in  many  districts  as  a  gangue  mineral  of 
gold,  silver,  and  zinc  veins,  but  in  these  deposits  in  the  United 
States  it  is  not  exploited.  The  chief  source  of  fluorite  is  southern 
Illinois,8  where  lower  Carboniferous  beds  are  extensively  faulted 
and  intruded  by  lamprophyre  dikes.  The  veins  dip  steeply 
and  cut  across  beds  of  limestone.  Some  of  them  are  nearly  40 
feet  wide;  some  solid  masses  are  10  feet  thick.  Some  galena, 
sphalerite,  pyrite,  and  chalcopyrite  are  associated  with  the 
fluorite.  The  gangue  minerals  are  quartz  and  calcite,  and  in 


,  W.  C.:  Celestite  Deposits  in  California  and  Arizona.     U.  S. 
Geol.  Survey  Bull.  540,  pp.  521-533,  1912. 
2PHALEN,  W.  C.:  Op.  tit.,  p.  351. 

3  PRATT,  J.  H.  :  Strontium  Ores.     U.  S.  Geol.  Survey  Mineral  Resources, 
1901,  part  2,  pp.  955-958,  1902. 

4  BURCHARD,  E.  F.  :  Fluorspar  and  Cryolite.     U.  S.  Geol.  Survey  Mineral 
Resources,  1907,  part  2,  pp.  607-620,  1908  (gives  bibliography). 

6  BAIN,  H.  F.  :  Fluorite  Deposits  of  Southern  Illinois.     U.  S.  Geol.  Sur- 
vey Bull.  255,  1905. 


560      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

some  of  the  veins  barite  is  present.  The  fluorite  does  not  dis- 
solve very  readily  from  the  outcrops  and  at  some  places  is  re- 
covered by  placer  mining.  At  the  Riley  mine,  Crittenden  County, 
Kentucky,1  fluorite  ore  fills  a  fault  fissure  between  Carboniferous 
limestone  and  quartzite.  Fluorite  is  found  also  in  Boulder 
County2  and  at  Wagon  Wheel  Gap,3  Colorado;  at  the  latter 
place  it  occurs  along  a  fissure  from  which  hot  springs  now  issue. 

The  principal  use  of  fluorite  is  for  flux.  About  80  per  cent, 
of  the  American  output,  according  to  Burchard,  is  used  in  the 
steel  trade  in  the  manufacture  of  open -hearth  steel.  Fluorite 
is  used  also  in  making  aluminum,  in  the  electrolytic  refining  of 
antimony  and  lead,  and  for  manufacturing  glass,  enamel,  and 
hydrofluoric  acid.  The  production  of  fluorite  in  the  United 
States  in  1915  was  136,941  tons,  valued  at  $764,475.  It  is  in 
good  demand. 

Cryolite  (NasAlFe)  is  a  comparatively  rare  mineral.  The 
principal  deposits  are  at  Ivigtut,  in  southern  Greenland,  where  a 
wide  vein  in  granite4  carries,  with  much  cryolite,  a  little  siderite, 
quartz,  pyrite,  chalcopyrite,  and  sphalerite.  Peripheral  portions 
of  the  vein  carry  also  feldspar,  cassiterite,  fluorite,  and  other 
minerals.  A  cryolite-quartz  vein  in  -granite  occurs  south  of 
Pikes  Peak,  Colorado.5  Cryolite  imported  from  Greenland  is 
used  in  making  sodium  salts,  aluminum,  glass,  and  enamel  ware. 

MINERAL  PAINTS 

Many  mineral  substances  are  used  for  pigments.  The  cheaper 
grades  are  generally  fine  clayey  material  highly  colored  with 
iron,  manganese,  or  other  minerals.  In  general  they  are  residual 
products  of  the  weathering  of  rocks  containing  these  metals. 
Some  earthy  iron  ore  containing  manganese  is  liver  brown  before 
and  reddish  brown  after  burning.  Such  material  before  burning 
is  called  "umber,"  and  after  burning  "burnt  umber."  Sienna  is 

1  FOHS,  JULIUS:  Fluorspar  Deposits  of  Kentucky.  Ky.  Geol.  Survey  Bull. 
9,  1907. 

MILLER,  A.  M. :  The  Lead  and  Zinc-Bearing  Rocks  of  Central  Kentucky. 
Ky.  Geol.  Survey  Bull.  2,  p.  25,  1905. 

1  BURCHARD,  E.  F.:  Op.  cit.,  pp.  612-617. 

3  EMMONS,  W.  H.,  and  LARSEN,  E.  S. :  The  Hot  Springs  and  Mineral 
Deposits  of  Wagon  Wheel  Gap,  Colorado.  Econ.  Geol,  vol.  8,  p.  242,  1913. 

*  QUALE,  PAUL:  An  Account  of  the  Cryolite  of  Greenland.  Smithsonian 
Inst.  Rept.,  1866,  p.  398. 

8  DANA,  J.  D.:  "System  of  Mineralogy,"  6th  ed.,  p.  167,  1892. 


DEPOSITS  OF  THE  NONMETALS  561 

a  similar  material,  but  of  lighter  color,  probably  due  to  hydrated 
iron  oxide,  limonite. 

Ocher  is  the  yellow  iron  oxide,  limonite,  incorporated  in  a  clay 
base.  Cinder  from  acid  making  (iron  oxide),  roasted  siderite, 
and  raw  hematite  are  ground  and  used  to  make  red  paint.  Black 
clay,  shale,  and  slate  are  ground  and  used  as  fillers  and  as  pig- 
ments. Some  slates  are  distilled  and  both  the  residue  and  the 
oil  recovered  are  used  in  making  paint.  Calcium  carbonate — 
limestone,  calcite,  and  shells — is  ground  and  used  as  "whiting." 
Slaked  lime  is  used  extensively  for  whitewash.  Iron  oxide, 
ground  shales,  and  culm  from  coal  washeries  are  used  for  mortar 
colors.  Graphite,  gypsum,  asbestos,  barite,  asphalt,  and  mag- 
nesite,  discussed  elsewhere,  are  all  used  as  pigments.  High-grade 
paints  are  made  from  compounds  of  lead,  zinc,  mercury,  cobalt, 
arsenic,  etc.  The  United  States  produced  57,442  short  tons  of 
natural  pigments  in  1915,  valued  at  $551,598.  More  detailed 
information  is  given  in  reports  of  the  United  States  Geological 
Survey.1 

SALT 

General  Occurrence. — When  sea  water  is  evaporated,  its 
mineral  salts  are  precipitated — the  least  soluble  first.  The  order 
of  precipitation  depends  not  only  on  the  solubility  of  the  salts 
in  pure  water,  but  also  upon  concentration,  temperature,  pres- 
sure, and  other  salts  dissolved  in  the  water.  If  certain  minor 
constituents  such  as  iron,  manganese,  and  phosphorus  are 
omitted  the  composition  of  the  ocean  may  be  stated  as  follows:2 

Composition  of  oceanic  salts  Composition  of  ocean 


NaCl  

77.76 

O  

85.79 

MgCl2  

10.88 

H  

10.67 

MgS04  

4.74 

Cl  

2.07 

CaSO4  

3.60 

Na  

1.14 

K2S04  

2.46 

Mg  

0.14 

MgBr2  

0.22 

Ca...  

.'  0.05 

CaC03  

0.34 

K  

0.04 

a 

0  09 

100.00 

Br  

0.008 

C  

0.002 

100.000 

^ee  particularly  PHALEN,  W.  C.:  Mineral  Paints.  U.  S.  Geol.  Survey 
Mineral  Resources,  1911,  part  2,  pp.  970-993,  1912  (contains  an  extensive 
bibliography). 

2  CLARKE,  F.  W. :  The  Data  of  Geochemistry,  3d  ed.  U.  S.  Geol.  Survey 
Butt.  616,  p.  23,  1916. 

36 


562      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

If  sea.  water  is  evaporated  to  dryness,  the  salts  will  separate 
approximately  in  the  following  order:  Calcium  carbonate  with 
a  little  iron  oxide,  calcium  sulphate,  sodium  chloride,  magnesium 
sulphate  and  magnesium  chloride,  sodium  bromide,  and  potas- 
sium chloride  and  sulphate.  At  some  stages,1  two  or  more  of 
the  salts  are  precipitated  simultaneously.  If  an  arm  of  the 
sea  is  shut  off  from  the  main  body,  its  water  evaporated,  and 
the  resulting  salts  covered  by  clay  or  some  other  impermeable 
bed,  the  saline  deposit  will  be  preserved.  Such  is  probably  the 
origin  of  the  famous  deposits  of  Stassfurt,  Germany.2  There 
the  beds  from  the  bottom  up  are  (1)  anhydrite  and  gypsum, 
(2)  rock  salt  and  anhydrite,  (3)  polyhalite  (hydrated  lime- 
magnesium,  potassium-sulphate),  (4)  kieserite  (MgSO4.H2O),  (5) 
carnallite  (KMgCl3.6H20),  locally  overlain  by  sylvite  (KC1). 
Above  the  potash  salts  is  a  bed  of  clay,  and  above  that  more 
anhydrite  and  salt,  indicating  probably  a  later  precipitation 
from  the  sea  water.  The  salt  deposits  are  covered  by  thick  beds 
of  sandstone  and  are  worked  through  deep  shafts.  They  were 
first  worked  for  salt  but  have  more  recently  furnished  the 
world's  chief  supply  of  potash  minerals  that  are  extensively  used 
for  fertilizer. 

Origin  of  Thick  Salt  Beds. — Some  salt  beds  are  1,000  feet  or 
more  thick.  The  volume  of  the  salts  precipitated  from  sea  water 
is  less  than  2  per  cent,  of  the  water.  For  1,000  feet  of  salt  to  be 
deposited  by  precipitation  over  the  floor  of  a  basin  it  is  neces- 
sary to  assume  a  volume  of  salt  water  having  the  concentration 
of  the  ocean  equal  to  a  depth  of  50,000  feet  over  the  basin.  So 
deep  a  basin  appears  improbable.  Even  if  we  assume  a  shrink- 
ing lake  or  inland  sea,  which  would  of  course  result  in  concen- 
trating the  salts  over  a  smaller  area,  the  hypothesis  still  appears 
improbable,  for  the  basin  required  would  be  much  deeper  than 
any  existing  today. 

To  account  for  the  great  thicknesses  of  some  salt  beds,  Och- 
senius  proposed  the  "bar"  hypothesis,  in  which  it  is  supposed 
that  sea  water  passes  over  a  bar  into  a  large  basin  near  shore, 
and  that  evaporation  is  balanced  by  a  flow  of  water  from  the 
sea  over  the  bar.  When  evaporation  reaches  a  certain  stage 
precipitation  begins.  Concentration  may  go  only  far  enough 

1  USIGLIO,  J.:  Annales  chim.  phys.,  3d  ser.,  vol.  27,  pp.  92,  172,  1849. 
1  For  a  clear  discussion  of  this  subject  with  numerous  references  see 
CLARKE,  F.  W.:  Op.  cit.,  pp.  221-228. 


DEPOSITS  OF  THE  NONMETALS  563 

to  precipitate  gypsum  or  only  far  enough  to  precipitate  gypsum 
and  salt,  and  the  more  soluble  salts  may  be  carried  by  reversed 
movement  of  water  back  to  the  sea.  If  a  bar  finally  rises  and  shuts 
in  the  bittern  or  magnesium  and  potassium  solutions  from  which 
salt  and  gypsum  have  been  precipitated,  then  the  sulphates  and 
chlorides  of  magnesium  and  potassium  will  be  precipitated  above 
the  sodium  chloride.  This  hypothesis  of  course  demands  a  nice 
adjustment  of  conditions,  which  is  probably  rarely  met;  it  is 
noteworthy,  however,  that  the  potash  salts  are  not  associated 
with  all  deposits  of  salt  and  gypsum.  In  fact,  thus  far  they 
have  not  been  developed  in  great  abundance  except  in  the 
Stassfurt  region. 

Salt  Deposits  of  the  United  States.— In  the  United  States  salt 
is  obtained  from  beds  of  Paleozoic  and  later  age  and  from  basins 
where  evaporation  is  now  going  on.  At  most  places  it  is  ob- 
tained by  the  evaporation  of  brines  pumped  from  the  saline 
rocks.  The  production  of  salt  in  1915  was  38,231,496  barrels, 
valued  at  $11,747,686.  In  New  York  salt  is  obtained  by  pump- 
ing brines  from  the  Salina  (Silurian)  beds,  where,  with  gypsum, 
it  occurs  as  lenses  in  shales.1 

In  Michigan  brines  are  obtained  from  Mississippian  sand- 
stones in  Saginaw  Valley.  Water  is  pumped  into  the  sands  and 
pumped  out  with  dissolved  salt.  The  brines  from  the  upper 
beds  of  the  Marshall  formation,  are  rich  in  bromine  and  supply 
most  of  the  bromine  output  of  the  United  States.  Much  of  the 
salt  is  utilized  in  manufacturing  sodium  carbonate.2 

In  Ohio3  salt  is  obtained  from  brines  from  the  Mississippian 
and  also  from  the  Salina  beds.  In  Kansas  salt  is  mined  from 
Permian  rOcks  by  means  of  shafts  and  obtained  from  salt 
springs.  In  the  Western  States  salt  is  obtained  from  undrained 
basins  where  it  is  forming  today.  At  Saline  Valley,  Inyo 
County,  California,4  a  deposit  of  salt  occupies  about  a  square 
mile  in  a  deep  basin.  This  pure-white  salt  (98.52  per  cent.  NaCl) 
requires  no  refining.  In  the  center  of  the  deposit  there  is  a 
small  pool  of  salt  water. 

1  MERRILL,  F.  J.  H.:  N.  Y.  State  Mus.  Bull.  11,  1893. 

2  COOK,  C.  W.:  The  Brine  and  Salt  Deposits  of  Michigan.     Mich.  Geol. 
and  Biol.  Survey,  Geol.  ser.  12,  pp.  1-188,  1914. 

3  BOWNOCKER,  J.  A.:  Salt  Deposits  and  the  Salt  Industry  in  Ohio.     Ohio 
Geol.  Survey,  4th  ser.,  vol.  8,  pp.  1-42,  1906. 

4  GALE,   H.  S. :  Salt,  Borax  and  Potash  in  Saline  Valley,  Inyo  County, 
California.     U.  S.  Geol.  Survey  Bull  540,  pp.  416-422,  1914. 


564      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

On  the  Gulf  coast  in  Louisiana  and  Texas  there  are  some  of 
the  most  remarkable  salt  deposits  in  the  world.  Shafts  and  bore 
holes  sunk  in  low  knobs  or  domes,  some  of  which  project  as  low 
islands  above  marshes,  encounter  bodies  of  salt  2,000  feet  thick 
or  more.  The  associated  rocks  are  Tertiary  and  Quaternary 
sands,  limestones,  and  clays.  The  beds  have  quaquaversal  dips 
away  from  the  salt  deposits,  which  structurally  resemble  lacco- 
lithic  intrusions.  Some  have  thought  that  the  salt  deposits  were 
formed  by  precipitation  along  fissures  by  ascending  hot  solu- 
tions, and  Harris1  believes  that  the  force  of  crystallization  has 
bowed  up  the  rocks  into  the  characteristic  domes.  -That  such  a 
force  should  be  so  potent  seems  incredible,  yet  the  "bar"  hy- 
pothesis seems  inadequate  to  account  for  these  unusual  deposits. 

Limestone  beds  are  found  above  the  salt  beds  in  several  of  the 
domes.  At  Spindletop,  Texas,2  and  in  some  other  salt  domes 
of  the  coast  region  petroleum  is  associated  with  the  salt,  and 
sulphur  also  is  found  in  some  of  the  deposits.  It  has  been  sug- 
gested that  the  salt,  gypsum,  and  petroleum  have  been  formed 
together,  and  that  the  petroleum  has  reduced  the  calcium 
sulphate,  forming  free  sulphur  from  it. 

As  Lindgren3  has  noted,  the  plasticity  of  salt  is  great.  It 
would  be  difficult  to  imagine  a  less  "competent"  bed  than  rock 
salt  soaked  with  water.  Pressure  on  the  rocks  that  formed  the 
anticlinal  folds  and  domes  may  possibly  have  caused  also  a 
marked  thickening  of  the  salt  beds  at  crests  of  anticlines.  Thin 
limestones  inclosed  in  sandy  rocks,  when  subjected  to  folding, 
are  generally  thickened  at  anticlines.  Wet  salt,  which  is  much 
more  yielding  than  calcite,  may  respond  to  this  process  under 
stresses  that  are  relatively  slight  and  where  the  rocks  are  under 
comparatively  light  load.  In  the  Stassfurt  field,  where  the  salt 
series  is  obviously  folded,  there  is  a  notable  thickening  of  the 
salt  bed  at  the  Engeln  anticline. 

One  other  remarkable  feature  of  these  domes  and  "islands" 
should  be  mentioned.  In  some  areas  several  of  them  are  in 

1  HARRIS,  G.  D.:  Rock  Salt:  Its  Origin,  Geology,  and  Occurrence.  La. 
Geol.  Survey  Bull.  7,  1908. 

"FENNEMAN,  N.  M.:  Oil  Fields  of  the  Texas-Louisiana  Gulf  Coastal 
Plain.  U.  S.  Geol.  Survey  Bull.  282,  pp.  119-121,  1906. 

HAHN,  F.  F.:  The  Form  of  Salt  Deposits.  Econ.  Geol.,  vol.  7,  pp.  120- 
135,  1912. 

*  LINDGREN,  WALDEMAR:  "Mineral  Deposits,"  p.  289,  New  York,  1913. 


DEPOSITS  OF  THE  NONMETALS  565 

alignment,  as  at  Vermilion  Bay,  La.  The  Avery  salt  dome,  on 
Petite  Anse  Island,  is  in  line  with  four  others.1  To  one  inclined 
to  regard  these  deposits  as  having  been  formed  by  precipitation 
from  ascending  waters  this  alignment  would  suggest  the  presence 
of  a  great  fissure.  Such  an  alignment  might  result,  also,  by  the 
development  of  small  domes  along  an  anticline.2  The  origin 
of  these  deposits  must  be  regarded  as  still  in  doubt.  The  hypo- 
thesis that  attributes  their  deposition  to  ascending  hot  waters 
is  not  entirely  satisfactory,  because  in  thousands  of  ore  veins 
formed  by  ascending  hot  waters  the  presence  of  salt  is  practically 
unknown,  except  in  small  fluid  inclusions  that  are  locally  de- 
veloped in  the  wall  rocks.  The  sedimentary  origin  of  salt  de- 
posits is  indicated  by  the  fact  that  essentially  all  of  them  occur 
in  sedimentary  rocks.  Igneous  and  metamorphic  rocks  are 
barren  of  salt  deposits. 

POTASH  SALTS 

Potash  salts3  are  important  economically  because  they  are 
necessary  for  plant  life.  Potash  is  one  of  the  abundant  constitu- 
ents of  the  earth's  crust;  several  per  cent,  of  it  is  present  in 
granites,  pegmatites,  and  leucite  rocks,  in  which,  however,  it  is 
relatively  insoluble  and  not  readily  available.  Potash  consti- 
tutes a  considerable  percentage  of  alunite,  a  mineral  that  is 
abundant  at  Goldfield,  Nev.,  at  Marysvale,  Utah,  and  at  several 
other  places  in  the  West.  By  calcination  alunite  is  dehydrated 
and  loses  some  SO3,  and  its  potassium  sulphate  is  rendered 
soluble.  Small  amounts  of  potash  are  obtained  by  leaching  wood 
ashes,  and  it  has  been  recovered  also  from  sea  weeds,  some  of 
which  contain  potash  in  notable  quantities. 

1  This  is  shown  by  Hilgard's  map  reproduced  in  MERRILL,  G.  P.:  "Non- 
metallic  Minerals,"  p.  51,  New  York,  1910.     The  original  is  not  accessible 
to  me. 

2  HAYES,   C.   W.,   and   KENNEDY,   WILLIAM:  Oil   Fields  of  the  Texas- 
Louisiana  Gulf  Coastal  Plain.     U.  S.  Geol.  Survey  Bull.  212,  pp.  143-144, 
1903. 

"ScHULTZ,  A.  R.,  and  CROSS,  WHITMAN:  The  Potash-Bearing  Rocks  of 
the  Leucite  Hills,  Wyoming.  U.  S.  Geol.  Survey  Bull.  512,  1912. 

BUTLER,  B.  S.,  and  GALE,  H.  S.:  Alunite.  U.  S.  Geol.  Survey  Butt.  511, 
1912. 

ZIEGLER,  VICTOR:  The  Potash  Deposits  of  the  Sand  Hills  Region  of 
Northwestern  Nebraska.  Colo.  School  of  Mines  Quart.,  vol.  10,  No.  3, 
pp.  6-26,  1915 -  •' 


566      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

Nearly  all  the  potash  used  in  the  United  States  has  come  for 
many  years  from  the  Stassfurt  salt  beds  in  Germany.  In  recent 
years  the  United  States  Geological  Survey  has  been  engaged  in 
a  search  for  potash1  in  some  form  readily  available  for  agricul- 
ture. Many  lakes,  marshes,  and  playa  deposits  of  California 
and  Nevada  have  been  drilled  and  sampled.  At  Searles  Marsh, 
in  San  Bernardino  County,  California,  a  thick  salt  bed  having 
an  area  of  11  square  miles  is  saturated  with  brine  exceptionally 
rich  in  potash.  Potash  was  obtained  on  a  commercial  scale 
from  alkali  lakes  in  Nebraska  in  1916. 

Some  igneous  rocks  carry  a  percentage  of  K2O  approximately 
equal  to  that  of  commercial  potash  salts  used  for  fertilizer. 
The  invention  of  some  smelting  process  by  which  the  potash  of 
silicates  might  cheaply  be  rendered  soluble  would  prove  a  great 
service  to  mankind. 

BROMINE 

Bromine  is  found  as  bromides  in  sea  water  and  in  some  brines. 
In  Michigan,  West  Virginia,  and  Pennsylvania  it  is  obtained 
from  brine  incidentally  to  the  manufacture  of  salt.  The  lower 
Carboniferous  salt  beds  are  locally  high  in  bromine.2 

Bromine  is  used  in  chemistry,  in  photography,  in  medicine, 
and  as  a  disinfectant.  The  United  States  in  1915  produced 
855,857  pounds,  valued  at  $856,307. 

SODIUM  SULPHATE 

Sodium  sulphate  (Na2S04.10H2O)  is  deposited  in  abundance 
in  some  inland  basins  where  mineral  waters  are  evaporated. 
Much  of  this  material  is  obtained  as  a  by-product  in  manufactur- 
ing hydrochloric  acid  and  other  salts,  and  the  natural  product 
sells  at  a  low  price.  Sodium  sulphate  is  precipitated  from  water 
relatively  low  in  calcium  and  magnesium  before  sodium  chloride 
is  formed.  Its  solubility  is  appreciably  affected  by  slight  changes 
in  temperature.  At  Great  Salt  Lake,  Utah,  it  is  precipitated 

1  GALE,  H.  S. :  The  Search  for  Potash  in  the  Desert  Basin  Region.  U.  S. 
Geol.  Survey  Bull.  530,  pp.  295-312,  1913. 

YOUNG,  G.  J. :  Potash  Salts  and  Other  Salines  in  the  Great  Basin  Region. 
U.  S.  Dept.  Agr.  Bull.  61,  pp.  1-96,  1914. 

LANE,  A.  C.:  "Mineral  Industry,"  vol.  16,  p.  123,  1907.  See  also 
volumes  of  U.  S.  Geol.  Survey  Mineral  Resources,  especially  an  article  by 
F.  J.  H.  MERRILL  in  volume  for  1904,  pp.  1029-1030,  1905. 


DEPOSITS  OF  THE  NONMETALS  567 

during  the  winter  and  cast  up  on  the  shore.  In  summer  it  is 
redissolved.  Formerly  the  harvesters  in  winter  removed  the 
salt  from  the  reach  of  the  waves  and  collected  it  at  leisure.1 

Near  Earamie,  Wyo.,  sodium  sulphate  has  been  gathered  from 
small  lakes  that  dry  up  in  the  summer.  Deposits  are  known  at 
many  places  in  the  arid  West.  Sodium  sulphate  is  used  in  the 
manufacture  of  paper  and  glass  and  for  medicine. 

GYPSUM 

Gypsum  (CaSO4.2H20)  occurs  in  beds  and  is  commonly 
associated  with  salt.  Sea  water  contains  a  considerable  propor- 
tion of  calcium  sulphate,  which  on  evaporation  is  deposited 
before  sodium  chloride.  Although  gypsum  is  commonly  formed 
by  the  superficial  alteration  of  sulphide  ores,  the  principal  gypsum 
deposits  are  of  sedimentary  origin.  Many  of  them  are  asso- 
ciated with  "Red  Beds"  and  have  been  formed  by  the  evapora- 
tion of  sea  water  under  arid  conditions  in  inclosed  basins  or  in 
arms  of  the  sea. 

Anhydrite  (CaSO^  is  likewise  precipitated  from  sea  water. 
It  is  not  so  desirable  as  gypsum  for  most  purposes.  By  long  ex- 
posure anhydrite  becomes  hydrated,  forming  gypsum,  and  some 
of  the  workable  gypsum  beds  have  been  formed  in  this  way. 

Extensive  gypsum  beds  occur  in  many  States  of  the  Union. 
They  are  widely  distributed  in  Paleozoic  and  Mesozoic  rocks 
and  are  found  also  in  the  Tertiary.  In  New  York  gypsum  is 
interbedded  with  shales  and  shaly  limestones  in  the  Salina  forma- 
tion. The  beds  worked  are  from  4  to  30  feet  thick.  Some  of 
the  deposits  are  worked  underground,  and  some  are  quarried. 
The  gypsum-bearing  area,  which  is  150  miles  long,  lies  a  few 
miles  south  of  and  parallel  to  the  south  shore  of  Lake  Ontario.2 

In  northern  Ohio  gypsum  is  found  in  the  Monroe  formation 
(Silurian).  In  Michigan3  gypsum  is  mined  from  deposits  inter- 
stratified  with  lower  Carboniferous  shale  and  limestone. 

In  Iowa,  near  Fort  Dodge,4  gypsum,  probably  of  Permian 

1  TALMAGE,  J.  E.:  Science,  new  ser.,  vol.  14,  p.  446,  1889. 

2  NBWLAND,  D.  H.,  and  LEIGHTON,  HENRY:  Gypsum  Deposits  of  New 
York.     N.  Y.  State  Mus.  Bull.  143,  1910. 

8  GRIMSLEY,  G.  P.:  The  Gypsum  of  Michigan  and  the  Plaster  Industry. 
Mich.  Geol.  Survey,  vol.  9,  part  2,  1904. 

4  WILDER,  F.  A. :  Geology  of  Webster  County.  Iowa  Geol.  Survey 
Ann,  Rept.,  vol.  12,  pp.  65-235,  1901. 


568      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

age,  is  mined.  Locally  it  rests  on  the  "Coal  Measures"  and  is 
covered  with  glacial  drift.  The  gypsum  bed  is  from  10  to  25 
feet  thick. 

In  southwestern  Virginia1  gypsum  associated  with  salt  and 
anhydrite  is  found  in  Carboniferous  gray  and  red  clays.  The 
rocks  dip  25°  to  45°,  and  some  beds  are  30  feet  thick. 

In  Kansas2  gypsum  deposits  are  mined  in  a  belt  that  extends 
northeastward  across  the  State.  The  beds  are  associated  with 
Permian  red  shales.  Some  of  the  Kansas  deposits  have  been 
formed  by  solutions  that  dissolved  rock  gypsum  from  the  beds 
and  precipitated  the  calcium  sulphate  as  gypsite  or  earthy 
gypsum  in  swamps  and  marshes  near  by.  The  material  is  used 
for  making  wall  plaster. 

Large  gypsum  deposits  are  found  in  western  Oklahoma  and 
around  the  Black  Hills,  South  Dakota. 

In  New  Mexico3  a  gypsum  bed  60  feet  thick  is  associated 
with  limestone,  red  shale,  and  pink  sandstone  in  the  "Red 
Beds." 

Gypsum  is  used  in  the  manufacture  of  wall  plaster,  land  plaster, 
stucco,  plaster  of  Paris,  and  various  cements.  The  ground 
powder,  unburned,  is  used  to  retard  the  set  of  Portland  cement. 
Land  plaster  is  powdered  gypsum,  unburned,  used  for  fertilizer. 
The  white  powder  is  used  in  making  paint,  crayons,  and  paper  and 
for  an  adulterant.  White  massive  gypsum  (alabaster)  is  used  by 
sculptors  for  making  ornaments.  Gypsite,  an  impure  uncon- 
solidated  mixture  of  gypsum  and  clay,  or  of  gypsum,  clay,  and 
sand,  is  used  for  making  wall  plaster,  in  which  the  clay  serves 
as  a  retarder.  Plaster  boards,  building  blocks,  and  various 
o.ther  forms  are  made  from  gypsum  plaster.4 

Gypsum  contains,  besides  combined  water,  a  variable  content 
of  absorbed  moisture.  If  it  is  heated  to  a  temperature  between 
212°  and  400°  F.,  depending  on  the  impurities  it  contains,  the 
free  moisture  and  part  of  the  combined  water  are  driven  off: 

1  ECKEL,  E.  C. :  Salt  and  Gypsum  Deposits  of  Southwestern  Virginia. 
U.  S.  Geol.  Survey  Bull.  213,  pp.  406-416,  1903. 

2  GRIMSLEY,  G.  P.,  and  BAILEY,  E.  H.  S.:  Special  Report  on  Gypsum  and 
Gypsum  Cement  Plasters.     Kans.  Univ.  Geol.  Survey,  vol.  5,  p.  82,  -1899. 

3SnALER,  M.  K.:  Gypsum  in  Northwestern  New  Mexico.  U.  S.  Geol. 
Survey  Bull.  315,  pp.  260-266,  1907. 

4  BURCHARD,  E.  F. :  Gypsum.  U.  S.  Geol.  Survey  Mineral  Resources, 
1910,  part  2,  pp.  717-733,  1911  (contains  bibliography  and  a  map  showing 
distribution  of  gypsum  plants  in  the  United  States). 


DEPOSITS  OF  THE  NONMETALS  569 

CaS04.2H2O  becomes  2CaS04.H2O.  The  partly  dehydrated 
material  is  ground,  and  when  water  is  added  it  sets  again,  form- 
ing more  highly  hydrated  calcium  sulphate.  If  it  is  raised  to 
too  high"  a  temperature  the  gypsum  becomes  "dead  burned" 
and  will  not  readily  set  on  addition  of  water. 

The  production  of  gypsum  in  the  United   States  in   1915 
amounted  to  2,447,611  tons,  valued  at  $6,596,893. 


SULPHUR 

Although  sulphur  is  present  in  sulphides  in  most  lode  deposits 
of  the  metals,  native  sulphur  forms  only  sparingly  by  oxidation 
of  sulphide  ores,  and  lode  deposits  produce  little  commercial 
sulphur.  The  sulphur  supply  is  obtained  from  sedimentary 
beds  that  carry  sulphur,  from  deposits  formed  near  volcanic 
vents  and  at  the  orifices  of  hot  springs  that  yield  hydrogen  sul- 
phide, and  from  extinct  volcanic  and  fumarolic  centers.  The 
sulphur  has  evidently  been  formed  near  the  surface  by  the  oxi- 
dation of  hydrogen  sulphide.  In  the  presence  of  oxygen  the 
following  reaction  probably  takes  place : 

H2S  +  O  =  H2O  +  S. 

The  largest  sulphur  deposits  are  found  in  Calcasieu  Parish, 
Louisiana.  Below  about  400  feet  of  unconsolidated  sandstone 
is  a  bed  of  gypsum  and  sulphur  with  some  organic  matter.  The 
deposits  were  discovered  in  boring  for  oil,  and  attempts  were 
made  to  sink  shafts  to  them,  but  that  was  found  to  be  imprac- 
ticable in  the  loose  sandy  beds  overlying  the  sulphur  beds.  The 
deposits  are  now  successfully  worked  by  the  Frasch  process. 
Two  pipes,  one  inside  the  other,  are  sent  down  to  the  sulphur  bed, 
and  superheated  water  or  steam  is  discharged  through  one  pipe 
at  the  bottom  of  the  well,  melting  the  sulphur,  which  rises  with 
the  hot  water  through  the  other  pipe  and  solidifies  on  cooling 
in  the  air.  The  process  also  refines  the  sulphur,  a  pure  product 
being  obtained. 

The  deposits  occupy  an  anticline  in  Cretaceous  sedimentary 
rocks.  The  area  has  been  outlined  by  drilling  and  is  rudely 
circular  and  over  half  a  mile  in  diameter.  The  sulphur  bed  is 
locally  more  than  100  feet  thick,  and  much  of  it  contains  about 
70  per  cent,  of  sulphur.  The  sulphur  is  associated  with  lime- 


570      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

stone  and  gypsum  and  is  underlain  by  gypsum.  These  deposits1 
contain  large  reserves. 

Valuable  deposits  of  sulphur  are  found  also  near  Bryan  Heights, 
Texas,2  in  a  structural  dome  that  has  yielded  oil  and  gas.  The 
sulphur  is  associated  with  salt,  gypsum,  and  limestone. 

Before  the  development  of  the  Frasch  process  and  the  exploita- 
tion of  the  Louisiana  deposits  Sicily  was  the  chief  source  of  the 
world's  sulphur  supply.  The  Sicilian  sulphur  deposits  lie  in 
tilted  strata  and  are  associated  with  limestone  and  gypsum. 
The  sulphur  in  these  deposits  is  believed  by  many  to  be  of  syn- 
genetic  origin,3  although  the  method  of  their  formation  is  not 
yet  clearly  elucidated. 

The  origin  of  sulphur  beds  has  long  been  a  subject  of  con- 
troversy. It  is  supposed  by  some  that  organic  matter  has  re- 
duced gypsum,  forming  calcium  sulphide,  which  was  further 
decomposed  by  carbon  dioxide  and  oxygen,  yielding  calcium 
carbonate  and  sulphur.  This  reaction  has  not  been  verified 
experimentally,  and  the  genesis  of  these  deposits  is  still  in  doubt. 
A  plausible  hypothesis  has  been  stated  by  Hunt,4  who  believes 
that  the  sulphur  is  formed  by  the  destruction  of  gypsum  through 
the  agency  of  certain  anaerobic  bacteria  that  consume  calcium 
sulphate  and  liberate  hydrogen  sulphide.  Such  organisms  are 
now  active  in  the  Black  Sea.  The  oxidation  of  the  hydrogen 
sulphide  would  set  free  sulphur,  as  noted  above. 

At  Sulphurdale,  Utah,5  and  at  Cody,  Wyo.,6  sulphur  has  been 
deposited  in  shattered  lavas,  probably  by  hydrogen  sulphide 
gases.  At  Cody  such  gases  now  issue  copiously  from  vents 

1  HATES,  C.  W.,  and  KENNEDY,  WILLIAM:  Oil  Fields  of  the  Texas- 
Louisiana  Gulf  Coastal  Plain.  U.  S.  Geol.  Survey  Butt.  212,  pp.  133-135, 
1903. 

HARRIS,  G.  D.:  Oil  and  Gas  in  Louisiana.  U.  S.  Geol.  Survey  Bull. 
429,  pp.  99-103,  1910. 

ZPHALEN,  W.  C.:  Sulphur,  Pyrite,  and  Sulphuric  Acid.  U.  S.  Geol. 
Survey  Mineral  Resources,  1912,  part  2,  pp.  931-953,  1913. 

"STUTZER,  OTTO:  "Die  wichtigsten  Lagerstatten  der  Nicht-Erze,"  p. 
474,  Berlin,  1911.  Translated  by  PHALEN,  W.  C.:  Econ.  Geol.,  vol.  7,  pp. 
732-745,  1912. 

4  HUNT,  W.  F.:  The  Origin  of  Sulphur  Deposits  in  Sicily.  Econ.  Geol., 
vol.  10,  pp.  543-579,  1915. 

6  LEE,  W.  T.:  The  Cove  Creek  Sulphur  Beds,  Utah.  U.  S.  Geol.  Sur- 
vey Bull.  315,  pp.  485-489,  1907. 

6  WOODRUFF,  E.  G.:  Sulphur  Deposits  at  Cody,  Wyo.  U.  S.  Geol.  Sur- 
vey Bull.  340,  pp.  451-456. 


DEPOSITS  OF  THE  NONMETALS  571 

near  sulphur  deposits.  At  Thermopolis,  Wyo.,1  sulphur  is  ob- 
tained below  travertine  that  rests  on  limestone. 

In  Hokkaido,  Japan,  sulphur  is  recovered  from  an  old  volcanic 
crater.  Sulphur  is  said  to  occur  in  commercial  quantites  near 
Popocatapetl,  Mexico,  and  other  volcanoes. 

A  large  part  of  the  sulphur  produced  is  utilized  in  the  paper 
industry,  in  which  it  is  converted  into  sulphite  and  used  for 
bleaching  paper  pulp.  A  little  sulphur  is  used  in  making  sul- 
phuric acid,  but  most  sulphuric  acid  is  made  by  burning  sulphide 
minerals.  Because  sulphur  ignites  at  a  low  temperature  it  is 
used  in  making  matches,  gunpowder,  and  fireworks.  Sulphur 
dioxide  is  used  extensively  for  bleaching.  Sulphur  is  used  for 
spraying  vegetation  to  protect  it  against  fungous  diseases,  as  a 
preservative,  and  for  vulcanizing  rubber. 

In  1914  the  United  States  produced  327,634  long  tons  of 
sulphur,  valued  at  $5,479,849.  Most  of  this  was  from  Louisiana; 
a  little  came  from  Texas,  Nevada,  and  Wyoming. 

PYRITE  AND  SULPHURIC  ACID 

Pyrite,  marcasite,  pyrrhotite,  sphalerite,  and  sulphur  are  used 
for  making  sulphuric  acid.  The  roasted  minerals  give  off  sulphur 
dioxide,  which  is  led  into  a  Glover  tower,  where  nitrogen  oxides 
and  steam  are  introduced.  Sulphur  dioxide  is  oxidized  to  sul- 
phur trioxide,  with  which  a  molecule  of  water  unites,  forming 
sulphuric  acid.  Part  of  the  nitrogen  compounds  after  having 
oxidized  the  sulphur  dioxide  are  recovered  in  Gay-Lussac  towers. 
Thus  the  process  with  respect  to  nitrogen  is  regenerative. 

The  huge  acid  plants  in  the  Ducktown  district,  Tennessee, 
use  blast-furnace  gases  obtained  from  smelting  pyrrhotite-chalco- 
pyrite  copper  ores.  Some  plants  make  acid  from  fumes  obtained 
by  roasting  sphalerite  as  a  preliminary  to  charging  it  in  zinc 
retorts.  Marcasite  is  obtained  by  separating  the  iron  sulphide 
from  zinc  sulphide  in  zinc  concentrates.  The  principal  output 
of  marcasite  comes  from  the  zinc  ores  of  southwestern  Wisconsin 
and  northwestern  Illinois. 

Pyrite  is  mined  in  several  Eastern  States,  and  considerable 
pyrite  is  now  obtained  as  a  by-product  of  coal  mining  in  Illinois, 
Indiana,  and  Ohio.  At  some  plants  the  cinder  obtained  from 

1  WOODRUFF,  E.  G. :  Sulphur  Deposits  near  Thermopolis,  Wyo.  U.  S. 
Geol.  Survey  Bull.  380,  pp.  373-380. 


572      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

burning  pyrite  for  acid  making  is  subsequently  leached  for  the 
extraction  of  copper  and  smelted  for  iron.  Pyrite  is  imported 
from  Canada  and  Spain.  The  Spanish  pyrite  from  the  Huelva 
or  Rio  Tinto  region  is  of  high  grade  and  abundant.  It  is  received 
in  large  quantities  at  eastern  ports. 

Virginia  is  an  important  producer  of  domestic  pyrite.  In 
Louisa  County  there  are  huge  lenticular  deposits  of  pyrite  in 
garnet-mica  schist  which  is  probably  a  metamorphosed  lime- 
stone.1 Minerals  associated  with  the  pyrite  include  chalco- 
pyrite,  galena,  sphalerite,  pyrrhotite,  magnetite,  actinolite, 
calcite,  and  garnet.  The  lenses  of  pyrite  ore  are  rudely  in  align- 
ment or  overlap.  According  to  Watson  they  have  replaced  sedi- 
mentary beds.  Pyritic  lenses  in  schist  are  worked  also  in  Prince 
William  County,  Virginia.  In  St.  Lawrence  County,  New  York,2 
near  Canton  and  Gouverneur,  deposits  of  low-grade  pyrite  in 
schist  are  mined  and  concentrated.  At  the  Davis  mine,  Franklin 
County,  Massachusetts,  a  deposit  of  high-grade  pyrite  in  schist 
was  mined  for  many  years  but  is  said  now  to  be  exhausted.  At 
the  Milan  mine,  New  Hampshire,3  a  pyritic  deposit  in  schist  is 
mined  and  concentrated  for  pyrite.  Considerable  pyrite  is 
mined  in  California  in  the  foothill  copper  belt  and  in  Shacta 
County. 

In  1915  the  United  States  produced  394,124  long  tons  of  pyrite, 
valued  at  $1,674,933. 

BORON  COMPOUNDS 

Boron  is  a  comparatively  rare  element.  It  is  present  in  the 
silicates,  tourmaline,  axinite,  and  datolite,  but  these  are  not 
important  as  sources  of  boron.  Borax  compounds  are  present 
also  in  many  hot  springs.  In  arid  regions  borax  salts  are  formed 
by  the  evaporation  of  water  in  closed  basins.  Nearly  all  the 
boron  compounds  of  commerce  are  obtained  from  bedding-plane 
deposits  of  borax  minerals  that  are  interlayered  with  sedimentary 
rocks.  These  are  found  principally  in  areas  of  late  volcanic 
activity. 

1  WATSON,  T.  L.:  "Mineral  Resources  of  Virginia,"  p.  190,  Lynchburg, 
1907. 

2  SMYTH,  C.  H.,  JR.  :  On  the  Genesis  of  the  Pyrite  Deposits  of  St.  Lawrence 
County.     N.  Y.  State  Mus.  Bull.  158,  p.  143,  1912. 

3EMMONS,  W.  H.:  Some  Ore  Deposits  of  Maine  and  the  Milan  Mine, 
New  Hampshire.  U.  S.  Geol.  Survey  Butt.  432,  pp.  50-60,  1910. 


DEPOSITS  OF  THE  NONMETALS  573 

The  water  of  Clear  Lake,  California,1  contains  an  unusual 
proportion  of  boron  compounds.  Formerly  the  water  was 
evaporated  for  borax  salts  on  a  commercial  scale.  The  solid 
matter  in  a  spring  on  the  margin  of  this  lake  contains  25.61  per 
cent.  B407. 

The  principal  borax  minerals  are  colemanite  (Ca2B6Oii.5H2O), 
borax  (Na2B4O7.10H2O),  ulexite  (CaNaB509.8H2O),  and  bora- 
cite  (Mg7Cl2Bi6O3o).  Of  these,  colemanite  supplies  practically 
the  total  output  of  borax  in  the  United  States.  A  little  boracite 
is  obtained  from  the  Stassfurt  region,  Germany,  and  some  borates 
are  recovered  as  by-products  from  the  evaporation  of  brines. 
Pandermite  is  a  lime  borate  near  colemanite  in  composi- 
tion, but  it  does  not  decrepitate  on  heating  and  is  not  used  at 
present. 

Boron  minerals  are  found  in  both  syngenetic  and  epigenetic 
deposits.  Some  have  been  formed  probably  by  the  evaporation 
of  waters  that  have  leached  older  bedded  deposits.  Some  of  the 
older  deposits  are  doubtless  the  result  of  desiccation  of  hot-spring 
waters  in  ancient  closed  basins;  others  have  been  formed  in 
fissures  and  fractured  zones.  The  soluble  borax  minerals2  are 
removed  readily  from  outcrops.  Some  outcrops  of  colemanite 
are  marked  by  gypsum. 

The  Lila  C.  borax  mine,  in  Death  Valley,  California,  is  at 
present  the  principal  source  of  borax.3  The  borax  beds  are  asso- 
ciated with  Tertiary  shales,  sandstones,  and  vesicular  lavas. 
The  strata  are  steeply  tilted  and  faulted.  The  borax  beds, 
which  occur  at  definite  stratigraphic  horizons,  dip  about  45.° 
The  principal  borax  mineral  is  colemanite,  the  beds  of  which  are 
locally  15  feet  or  more  thick. 

At  Stauffer,  Ventura  County,  California,  colemanite  deposits 
occur  in  folded  and  faulted  sedimentary  rocks  closely  associated 
with  beds  of  basalt  that  inclose  zones  of  shale  and  massive  lime- 
stone. The  limestone  beds  contain  the  largest  borate  deposits 

1  BECKER,  G.  F. :  Geology  of  the  Quicksilver  Deposits  of  the  Pacific 
Slope.  U.  S.  Geol.  Survey  Mon.  13,  pp.  265,  440,  1888. 

2 GALE,  H.  S.:'  The  Origin  of  Colemanite  Deposits.  U.  S.  Geol.  Survey 
Prof.  Paper  85,  pp.  1-9,  1913. 

3  GALE,  H.  S.:  The  Lila  C.  Borax  Mine  at  Ryan,  Calif.  U.  S.  Geol. 
Survey  Mineral  Resources,  1911,  part  2,  pp.  861-866,  1912. 

CAMPBELL,  M.  R.:  Reconnaissance  of  the  Borax  Deposits  of  Death  Valley 
and  Mohave  Desert.  U.  S.  Geol.  Survey  Bull.  200,  pp.  1-23,  1902. 


574      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

in  the  field.     The  limestone  is  sheeted,  shattered,  and  cemented 
and  partly  replaced  by  colemanite. 

Borax  compounds  are  used  for  preservatives,  in  chemistry, 
as  cleansers  and  insecticides,  and  for  making  glass,  paints,  cos- 
metics, etc.  The  United  States  produced  in  1915  about  67,003 
tons  of  borax,  valued  at  $1,677,099. 

NITRATES 

Sodium  nitrate  (NaN03,  Chile  niter)  and  potassium  nitrate 
(KNOs,  saltpeter)  are  readily  soluble  in  water  and  therefore 
are  found  only  in  arid  regions  or  in  caves  or  other  places  where 
they  are  protected  from  rain.  These  salts  are  formed  by  the 
decomposition  of  organic  waste  through  the  agency  of  nitrifying 
bacteria.  Nitrate  deposits  occur  in  Death  Valley,  California, 
and  at  other  places  in  the  United  States,  but  these  are  probably 
of  small  value.  The  world's  supply  of  nitrates  is  obtained  mainly 
from  the  Atacama  and  Tarapaca  desert  region  of  Chile.  In 
1912  Chile  produced  2,585,850  metric  tons  of  sodium  nitrate, 
valued  at  $107,054,090. l 

The  deposits  are  found  here  and  there  along  the  coast  for  a 
distance  of  about  450  miles.  They  are  on  a  plateau  about  2,500 
feet  above  the  sea,  west  of  the  Andes.  The  country  is  arid: 
rain  falls  at  intervals  of  about  ten  years.  The  arid  plateau  is 
composed  of  Jurassic  rocks  covered  with  marine  sands,  gravel, 
clay,  and  mud  and  angular  fragments  from  the  hills.  The  nitrate- 
bearing  mixture,  or  caliche,  is  associated  with  the  unconsolidated 
material.  As  a  rule,  sodium  chloride,  alkali  and  alkali  earth 
sulphates,  sodium  borate,  and  other  salts  are  present.  The 
impure  nitrate  mixtures  are  purified  by  dissolving  in  hot  water 
and  by  fractional  crystallization. 

Over  the  origin  of  the  Chile  nitrate  deposits  there  is  much 
controversy.  One  hypothesis  is  that  the  nitrates  of  the  caliche 
have  been  derived  from  the  air  by  oxidation  of  nitrogen  through 
the  agency  of  static  electricity.2  In  Europe  nitrates  are  pro- 
duced artificially  by  oxidation  of  nitrogen  of  the  air  in  the  electric 
arc.  It  is  said,  however,  that  electrical  effects  are  not  more  pro- 
nounced in  the  desert  regions  of  Chile  than  in  other  desert  regions 

1  STRAUSS,  L.  W. :  The  Chilean  Nitrate  Industry.     Min.  and  Sci.  Press, 
vol.  108,  p.  972,  1914. 

2  SEMPER,  E.,   and    MICHELS:  Die   Saltpeterindustrie   Chiles.     Zeitschr. 
Berg-,  Huiien-  u.  Salinenwesen  preuss.  St.,  vol.  52,  pp.  359-482,  1904. 


DEPOSITS  OF  THE  NONMETALS  575 

where  nitrates  do  not  accumulate.  The- most  generally  accepted 
theory  is  that  the  nitrate  mixture  has  been  derived  from  the 
leaching  of  material  formed  by  the  action  of  nitrifying  bacteria 
on  organic  matter  in  the  soil.1  This  organic  matter  is  supposed  to 
be  in  part  of  animal  origin.  Guano  deposits  occur  in  parts  of 
the  nitrate  region.  Penrose2  considers  it  probable  that  the  de- 
posits have  originated  through  the  leaching  of  nitrate  from  ex- 
tensive deposits  of  bird  guano  that  accumulated  before  the  coast 
range  was  thrown  up,  the  leachings  having  mingled  with  the 
salines  of  a  closed  basin.  Although  the  country  is  extremely 
arid,  the  ground-water  nevertheless  stands  near  the  surface, 
being  supplied  by  the  mountains  near  the  border.  Singewald 
and  Miller3  note  that  the  nitrate  deposits  occur  at  the  places 
where  the  ground  water  comes  near  the  surface  and  where  the 
soil  is  porous  and  believe  that  the  accumulation  of  nitrates  is 
due  to  abnormally  rapid  evaporation  of  great  quantities  of 
ground  water  under  extremely  arid  conditions.  They  also  believe, 
however,  that  the  quantities  of  nitrates  carried  by  the  water 
are  above  the  average,  and  they  note  the  presence  of  skeletons 
of  birds  and  guano  in  the  desert  basin. 

The  uses  of  nitrates  for  fertilizer  and  for  making  sulphuric 
acid  and  explosives  are  well  known. 


MINERAL  FERTILIZERS 

Plants  subtract  from  soils  each  year  appreciable  amounts  of 
mineral  matter.  Unless  this  is  replenished,  the  productivity 
of  the  soil  will  ultimately  be  impaired.  For  some  crops,  especially 
for  grains,  lime,  potash,  phosphorus,  and  nitrates  are  necessary. 
Any  one  or  all  of  these  may  be  insufficient  in  the  soil.  Lime  is 
supplied  as  limestone,  marl,  or  gypsum.  Limestone  and  marl 
are  especially  useful  when  it  is  desired  to  correct  acid  soils.  The 
limestone  is  generally  ground  to  fine  dust  and  sometimes  it  is 
calcined  and  slaked  before  grinding.  Gypsum  is  sold  in  the  raw 

1  MiiNTz,  A. :  Recherches  sur  la  formation  des  gisements  du  nitrate  de 
soude.  Compt.  Rend.,  vol.  101,  pp.  1265-1267,  1885. 

2 PENROSE,  R.  A.  F.,  JR.:  The  Nitrate  Deposits  of  Chile.  Jour.  Geol., 
vol.  18,  pp.  1-32,  1910. 

3  SINGEWALD,  J.  T.,  JR.,  and  MILLER,  B.  L. :  The  Genesis  of  the  Chilean 
Nitrate  Deposits.  Econ.  Geol,  vol.  11,  pp.  103-114,  1916. 


576      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

state  finely  ground.  When  used  for  fertilizer,  it  is  called  land 
plaster.  Potash  is  supplied  as  the  sulphate  (kainite)  and  as 
niter  (potassium  nitrate),  though  niter  is  too  expensive  for 
general  use.  Greensand  marl,  which  contains  some  potash  in 
the  mineral  glauconite  (a  hydrous  iron  and  potassium  silicate), 
is  also  used  as  a  fertilizer.  The  potash  in  glauconite  is  less  readily 
available,  however,  for  it  dissolves  but  slowly.  Phosphorus  is 
supplied  by  bone  phosphate,  rock  phosphate,  apatite,  and  wavel- 
lite;  all  are  generally  treated  with  sulphuric  acid  to  make  the 
phosphate  more  soluble.  Recently  finely  ground  phosphate 
rock  has  been  applied  to  soils  in  the  raw  state,  but  as  it  is  slowly 
soluble  a  longer  time  must  elapse  before  the  investment  yields 
returns.  Mineral  phosphates  compete  with  bone  meal  and  fish 
waste,  which  contain  calcium  phosphate.  In  Europe  the  phos- 
phates obtained  by  the  basic  open-hearth  process  of  making  steel 
are  used  for  fertilizer. 

Nitrates  are  formed  in  soils  by  certain  bacteria  working  in 
conjunction  with  leguminous  plants;  deficiency  in  nitrates, 
therefore,  may  be  corrected  by  suitable  rotation  of  crops.  Some 
commercial  fertilizers  contain  sodium  or  potassium  nitrates  or 
ammonia  salts. 

Guano,  used  for  fertilizer,  is  the  excrement  of  animals,  chiefly 
of  birds  and  bats.  Some  islands  of  the  sea  are  thickly  covered 
with  a  mantle  of  guano,  and  formerly  large  quantities  of  this 
material  were  imported,  especially  from  Peru.  In  Texas  bat 
guano  has  been  recovered  from  caves.1 

Sulphuric  acid  is  extensively  used  to  convert  rock  phosphate 
and  bones  into  the  more  soluble  superphosphate.  Thus  there  is 
a  close  relation  between  the  acid  and  phosphate  fertilizer 
industries,  and  many  fertilizer  companies  operate  acid  plants. 

Phosphate  rock  is  a  noncrystalline  material,  principally  lime 
phosphate;  it  contains  as  a  rule  25  to  35  per  cent.  P2O5,  and  about 
40  to  50  per  cent.  CaO.  Other  radicles  commonly  present  are 
SiO2,  A1203,  Fe2O3,  and  CaC03.  Apatite  [(CaF)  Ca4(P04)3] 
contains  42.3  per  cent.  P2O5.  Wavellite  has  the  formula 
4A1PO4.2A1(OH)3  +  9H2O,  which  corresponds  to  35.2  per  cent. 
P2O5.  Of  these  products  phosphate  rock  yields  much  the  greatest 
quantity  of  fertilizer.  In  1915  the  United  States  produced 


PHILLIPS,  W.  B.:  Bat  Guano  Caves.     Mines  and  Minerals,  vol.  21,  p. 
440,  1901. 


DEPOSITS  OF  THE  NONMETALS  577 

1,835,667  tons  of  phosphate  rock,  valued  at  $5,413,449;  nearly  all 
of  this  was  from  Florida,  Tennessee,  and  South  Carolina.  A  little 
apatite  is  produced  as  a  by-product  of  the  magnetic  separation 
of  iron  fires  in  the  Adirondack  region,  New  York. 

Phosphate  rock  occurs  in  sedimentary  beds  or  as  surface  con- 
centrations formed  by  the  weathering  of  such  beds.  It  is  com- 
monly associated  with  marine  limestone  and  is  generally  of 
marine  origin.  Its  color  is  white,  gray,  brown,  blue,  or  black, 
depending  on  impurities.  Some  of  it  is  made  up  of  numerous 
shells  or  fragments  of  shells.  The  pisolitic  or  oolitic  texture 
is  very  common. 

Of  the  origin  of  phosphate  rock  there  is  yet  much  to  be  learned. 
Some  is  doubtless  formed  by  the  bodies  and  waste  of  marine 
animals.  Some  is  probably  precipitated  directly  from  sea  water. 
Richards  and  Mansfield1  regard  the  oolitic  texture  of  Idaho  phos- 
phate as  original.  Shells  of  animals  that  normally  have  lime  car- 
bonate shells  are  found  to  be  made  up  of  mixtures  of  lime  car- 
bonate and  lime  phosphate. 

The  textural  features  of  some  phosphate  rocks  are  similar  to 
those  found  in  Alabama  hematites  (page  323).  The  beds  of 
phosphate  and  hematite  are  likewise  very  extensive.  Doubtless 
the  origin  of  both  is  closely  similar,  and  probably  some  phosphate 
is  precipitated  directly  from  sea  water  and  other  phosphate  re- 
places calcareous  material  on  the  sea  bottom.  Where  phosphatic 
limestone  weathers  lime  carbonate  is  removed  more  rapidly  than 
lime  phosphate. 

Some  deposits  form  residual  masses  due  to  surface  concentra- 
tion. The  western  deposits  (Fig.  207)  are  generally  richer  at 
the  outcrop  than  in  depth,  owing  to  the  removal  of  lime  car- 
bonate. There  are  many  beds  in  the  West,  however,  that  are 
high  grade  in  what  appears  to  be  the  original  concentration. 
Here  also,  in  their  enrichment  these  deposits  show  a  similarity 
to  the  Clinton  type  of  iron  ores. 

Apatite  is  not  important  as  an  ore  of  phosphate.  In  the 
Adirondacks  it  occurs  with  magnetite  in  deposits  formed  by  mag- 
matic  segregation  (page  337).  The  production  from  these  de- 
posits is  small.  In  Ontario  and  Quebec,  notably  at  Templeton, 

1  RICHARDS,  R.  W.,  and  MANSFIELD,  G.  R.:  Preliminary  Report  on  a 
Portion  of  the  Idaho  Phosphate  Reserve.     U.  S.  Geol.  Survey  Bull.  470, 
p.  371,  1911.     BLACKWELDER,  ELIOT:  Origin  of  Rocky  Mountain  Phosphate 
Deposits.     Geol.  Soc.  America  Bull.,  vol.  26,  p.  100,  1916. 
37 


578      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 


Quebec,1  considerable  deposits  of  apatite  are  found  with  pyroxene, 
mica,  and  calcite  in  what  appears  to  be  a  contact-metamorphic 
or  nearly  related  deposit.  The  Canadian  apatites  have  been 
almost  driven  from  the  market  by  the  higher-grade  phosphate 
rock,  and  now  the  deposits  are  worked  principally  for  mica. 


-•i-v^V -«;=~.-..~  .-,.-.  ^V''  ^     '      -.^^3^Hl? 


FIG.  207. — View  of  phosphate  mountain  near  Melrose,  Mont.,  looking 
from  the  direction  of  Melrose,  with  a  diagrammatic  section  showing  the 
geologic  structure.  (After  Gale,  U.  S.  Geol.  Survey.) 

Apatite  is  the  most  abundant  rock-making  mineral  that  con- 
tains phosphorus  and  doubtless  is  the  chief  original  source  of  the 
element.  Clarke2  cites  many  analyses  of  river  water  which  carry 
from  a  trace  to  1.6  per  cent.  PO4.  Sea  water,  on  the  other  hand, 
contains  only  traces  of  phosphorus.  It  is  removed  to  form  the 
bones  of  fishes  and  other  aquatic  vertebrates  and  is  precipitated 
in  phosphatic  nodules  that  are  found  on  the  sea  bottom.  It  is 

1  PENROSE,  R.  A.  F.,  JR.:  Nature  and  Origin  of  Deposits  of  Phosphate  of 
Lime.     U.  S.  Geol.  Survey  Bull.  46,  pp.  34-39,  1888. 

CIRKEL,  FRITZ:  Mica;  Its  Occurrence,  Exploitation,  and  Uses,  pp.  1-148. 
Canada  Dept.  Mines,  Mines  Branch,  1905. 

2  CLARKE,  F.  W. :  The  Data  of  Geochemistry,  3d  ed.     U.  S.  Geol.  Survey 
Bull.  616,  pp.  75-106,  1916. 


DEPOSITS  OF  THE  NONMETALS  579 

probably  precipitated  also  by  the  metasomatic  replacement  of 
lime  carbonate  shells. 

The  deposits  of  phosphate  rock  in  Utah,  Idaho,  Wyoming, 
and  Montana1  are  the  most  extensive  known.  These  deposits, 
although  of  high  grade,  are  not  now  worked  except  on  a  small  scale. 
But  they  will  probably  be  worked  extensively  within  a  few 
decades,  when  western  soils  will  require  fertilizers,  or  as  soon 
as  low  freight  rates  permit  shipment.  The  deposits  are  princi- 
pally in  the  limestones  of  the  Pennsylvanian  (upper  Carbon- 
iferous). They  are  sedimentary  beds  associated  with  limestones, 
sandstones,  and  shales  and  are  but  little  altered  by  surface 
leaching.  '  • 

MAGNESIAN  MINERALS 

Olivine (FeMg)2.SiO4 

Enstatite (FeMg).SiOs 

Serpentine H4Mg3.Si2O9 

Magnesite MgCOs 

Tremolite CaMg3.Si4Oi2 

Talc H2Mg3.Si4Oi2 

Meerschaum H4Mg2.Si3Oio 

Olivine  and  Serpentine. — Of  the  magnesian  minerals  olivine 
generally  has  no  marketable  value,  though  some  varieties  are 
used  in  a  small  way  as  gem  material.  It  is,  however,  an  im- 
portant source  of  serpentine  and  other  magnesian  minerals.  It 
is  the  principal  constituent  of  peridotites  and  is  present  in  many 
other  basic  rocks  that  alter  to  serpentine.  It  is  a  valuable 
protore  of  iron. 

Olivine  is  one  of  the  most  unstable  minerals  and  changes  into 
serpentine  with  great  facility.  Peridotites  and  other  basic 
rocks  that  in  hand  specimens  appear  to  be  perfectly  fresh  are 
almost  universally  found,  when  examined  microscopically,  to  be 
serpentinized  along  cracks  of  olivine.  As  secondary  products, 
both  talc  and  serpentine  are  formed  at  very  great  depths.  In 
California  canyons  thousands  of  feet  deep  expose  serpentine 

1  RICHARDS,  R.  W.,  and  MANSFIELD,  G.  R.:  Preliminary  Report  on  a 
Portion  of  the  Idaho  Phosphate  Reserve.  U.  S.  Geol.  Survey  Bull.  470, 
p.  371,  1910. 

GALE,  H.  S. :  Rock  Phosphate  near  Melrose,  Mont.     Idem,  p.  440. 

BLACKWELDER,  ELIOT:  A  Reconnaissance  of  the  Phosphate  Deposits  of 
Western  Wyoming.  Idem,  p.  452. 


580      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

rocks.  Either  they  were  formed  far  below  the  surface,  or  altera- 
tion was  more  rapid  than  erosion,  which  in  the  surroundings  is 
almost  incredible.  Lindgren1  has  suggested  that  olivine  has 
changed  to  serpentine  through  the  agency  of  ascending  waters 
associated  with  igneous  intrusives.  The  alteration  of  olivine 
to  serpentine  involves  no  great  chemical  change,  as  is  indicated 
by  the  following  equation:2 

2Mg2SiO4  +  CO2  +  2H20  =  H4Mg3Si2O9  +  MgC03 

Forsterite  Serpentine  Magnesite 

Serpentine  develops  from  pyroxenic  rocks  that  have  replaced 
limestones,  and  also  in  ferromagnesian  schists.  The  formation 
of  serpentine  is  attended  by  increase  of  volume,  and  some  serpen- 
tine rocks  disintegrate  readily  when  piled  in  dumps.  Thus 
diamond-bearing  serpentines  in  South  Africa  crumble  down  in  a 
few  months.  Serpentine,  particularly  the  green  translucent 
variety,  is  used  for  ornamental  purposes.  The  asbestiform 
varieties  of  serpentine  are  mentioned  on  page  581. 

Magnesite.  —  Magnesite  develops  from  olivine  and  serpentine, 
in  part  after  the  manner  indicated  in  the  equation  above.  It 
occurs  in  veins  and  fractured  zones,  filling  cavities  and  replacing 
the  serpentine  wall  rock.  Commonly  associated  minerals  are 
chalcedony  and  quartz.  The  change  is  probably  effected  through 
the  agency  of  carbonated  water,  as  indicated  by  the  equation 
below,  serpentine  yielding  magnesite  and  silica. 

H4Mg3Si2O9  +  3C02  =  3MgC03  +  2H20  +  2SiO2, 

Considerable  deposits  are  found  in  California,3  especially  in 
Lhe  Coast  Range.  Almost  if  not  quite  invariably  the  magnesite 
is  associated  with  serpentine.  The  deposits  of  the  United  States 
supplied  about  30,499  tons  in  1915,  valued  at  $274,491.  This, 
however,  is  only  a  small  part  of  the  magnesite  used  in  this  country. 
Imports  come  from  Austria  and  Greece. 


,  WALDEMAR:  "Mineral  Deposits,"  p.  343,  New  York,  1913. 

*  For  simplicity  I  have  used  forsterite,  the  magnesium  olivine,  free  from 
iron.  Fayalite  (FeaSiCh)  is  the  corresponding  iron  olivine.  Both  species 
are  comparatively  rare.  Common  olivine  generally  contains  both  iron  and 
magnesium. 

3  HESS,  F.  L.:  The  Magnesite  Deposits  of  California.  U.  S.  Geol.  Survey 
Bull.  355,  1908. 

GALE,  H.  S.  :  Late  Developments  of  Magnesite  Deposits  in  California  and 
Nevada.  U.  S.  Geol.  Survey  Bull  540,  pp.  483-520,  1912. 


DEPOSITS  OF  THE  NONMETALS  581 

Magnesite  is  used  principally  for  making  refractory  brick, 
for  smelting,  for  packing  steam  pipes,  and  in  the  manufacture  of 
paper,  ^carbon  dioxide,  oxychloride  cement,  medicines,  etc. 

Talc. — Tremolite,  enstatite,  and  other  magnesian  minerals 
break  down  readily,  forming  talc.  The  reactions,  according  to 
Clarke,1  are  as  follows: 

CaMg3Si4012  +  H2O  +  CO2  =  H2Mg3Si4Oi2  +  CaC03 
Mg4Si4Oi2  +  H20  +  C02  =  H2Mg3Si4012  +  MgC03 

Compact  and  impure  talc  is  called  soapstone.  In  many  places 
talc  is  formed  by  the  alteration  of  tremolite-bearing  limestone  or 
from  basic  igneous  rocks.  Talc  is  found  in  schists  or  dynamically 
metamorphosed  rocks.2  Dynamic  metamorphism  is  not  neces- 
sary for  its  development,  however,  as  it  is  found  also  in  rocks  that 
are  not  highly  schistose. 

Talc  and  soapstone  are  used  as  refractories,  in  laboratory 
tables,  gas  burners,  electric  insulators,  etc.  Ground  to  powder 
they  are  used  in  making  paper,  paint,  toilet  powder,  and  dynamite. 
The  United  States  in  1915  produced  186,891  tons  of  talc  and 
soapstone,  valued  at  $1,891,582. 

Meerschaum. — Meerschaum  is  probably  derived  from  serpen- 
tine and  from  impure  dolomites.  Its  uses  for  smokers'  articles 
are  well  known.  Most  of  it  is  imported  from  Asia  Minor.  In 
New  Mexico  it  forms  veins  and  balls  in  cherty  limestone.3 

ASBESTOS 

Asbestos  is  a  trade  term  that  is  applied  to  minerals  that  are 
fibrous  and  poor  conductors  of  heat  and  that  may  be  used  in 
making  certain  products  for  protection  against  fire.  Most  of 
them  are  magnesian  minerals.  Serpentine  (H4Mg,Si2O9),  amphi- 
bole  (Ca(Mg,Fe)3(Si03)4),  anthophyllite  ([Mg,Fe]SiO3),  and  cro- 
cidolite  (NaFeSi2O6.FeSi03)  have  asbestiform  varieties. 

Several  types  of  asbestos  are  recognized — cross-fiber,  slip- 
fiber,  and  mass-fiber.  The  cross-fiber  asbestos  occurs  in  veins 
as  much  as  several  inches  wide,  and  the  fibers  are  about  normal 

1  CLARKE,  F.  W. :  The  Data  of  Geochemistry,  3d  ed.     U.  S.  Geol.  Survey 
Bull.  616,  p.  415,  1916. 

2  DILLER,  J.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1908,  part  2,  pp. 
869-878. 

3STERRETT,  D.  B. :  Meerschaum  in  New  Mexico.  U.  S.  Geol.  Survey 
Bull.  340,  pp.  466-473,  1908. 


582      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

to  the  walls  of  veins.  The  slip-fiber  occurs  on  slipping  planes,  and 
the  fibers  are  parallel  to  the  planes  of  movement.  Mass-fiber  is 
found  as  masses  not  occupying  veins  or  slipping  planes,  and  the 
threads  are  arranged  haphazard  or  are  radiating.  Most  of  the 
cross-fiber  asbestos,  which  is  the  highest  grade,  is  chrysotile. 
Much  of  the  mass-fiber  is  anthophyllite. 

As  a  rule  much  waste  rock  is  mined  with  asbestos.  The  ore 
is  crushed  and  separated,  usually  by  means  of  an  air  blast. 

Serpentine  asbestos  is  formed  by  the  alteration  of  peridotite 
or  certain  other  rocks.  The  highest-grade  asbestos  deposits 
are  veinlets  in  serpentine.  It  is  believed  that  they  have  been 
formed  through  the  agency  of  waters  that  coursed  through 
cracks  and  fissures,  either  hot  waters  soon  after  the  rocks  had 
formed  or  surface  waters  later,  when  the  rocks  were  eroded. 

The  most  productive  deposits  of  asbestos  in  North  America 
are  at  Thetford,  Quebec,1  in  a  belt  of  igneous  rocks  that  extends 
southwestward  into  northern  Vermont2  (see  Fig.  208).  The 
asbestos  occurs  as  cross-fiber  veins,  closely  spaced  in  a  serpentine 
that  has  probably  been  formed  by  alteration  from  peridotite. 
The  veinlets,  which  are  from  a  fraction  of  an  inch  to  several  inches 
thick,  are  believed  to  be  alteration  products  of  the  igneous  rocks 
in  which  they  are  found.  Veins  of  chrysotile  asbestos  in  serpen- 
tine are  worked  in  Orleans  and  Lamoille  counties,  Vermont. 
Near  Casper,  Wyo.,3  veins  of  cross-fiber  asbestos  occur  in  serpen- 
tine, which  also  is  probably  an  alteration  product  of  peridotite. 

1  CIEKEL,    FRITZ:  Chrysotile    Asbestos,    Its    Occurrence,    Exploitation, 
Milling,  and  Uses,  2d  ed.     Canada  Dept.  Mines,  Mines  Branch,  1910. 

DRESSER,  J.  A. :  On  the  Asbestos  Deposits  of  the  Eastern  Townships  of 
Quebec.  Econ.  Geol,  vol.  4,  p.  130,  1909. 

ELLS,  R.  W.:  Bulletin  on  Asbestos,  Canada  Geol.  Survey,  1903. 

2  DILLER,  J.  S. :  The  Types  and  Modes  of  Occurrence  of  Asbestos  in  the 
United   States.     Canadian   Min.   Inst.   Quart.   Bull.   No.    13,   pp.    45-58, 
February,  T911. 

MARSTERS,  V.  F. :  Petrography  of  the  Amphibolite,  Serpentine,  and  Asso- 
ciated Asbestos  Deposits  of  Belvedere  Mountain,  Vermont.  Geol.  Soc. 
America  Bull,  vol.  16,  pp.  419-446,  1905. 

RICHARDSON,  C.  H.:  Asbestos  in  Vermont.  Vermont  State  Geologist 
Seventh  Rept.,  pp.  315-330,  1910. 

3 LAKES,  ARTHUR:  The  Wyoming  Asbestos  Deposits  and  Mills.  Min. 
Sri.,  October  28,  1909. 

DILLER,  J.  S. :  The  Types,  Modes  of  Occurrence,  and  Important  Deposits 
of  Asbestos  in  the  United  States.  U.  S.  Geol.  Survey  Bull.  470,  pp.  512- 
516,  1911. 


DEPOSITS  OF  THE  NONMETALS 


583 


In  the  Grand  Canyon  of  Arizona  asbestos  occurs  with  serpentine 
in  limestone.  Diller  considers  it  to  be  derived  from  material 
of  sedimentary  origin.  The  asbestos  rocks  are  near  an  intruding 
diabase  sill,  and  the  parent  olivine  minerals  may  have  been  formed 
by  contact-metamorphic  processes. 

In  Idaho,  about  14  miles  southeast  of  Kamiah,  ledges  of  antho- 


FIG.  208. — Map  showing  location  of  asbestos  deposits  in  northern  Vermont 
and  adjoining  portion  of  Canada.     (After  Diller,  U.  S.  Geol.  Survey.) 

phyllite  rock  are  found  intruded  in  mica  schist.  This  rock  is 
quarried  and  shipped  to  Spokane,  Wash. 

Most  of  the  asbestos  produced  in  the  United  States  is  mined  in 
Georgia.  At  Sail  Mountain,  Ga.,  according  to  Diller,  antho- 
phyllite  asbestos  occurs  in  lenticular  masses  in  gneiss,  which 
is  believed  to  be  an  altered  igneous  rock.  Near  Bedford,  Va., 
there  are  deposits  of  mass-fiber  presumably  derived  from  basic 
rocks. 

The  production  of  asbestos  in  the  United  States  is  small.  In 
1915  it  amounted  to  1,731  short  tons,  valued  at  $76,952.  The 
value  of  the  annual  Canadian  output  is  about  $3,000,000.  As- 


584      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

bestos  is  used  for  making  fireproof  theater  curtains,  ropes, 
clothing,  etc.  When  felted  it  is  a  good  nonconductor  of  heat 
and  electricity  and  finds  many  uses  as  an  insulator.  The  lower 
grades  are  mixed  with  cement  and  manufactured  into  fireproof 
shingles.  These  are  cheap  and  for  some  purposes  are  superior 
to  tile  and  slate.  Asbestos  plaster  is  used  in  theaters  to  deaden 
noise.  Boards  are  made  of  asbestos  and  cement.  The  demand 
is  increasing,  especially  for  the  low-priced  materials.1 

MONAZITE  AND  XENOTIME 

Monazite  is  normally  cerium  phosphate  (CePC^),  but  rare- 
earth  metals — thorium,  praseodymium,  lanthanum,  neodymium 
— are  generally  present  in  it.  Xenotime  (YtP04)  is  a  phosphate 
of  yttria,  with  rare  earths  of  the  cerium  group.  These  minerals 
are  exploited  for  the  rare  earths  they  contain,  especially  for 
thorium,  which  is  used  for  making  glowers  and  mantles  for 
lights.  The  principal  production2  is  derived  from  the  Carolinas, 
east  of  the  Blue  Ridge.  The  country  is  an  area  of  gneiss, 
schist,  and  granite,  cut  by  various  intrusive  rocks,  including 
extensive  dikes  of  pegmatite.  The  monazite  is  mined  mainly 
from  placers,  but  some  is  mined  from  pegmatized  gneiss  in  place, 
especially  where  the  gneiss  has  rotted. 

ZIRCON 

Small  amounts  of  zircon  (ZrO2)  are  present  in  many  igneous 
rocks.  Some  pegmatites  contain  considerable  amounts.  Zircon 
resists  weathering  processes  and  is  concentrated  in  stream  gravels 
derived  from  zircon-bearing  rocks.  In  the  Carolinas  zircon  is 
associated  with  monazite  in  gravels.  Near  Zirconia,  Henderson 
County,  North  Carolina,  zircon  is  obtained  by  washing  the  de- 
composed croppings  of  a  pegmatite  dike.3  Zircon-bearing  pegma- 
tites occur  also  near  Cash  in  the  Wichita  Mountains,  Oklahoma.4 

Zircon  is  used  for  the  manufacture  of  zirconium  compounds, 
which  are  used  in  making  glowers  for  the  Nernst  electric  light. 

1DiLLER,  J.  S.:  U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2,  p. 
995,  1912. 

2  PRATT,  J.  H.,  and  STERRETT,  D.  B.:  Monazite  and  Monazite  Mining 
in  the  Carolinas.     Am.  Inst.  Min.  Eng.  Trans.,  vol.  40,  pp.  488-511,  1909. 

3  STERRETT,  D.  B. :  Monazite  and  Zircon.     U.  S.  Geol.  Survey  Mineral 
Resources,  1911,  part  2,  pp.  1193-1196,  1912. 

4  STERRETT,  D.  B.:  Monazite  and  Zircon.     U.  S.  Geol.  Survey  Mineral 
Resources,  1907,  part  2,  p.  792,  1908. 


DEPOSITS  OF  THE  NON METALS  585 


WATER 

Occurrence. — Water  (H20)  is  essential  for  animal  and  vegetable 
life,  and  it  plays  a  part  in  the  genesis  of  all  minerals.  A  part  of 
the  rain  water  that  falls  upon  the  earth  soaks  into  the  ground 
and,  where  openings  are  available,  reissues  as  springs.  The  dis- 
tribution and  movements  of  ground  water  are  mentioned  under 
weathering  and  also  in  the  discussion  of  the  enrichment  of  ores. 
Nearly  everywhere  the  ground  contains  some  water,  but  the 
amount  is  exceedingly  variable,  depending  on  the  climate,  the 
structure  of  the  rocks,  the  distribution  of  fissures,  etc. 

Below  the  vadose  zone  (page  132)  the  rocks  are  generally 
saturated  with  water.  At  greater  depths,  however,  the  rocks 
because  of  pressure  are  tight,  and  water  if  present  is  nearly  static. 
Estimates  of  the  amount  of  water  in  the  earth's  crust, l  stated  in 
terms  of  the  thickness  of  a  uniform  blanket  of  water  spread  over 
the  surface,  range  from  less  than  100  feet  to  nearly  1,000  feet. 

Almost  all  deep  mines  encounter  more  or  less  water.  Wells 
sunk  for  water  frequently  encounter  a  supply  only  a  few 
feet  below  the  surface.  Some,  however,  go  hundreds  of  feet 
to  water,  the  depth  depending  on  local  conditions.  On  the  flood 
plains  of  rivers  water  is  generally  reached  near  the  surface. 
Many  wells  in  glacial  drift  encounter  water  at  shallow  depths. 
Some  deep  borings  are  dry. 

Artesian  Water. — Deep  borings  encountering  abundant  water 
supply  are  commonly  termed  artesian  wells  (from  Artois,  France, 
where  wells  of  this  sort  have  long  been  used).  Originally,  how- 
ever, this  term  was  applied  only  to  flowing  wells.  The  condi- 
tions that  are  favorable  to  the  development  of  an  artesian  circu- 
lation are  (1)  a  porous  bed  of  size  sufficient  to  serve  as  a  carrier 
and  reservoir,  (2)  an  impervious  bed  above  it,  (3)  a  collecting 
area  higher  than  the  point  of  issue.2 

Sandstones  and  gravels  are  good  water  carriers.  The  total 
pore  space  of  such  rocks  commonly  amounts  to  10  per  cent,  or 
more.  Because  of  their  storage  capacity  thick  sandstones  are 

1  VAN  HISE,  C.  R. :  A  Treatise  on  Metamorphism.  U.  S.  Geol.  Survey 
Mon.  47,  pp.  123-657,  1904. 

FULLER,  M.  L.:  The  Amount  of  Free  Water  in  the  Earth's  Crust.  U.  S. 
Geol.  Survey  Water-Supply  Paper,  160,  1906. 

2CHAMBEKLiN,  T.  C. :  Requisite  and  Qualifying  Conditions  of  Artesian 
Wells.  U.  S.  Geol.  Survey  Fifth  Ann.  Rept.,  pp.  131-173,  1884. 


586      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

better  carriers  than  thin  sandstones.  Fractured  limestones  or 
other  fractured  rocks  may  be  carriers  or  reservoirs  of  water. 
Amygdaloidal  lavas  have  high  pore  space  and  may  carry  water, 
but  because  their  openings  are  not  connected  they  are  generally 
inferior  to  sandstones.  The  shafts  that  penetrate  many  tilted 
vesicular  flows  at  copper  mines  of  Keweenaw  Point,  Mich.,  are 
dry  in  the  deeper  levels. 

For  an  artesian  flow  it  is  necessary  that  the  pervious  beds  be 
capped  by  impervious  or  nearly  impervious  rocks  (Figs.  209, 


FIG.  209. — Diagram  illustrating  chief  requisite  conditions  of  artesian  wells. 
(After  T.  C.  Chamberlin,  U.  8,  Geol.  Survey.) 

210),  for  otherwise  the  water  would  escape  and  the  water  pres- 
sure would  be  lowered  to  a  point  where  water  would  not  rise  in 
openings.  In  general  shale  forms  the  best  cap  above  the  per- 
vious beds.  The  character  of  the  rocks  below  the  water  carrier 
is  not  important  if  such  rocks  do  not  crop  out  below  the  carrier. 
If  the  underlying  rocks  are  impervious  they  will  hold  the  water 
in;  if  they  are  pervious  and  have  no  outlet  they  will  be  filled  with 
stagnant  water  that  will  prevent  the  downward  escape  of  water 


FIG.  210. — Diagram  illustrating  the  thinning  out  of  a  porous  bed  between 
two  impervious  beds.     (After  T.  C.  Chamberlin,  U.  S.  Geol.  Survey.) 

in  the  carrier.  The  water-bearing  bed  should  be  exposed  at  a 
point  above  the  point  of  issue.  The  water  will  riot  rise  as  high 
as  the  point  of  entry  because  of  loss  of  pressure  due  to  friction. 
In  crystalline  rocks,  such  as  granites  and  schists,  where  joints 
and  fissures  are  numerous  and  closely  spaced,  these  openings  may 
serve  as  reservoirs  of  underground  water.  In  general  it  is  neces- 
sary to  pump  the  water,  and  as  a  rule  the  flow  is  not  great.  In 
some  mines  in  crystalline  rocks,  however,  the  flow  of  water  is  very 
large  after  numerous  drifts  and  crosscuts  have  been  run  into  the 
surrounding  rocks.  As  a  rule  small  pumps  can  handle  the  water 


DEPOSITS  OF  THE  NONMETALS  587 

encountered  in  sinking  shafts  in  crystalline  rocks,  and  unless 
master  fissures  are  cut,  the  amount  per  foot  of  opening,  of  water 
flowing  to  the  shaft  decreases  a  few  hundred  feet  below  the 
surface."1 

Mineral  Waters.— Natural  waters  are  rarely  pure.  Even 
rain  water  carries  some  mineral  salts,  as  well  as  carbon  dioxide 
and  a  little  oxygen.  Many  minerals  are  attacked  by  water,  and 
as  rain  water  percolates  through  soil  or  rocks  it  gathers  mineral 
matter,  which  it  carries  in  solution. 

Mineral  waters  are  grouped  as  table  and  medicinal  waters. 
Table  waters  are  generally  not  high  in  mineral  salts;  if  they  were 
their  taste  and  physiologic  effects  would  render  them  unpopular. 
Waters  low  in  mineral  salts  and  high  in  carbonic  acid  are  highly 
prized  for  table  use  because  of  the  palatable  quality  imparted 
by  the  acid.  Nearly  all  the  carbonated  waters  of  the  trade  are 
simply  pure  water  or  water  of  low  salinity  artificially  carbonated. 
The  United  States  in  1915  produced  52,113,503  gallons  of  mineral 
waters  valued  at  $5, 138,794. l 

Medicinal  waters  are  prized  because  of  their  therapeutic 
qualities.  Waters  carrying  less  than  150  parts  per  million  of 
total  mineral  matter  are  classed  as  "low  in  mineral  matter,"2 
those  carrying  more  than  500  parts  are  "high,"  and  those  carrying 
more  than  2,000  parts  are  "very  high."  As  a  rule  waters  that 
contain  more  than  1,000  parts  of  salts  per  million  are  not  palata- 
ble, yet  waters  with  2,500  parts  per  million  may  be  used  for  many 
days  without  discomfort,  and  some  persons  can  withstand  waters 
with  more  than  3,300  parts  per  million.  Waters  with  as  much 
as  5,000  parts  per  million  are  inimical  to  health. 

It  has  been  questioned  whether  the  mineral  waters  of  some 
resorts  possess  the  therapeutic  effects  that  are  ascribed  to  them. 
Some  of  these  waters  carry  a  mineral  content  lower  than  that  of 
city  supplies.  Some  of  the ' '  cures ' '  reported  to  have  been  effected 
at  these  resorts  are  doubtless  due  to  changes  in  the  environment 
and  manner  of  living  of  the.  patients  and  to  copious  drinking  of 
water,  which  under  some  conditions  is  beneficial.  Lithium  salts 
are  commonly  supposed  to  relieve  uric  acid  poisoning,  but  this 

1  Discussions  of  the  water  supply  of  the  United  States  are  given  in  the 
water-supply  papers  of  the  United  States  Geological  Survey. 

2  DOLE,  R.  B. :  The  Concentration  of  Mineral  Water  in  Relation  to  Thera- 
peutic Activity.     U.  S.  Geol.  Survey  Mineral  Resources,  1911,  part  2,  pp. 
1175-1192,  1912. 


588      THE  PRINCIPLES  OF  ECONOMIC  GEOLOGY 

has  been  questioned.  Sulphates  in  moderately  large  doses  are 
cathartic.  Magnesium  carbonate  arid  bicarbonate  are  alkaline 
and  if  given  in  small  doses  will  correct  acidity  of  the  stomach. 
Boron  salts  are  antiseptic,  and  some  barium  salts  are  supposed  to 
correct  arterial  troubles.  Iron  salts  are  tonic.  Arsenic  salts 
above  small  amounts  are  poison. 


INDEX 


Abrasives,  548 

Acmite,  13,  23,  148 

Actinolite,  32,  148,  290 

Additions     during     contact     meta- 

morphism,  37 

Adirondack  region,  N.  Y.,  344 
Adularia,  71 
^Egirite,  13,  148 

Affinity  of  metals  for  sulphur,  157 
African  gold  deposits,  420 
Age,  of  certain  metalliferous  deposits, 

403 
of   deposits,  of  deep  vein  zone, 

57 
of  dynamically  metamorphosed 

deposits,  123 
of  earth,  287 
of  intermediate  zone,  67 
Agents  of  mineralization,  20 
Aggradational  deposits,  8 
Ajo,  Ariz.,  384 
Alabandite,  508 
Alaska  copper  deposits,  394 
Albite,  13,  23,  32,  51,  148 
Allanite,  13,  23,  32,  148 
Alaska  Gold   Mines  Company,  de- 
posits, 433 
Albemarle  County,  Va.,  weathering 

of  diorite  of,  127 
Alexo,  Ont.,  514 
Alkalies  in  thermal  waters,  271 
Altenberg,  Saxony,  tin  deposits,  51, 

235 

Alteration  of  ore  deposits,  124 
Altitude,   influence  on  enrichment, 

135 

Alum,  148 
Aluminum,  502 
uses  of,  503 

ore,  formed  by  weathering,  125 
Alunite,  71,  148,  502 


Alunitic  kaolinic  gold  veins,  255 
Amalgam,  401 

Amethyst  vein,  Creede,  Colo.,  194 
Amphibole,  13,  23,  32,  51,  119,  148 
Amygdaloidal  copper  deposits,  395 
Analcite,  13,  71,  148 
Analyses  of  fresh  and  altered  diorite, 
127 

of  hot  springs,  272 

of  mine  waters,  272 
Andalusite,  23,  32.  117,  148 
Andradite,  32,  51,  148 
Anglesite,  148,  490 
Anhydrite,  148,  567 
Ankerite,  32,  51,  65,  71,  148 
Annabergite,  511 
Anna    Lee    ore    chimney,    Cripple 

Creek,  Colo.,  96 
Anorthite,  13,  23,  32,  148 
Anthophyllite,  32,  148 
Anticlinal  deposits,  201 
Antimony,  148,  517 
Apatite,  13,  23,  32,  51,  65,  148,  577 
Apophyllite,  148 
Appalachian  bauxite  deposits,  506 

gold  deposits,  424 

depth  of  formation,  54 

region,  secondary  sulphide  zones 
in,  141 

valley,  iron  ores,  334 
Aquamarine,  23,  148 
Aqueo-igneous  deposits,  4 
Aragonite,  148 
Arfvedsonite,  13 
Argentite,  65,  148,  439,  443 
Arkansas,  aluminum  deposits,  505 

antimony  deposits,  518 

diamond  deposits,  554 
Arsenic,  148,  519 
Arsenopyrite,  23,  32,  51,  65,  71,  148, 

519 

Artesian  water,  585 
Asbestos,  581 


590 


INDEX 


Asbolite,  515 

Ascending   hot   solutions,    composi- 
tion, 271 
origin,  276,  283 
Aspen,  Colo.,  453 
Atacamite,  148,  345,  355 
Atlin  district,  B.  C.,  fractures  of,  212 
Atmosphere,  124 
Augite,  13,  23,  32,  148 
Aurichalcite,  148,  475 
Auriferous  conglomerates,  419 
Autunite,  526 
Axinite,  32 
Azurite,  148,  345,  355 


B 


Balaklala,  Calif.,  387 
Banded  ore,  2 

Banding,    in    contact   metamorphic 
deposits,  36 

in  replacement  deposits,  225 
Barite,  65,  71,  148,  557 
Baritic  fluorite  veins,  263 
Basal  conglomerate,  ore  of,  98 
Base-leveling,   influence  of,   on  ore 

enrichment,  135 
Basic  rocks,  magmatic  segregations 

in,  12 

Bassic  mine,  Colo.,  97 
Bastite,  148 

Bauxite,  65,  71,  88,  148,  502 
Bedding  planes,  173 
Bedding  plane  deposits,  66,  70,  71, 
81,  105,  111,  198 

faults,  105,  111 

Bendigo,    Victoria,    false    inverted 
saddles,  208 

saddles,  204 
Beryl,  23,  32,  148 
Bindheimite,  517 
Bingham,  Utah,  copper  deposits,  365 

depth  of  secondary  zones,  141 

wall  rock  alteration  at,  247 
Biotite,  13,  23,  32,  51,  148 
Birmingham,  Ala.,  323 

anticlinorium,  326 
Bisbee,  Ariz.,  deposits,  367 

ore  in  syncline,  208 


Bismite,  520 

Bismuth,  23,  65,  148,  520 

Bismuthinite,  23,  32,  51,  65,  71,  148, 

520 

Bismutite,  520 
Bituminous  matter  as  a  precipitating 

agent,  76 

Biwabik  iron  formation,  89,  304 
Black  Hills,  S.  Dak.,  gold  deposits, 

426 

tungsten  deposits,  525 
verticals  of,  197 
Blades  of  silica,  72 
Blind  chimney,  201 
Blind  deposit,  defined,  94 
Block  of  oak,  deformed,  180 
Bonanza  mine,  Alaska,  394 
Bonanzas,  72,  93 
Boracite,  573 
Borax,  572 
Bornite,  23,  32,  51,  65,  71,  148,  345, 

357 

deposits  in  sandstone,  77 
Bort,  148 

Boron  compounds,  571 
Boulder,    Colo.,    tungsten    deposits, 

525 

Boulder  Hot  Springs,  Mont.,  267 
Boulder-Leadville   belt,    Colo.,    447 
Boundaries  of  replacement  deposits, 

226 

Bournonite,  517 
Breccia  vein,  188 
Brecciated  vein,  191 
Breckenridge,  Colo.,  449 
Bremen  mine,  New  Mex.,  97 
Brewster  County,  Texas,  quicksilver 

deposits,  500 

Brinton,  Va.,  arsenic  deposits,  519 
Brochantite,  345,  355 
Bromine,  566 
Bromyrite,  148,  439 
Brookite,  148 
Brown  iron  ore,  334 
Brucite,  148 
Building  stones,  53-3 
Bullfrog,  Nev.,  deposits  of,  72 
Bullion,  Nev.,  garnet  vein  at,  34 
Bully  Hill,  Calif.,  deposits  of,  387 


INDEX 


591 


Bunker    Hill    and    Sullivan    lode, 

Idaho,  193,  495 
Buried  placers,  416 
Burnt  umber,  560 
Burro  Mountain  district,  New  Mex., 

383 

Butte,  Mont.,  copper  deposits,  357 
faulting  at,  102,  109 
fractures  of,  212 
mine  waters,  159 
ore  shoots,  97 

relation     of    quartz    porphyry 
dikes  and  ore  deposits,  279 
wall-rock  alterations,  242 
zinc  deposits,  485 
zonal  distribution  of  ores,  243, 
280 


Cable  mine,  near  Anaconda,  Mont., 

46,  466 
magnetite  ores,  38 

Cactus  mine,  Utah,  236 

Cadmium,  529 

Calamine,  148,  474,  475 

Calaverite,  148,  401 

Calcasieu  Parish,  La.,  sulphur   de- 
posits, 569 

Calcite,  23,  32,  51,  65,  71,  148 

Calcitic  cinnabar  veins,  261 

California,  antimony  deposits,  519 
gold  belt,  60,  430 
quicksilver  deposits,  500 

Calomel,  148,  499 

Calumet  copper  deposits,  Mich.,  395 

Camp  Bird  vein,  Colo.,  455,  456 

Cancrinite,  13,  148 

Capillary  openings,  170 

Carbon    dioxide,    precipitation    by 
escape  of,  76 

Carbonaceous  matter  as  a  precipi- 
tating agent,  76 

Carbonate  iron  ore,  333 

Carinate  folds,  118 

Carnotite,  527,  528 

Carolina  tin  belt  523 

Cassiterite,  13,  23,  32,  51,  88,  148, 

521 
veins,  233 


Cave  deposits  in  limestone,  80 
Cavities  in    replacement    deposits, 

225 

Celestite,  65,  71,  558 
Cement  materials,  546 
Cements,  542 
Cerargyrite,  148,  439,  443 
Cerusite,  148,  490 
Cervantite,  517 
Ceylon  graphite  deposits,  557 
Chalcanthite,  148,  345,  355 
Chalcedonic    and   calcitic   cinnabar 

veins,  261 
Chalcedony,  71 
Chalcocite,  65,  345,  355 

deposited  on  coal,  78 

deposits  in  sandstone,  77 
Chalcopyrite,  13,  23,  32,  51,  65,  71, 

119,  345,  357 
Chambered  deposit,  201 

vein,  201,  262 
Chamberlin  hypothesis,  288 
Chamberlin,  Tenn.,  iron-ore,  330 
Chambers,  201 

Champion,  deposits,  Mich.,  317 
Channels,  ore  shoots  along,  96 
Chattanooga,  Tenn.,  iron-ore  depos- 
its, 87,  329 

Chemical  concentration  by  weather- 
ing, 85 

Chemical  denudation,  287 
Chert,  65,  71,  148 
Chile,  niter,  574 

tourmaline  veins,  236 
Chimneys  of  ore,  199 
Chino  Copper  Co.,  deposits,  381 
Chitina    copper    deposits,    Alaska, 

394 

Chloanthite,  511 
Chlorite,  32,  51,  65,  71,  119,  148 
Chloritic  alteration,  248 

in  granite,.  249 

in  lavas,  249 

Chroniite,  13,  23,  32,  88,  148,  507 
Chromium,  507 
Chrysocolla,  148,  345,  355 
Cinnabar,  499 

veins,  51,  65,  71,  148,  261,  499 
Circulation  of  ground  water,  75,  132 


592 


INDEX 


Classification    of    primary    ore    de- 
posits, 4,  7 
Clay,  535 
Clifton,  Ariz.,  deposits  of,  376 

hydrothermal  alteration  at,  245 
Climatic  influences  on  enrichment. 

135 

Clinton  hematite  iron  ore,  323,  330 
analyses,  330 
genesis  of,  89 

Clinton,  New  York,  iron  ore,  331 
Cobalt,  Ont.,  deposits,  63,  444,  515 
cobalt  in,  516 
fracture  pattern,  214 
Cobaltite,  65,  71,  148,  515 
Coeur  d'Alene  district,  Idaho,  deep 
zone  veins  grade  into  inter- 
mediate zone  veins,  60 
lead-silver  deposits,  493 
relation  of  lead-silver  deposits 

to  igneous  contacts,  282 
wall-rock  alteration,  241 
zinc  deposits,  486 
Cold  solutions,  deposits  formed  by, 

4,  6,  74 

Colemanite,  573 
Collapse  during  alteration,  220 
Colorado  uranium  deposits,  528 
Colorados,  138 
Columbite,  23 

Combination  mine,  Mont.,  Ill 
Comb  structure,  2 
Composition  of  descending  thermal 
metalliferous  waters,  271 
of  magmatic  segregations,  13 
Compression  fractures,  178 
Compressive  stress,  179 
Comstock  Lode,  Nevada,  70,  71,  467 
fissures  of,  193 
mine  waters,  160 
Concrete,  544 
Conglomerate,  ores  of,  86,  89 

copper  deposits,  395 
Conjugated  fractures,  179,  208 
Connections    with    surface    during 

vein  formation,  56 
Contact  metamorphic  deposits,  4,  29 
depth  of  formation,  43 
distance  from  intrusions,  29 


Contact  metamorphic  deposits,  func- 
tion of  mineralizers,  43 
minerals  of,  44 
occurrence,  29 

ore  of,  relation  to  porosity,  223 
rocks  altered,  32 
rocks  causing,  32 
surface  enrichment  of,  135 
temperature  of  formation,  45 
texture  of,  36 
metamorphism,  29-48 
age  of,  42 

at  Cable,  Montana-,  30 
at  Cananea,  Mexico,  31 
at  Coeur  d'Alene,  Idaho,  32 
at  Morenci,  Arizona,  32 
at  Philipsburg,  Montana,  30 
at  Valardena,  Mexico,  31 
depth  of,  43 

endomorphism  attending,  47 
relation  to  fissuring,  33,  46 

Contacts,  relation  of  fissures  to,  197 
of   replacement   deposits,  226 

Contour  of  water  table,  131 

Convection  currents  in  magmas,  10 

Cooling  cracks,  175 

Coordinated  fractures,  208 

Copper  deposits,  71,  149,  345,  354 
age  of,  351 
genesis  of,  348 

mineral  composition,  345,  354 
outcrops,  351 
production,  346,  347 
porphyry  deposits,  349 

Copper  pitch  ore  356 

Copperopolis,  Calif.,  389 

Cordierite,  13,  32,  148 

Corkscrews,  201 

Cornwall,  Pa.,  magnetite  ores,  39 

Cornwall  tin  veins,  52,  233,  234 

Coronado  vein,  Ariz.,  63,  194 

Corundum,  13,  23,  32,  149,  502,  549 
gem,  554 

Country  rock,  2 

Covellite,  65,  345,  356 

Creede,  Colo.,  deposits,  70,  194,  458 
banded  ore  of,  3 
hot  springs,  265 
mine  waters,  160 


INDEX 


593 


Cretaceous  gold  veins,  403 
Crevices  in  Wisconsin  zinc  region,  80 
Cripple  Creek  district,  Colo.,  70,  434 

mine  gases,  285 

ore  chimneys,  96 

wall-rock  alteration,  254 

water  circulation,  278 
Crown    Point,  New  York,  graphite 

deposits,  556 

Crushing  strength  of  rocks,  115 
Gratification,  2,  225 

figure  showing,  3 
Crustified  banding,  2 
Cryolite,  51,  149,  502,  559,  560 
Crystal  boundaries,  in  replacement 

deposits,  225 
Crystal  Falls,  Mich.,  315 
Crystallization,  force  of,  176 
Crystallizers,  21 
Cuban  laterites,  origin  of,  127 
Cuprite,  149,  345,  355 
Curry  iron-bearing  formation,  Mich- 
igan, 316 

Cuyuna  Range,  Minn.,  iron  deposits, 
90,  309 

manganese  deposits,  510 


Dahlonega  mines,  Ga.,  52,  232,  425 
Dale,  Calif.,  deposits,  341 
Daubree,  experiment  of,  179 
Deep  vein  zone  deposits,  4,  5,  49 
age  of,  57 

depth  of  formation,  54 
gradation    into    contact    meta- 

morphic  deposits,  61 
into  pegmatites,  57 
into  veins  formed  at  moderate 

depths,  59 

mineral  composition  of,  50 
occurrence,  50 
size  of  deposits,  52 
texture  of  deposits,  52 
Deeper  circulation,  132 
Deer  Isle,  Maine,  schistose  ores  of, 

121 
Deerwood    iron-bearing    formation, 

310 
13 


Definitions,  1,  2,  3 
Deformation,  100 
provinces,  101 
Dehydration  cracks,  175 
De  Lamar,  Idaho,  veins,  72,  248 
Deposition  of  secondary  sulphides 

above  water  level,  143 
Deposits,  formed  at  moderate  depths 

by  hot  solutions,  62 
formed  at  shallow  depths  by  hot 

solutions,  69 
distance     of,     from     intrusive 

rocks,  63 

formed  by  magmatic   segrega- 
tion, 9 

of  deep  vein  zone,  49 
Depth,  factor  in  deformation,  100 
of  contact  metamorphism,  43 
of  formation  of  deposits  of  deep 

vein  zone,  53 
of  oxidized  zone,  140 
Desilverizing  lead  ore,  11 
Diallage,  13,  149 
Diamond,  13,  23,  552 
Diaspore,  502 
Diatomaceous  earth,  550 
Differentiation  of  magmas,  10 
Diffusion  in  magmas,  10 
Diopside,  13,  23,  32,  51,  149 
Diorite,  behavior  of,  during  weather- 
ing, 127 

Direction  of  movement,  110 
Dislocation  along  faults,  105 
Displacement  along  faults,  104,  105 
Disseminated   copper   ores,    hydro- 
thermal  alteration,  244 
Disseminated  deposits,  67,  188 
Dolcoathlode,  Cornwall,  Pa.,  52,  233 
Dolomite,  32,  51,  65,  71,  149 
Dolomibization  cracks,  175 
Downward  enrichment,  130 
Drag  ore,  109 
Druse,  2 

Ducktown,  Term.,  336,  390,  425 
depth  of  secondary  zone  at,  141 
figure     showing     oxidation    of 

deposits,  164 

mineral  changes  during  oxi- 
dation, 165 


594 


INDEX 


Ducktown,  Tenn.,  mine  waters,  159 
table   showing    oxidation    of 

ores,  165 

Dunnville  sandstone,  172 
Dynamic  metamorphism  of  ore    de- 
posits, 114 
material  added,  119 

subtracted,  119 

Dynamically     metamorphosed     de- 
posits, age  of,  123 

E 

Eagle    Mountain,    Calif.,    iron    de- 
posits, 339 
Earth,  failure,  178 

heat  gradient,  75 

origin,  288 

rigidity,  288 

self  compression,  288 

shrinkage,  114 

volume,  286 

water,  286 
East  Tennessee,  iron  deposits,  328 

zinc  deposits,  484 
Economic  geologist,  work  of,  1 
Eglestonite,  501 
Elaeolite,  13,  149 
Electrum,  401 
Elements  of  faults,  105 
Elkhorn,  Mont.,  deposits,  206 
Elongation   during   dynamic   meta- 
morphism, 117 

El  Paso,  Texas,  tin  deposits,  236,  524 
Ely  greenstone,  321 
Ely,  Minn.,  iron  deposits,  322 
Ely,  Nevada,  copper  deposits,  379 
Embolite,  439 
Emerald,  23,  32,  149,  554 
Emery,  13,  23,  32,  149,  550 
Enargite,  65,  149,  345,  357,  519 
End  products  of  weathering,  84 
Endomorphic    changes    in    contact 

metamorphism,  47 
Enduring  surface,  216 
Encampment,  Wyo.,  389 

extent  of  chalcocite   zones  at, 

141 

pegmatites   grade   into   quart? 
veins,  26 


England,  tin  deposits  of,  235 
Enrichment,    diagram   showing  im- 
portance of,  with  respect  to 
several  metals,  167 

general  features,  124 

silver  deposits,  441 

sulphide,  130 
Enstatite,  579 
Eolian  deposits,  8 
Eolian  gold  deposits,  416 
Epidote,  33,  51,  119,  149 
Epigenetic  deposits,  4 

structural  features,  184 
Equidimensional  deposits,  199 
Erosion,  influence  of  on  enrichment, 
136 

of  lodes,   estimate  of  amount, 

153 

Erythrite,    515 

Erzgebirge  uranium  deposits,  526 
Eureka,  Nevada,  446 
Eutectics,  18 
Experiments,  in  compression,  121 

in  contact  metamorphism,  44 

in  precipitation"  of  metals,  154 

in  solution  of  metals,  154 


Fahlband,  190 

False  inverted  saddle,  207 

saddle,  203 
Famatinite,  357 
Farncombe  Hill,  Colo.,  449 
Fault,  definition,  103 

dip,  definition,  103 

fissure,  definition,  184 
vein,  186 

mozaic,  107 

patterns,  110 

stria?,  107,  110' 

strike,  definition,  103 
Faulted  segments,  search  for,  107 
Faulting,  101 

combined  with  flowage,  118 

of  ore  deposits,  102 
Faults,  deposition  in,  192 

mineralized,  in  limestone,  194 
Fayalite,  149 


INDEX 


595 


Feldspars,  502,  640 

abrasive,  550 
Ferberite,  524 
Fertilizer^,  575 
Fierro,  New  Mexico,  339 
Fissures,  184 

size  of,  185 

Flat  River,  Mo.,  deposits,  491 
Flats  and  pitches,  81,  190 
Flaxseed  ore,  329,  331 
Florence  district,  Wis.,  315 
Flowage,  zone  of,  101 

of  minerals  compressed  in  tubes, 

121 

of  ore  deposits,  113 
Fluid  inclusions,  in  igneous  rocks, 

284 

in  veins,  282 
Fluorite,  13,  23,  33,  51,  65,  71,  149, 

529 
deposited    in    veins    near    hot 

springs,  266 

Fluorite-barite  veins,  263 
Fluoritic     tellurium-adularia     gold 

veins,  254 

Folding,  of  ore  deposits,  102,  113 
in  zone  of  flowage,  101 
in  zone  of  fracture,  101 
Foothill  copper  belt,  Calif.,  388 
Foot  wall,  103 
Force    of    crystallization    in    vein 

formation,  55,  176 
Forsterite,  33,  149 
Fossil  ores,  89,  329,  331 
Fourche  Mountain  aluminum  depos- 
its, Ark.,  503 

Fractional   crystallization    in    mag- 
mas, 10 
Fracture,  systems,  208 

zone  of,  101,  115 
Fractured  zone,  187 
Fractures,  formed  by  compression, 

178 

on  anticlines,  181 
in  synclines,  207 
tensional,  180 

Fracturing,  by  pressure,  179 
Fragments  in  replacement  deposits, 
227 


Franklin  Furnace,   N.  J.,  deposits, 

208,  487,  511 

Franklinite,  33,  149,  290,  470 
Freibergite,  444 
Frisco  district,  Utah,  496 
Fuller's  earth,  539 

G 

Gahnite,  51,  149 

Galena,  15,  23,  33,  51,  65,  71,  149, 

489,  490 
Gangue,  2 

mineral,  2 
Gap,  106 
Garnet,  13,  23,  33,  51,  88,  117,  149, 

549,  556 

veins,  Dahlonega,  Ga.,  53 
zones,  39 

Garnetiferous  gold  veins,  232 
silver-copper  veins,  232 
silver-lead  veins,  233 
Garnierite,  511 
Gas,  in  lavas,  286 

in  metal  mines,  285 
Gash  vein,  189 

in  Wisconsin,  80 
Gay-lussite,  149 
Gems,  552 

Geological  Society  of  America,  re-" 
port  of  committee  on  faults, 
102 

Georgetown,  Colo.,  relations  of  igne- 
ous dikes  and  ore  deposits, 
279 

Gersdorffite,  511,  512 
Gibbsite,  149,  502 
Gilpin    County,    Colo.,    gold-silver 

deposits,  450 
pitchblende  deposits,  526 
Glacial  deposits,  8      . 
gold  deposits,  416 
Glaciation,   relation  of  enrichment 

to,  137 

Glass  sand,  551 
Glasses,  19 

Glauconite,  32,  149,  290 
Globe,  Ariz.,  deposits,  371 

range  of  chalcocite  ore,  141 
Goethite,  290 


596 


INDEX 


Gogebic  range,  Mich.,  311 

length  of,  89 

Gold,  13,  23,  33,  51,  65,  71,  88,  149 
deposits,  401 
age  of,  403 
concentration      in      oxidized 

zone,  408 
enrichment,  405 
enrichment  and  chalcocitization, 

410 

in  hot  spring  deposits,  268 
lodes,  relation  to  placers,  415 
ores,  genesis  of,  402 

tenor  of,  401 
placers,  87,  408,  411 
minerals  in,  414 
relation  to  gold  lodes,  415 
Goldfield,  Nev.,  70,  71,  437 
fracture  patterns,  213,  214 
source  of  metal-bearing  waters, 

279 

wall-rock  alterations,  256 
Gold-silver  adularia  veins,  248 
Goslarite,  149,  474 
Gossan,  138 
Gossan  Lead,  Va.,  336 
Gouge,  2 
Graben,  107 

'  Gradient  of  earth's  heat,  75 
Granite,  533 
Granite-Bimetallic      mine,      Mont., 

secondary  ore  of,  146 
Graphic  intergrowths  of  sulphides, 

147 

Graphite,  13,  23,  33,  51,  556 
Grass  Valley,  Calif.,  deposits  of,  433 

wall  rock  alteration,  239 
Greenalite,  290,  307 
Greenockite,  149,  529 
Greisen,  235  . 
Grindstones,  548       ~ '•: 
Grooves,  107 
Grossularite,  33 
Groundwater,  131 
circulation,  75 
Guano  fertilizer,  576 
Gummite,  526 
Gypsum,.  149,  567 

fertilizer,  576          '  • 


Hachita,  N.  M.,  garnet  vein  at,  34 

Haile  mine,  S.  C.,  425 

Hailey,  Idaho,  wall  rock  alteration, 

240,  241 
Halite,  149 

Hanging  wall,  definition  of,  102 
Hanover,  N.  M.,  deposits  of,  339 
Haiiynite,  13,  149 
Hartville,  Wyo.,  iron  ores,  338 
Hausmannite,  508 
Hazel  Green  mine,  Wis.,  79 
Headlight  vein,  Mont.,  Ill 
Heat,  gradient  of  earth,  75 
transfer  in  earth,  289 
Heave  of  faults,  105 
Heavy  residuals  in  placers,  90 
Heavy  silicate  zones,  39 
Hedley,  B.  C.,  contact  metamorphic 

deposits,  32 

Hematite,  13,  23,  33,  51,  88,  149,  290 
deposits     of     western     United 

States,  338 

Hibbing,  Minn.,  deposits  of,  302 
Homestake  mine,  S.  D.,  426 
Horizontal  shear,  181 
Hornblende,  13,  23,  33,  51,  149 
Horse,  diagram  showing,  103 
Hope  mine,  Mont.,  202 
Horst,  107 

Horn  Silver  mine,  Utah,  497 
Hot  springs,  265,  275 

analyses  of  waters,  265 
Anaconda,  Mont.,  267 
Boulder  Hot  Springs,  Mont., 

260 
Norris  Geyser  Basin,  Mont., 

267 
Ojo    Caliente,   New   Mexico, 

266 
Wagon    Wheel    Gap,    Colo., 

265 

deposits,  263 
smelted  for  gold,  268 
Hot  waters  of  Wagon  Wheel  Gap, 

Colo.,  265 
Hubnerite,  524 
Humites,  33,  149 


INDEX 


597 


Hurley,  Wis.,  deposits,  311 
Hydrogen  sulphide,  generation  with 

acid,  158 

precipitation  by,  76 
Hydrolysis  of  ferric  sulphate,  154 
Hydrometamorphisra,  123 
Hydrosphere,  124 

Hydrathermal  alteration,  3,  230,  231 
wanting  in  dynamically  meta- 
morphosed deposits,  122 
widths  of  zones  of,  221 
Hydrous  minerals  in  igneous  rocks, 

284 
Hydrozincite,  149,  474 


Idaho  Springs,  Colo.,  450 

mine  waters,  160 
Igneous  emanations,  12 

intrusion,  mechanism  of,  289 

rocks,  gases  in,  286 

metals  in,  287 

Ilmenite,  13,  23,  33,  51,  149,  290,  513 
Ilvaite,  33,  149 

Imbricating  blades  of  silica,  73,  144 
Incompetent  beds,  behavior  of,  118 
Infusorial  earth,  550 
Intergranular  spaces,  172 
Intermediate  depths,  deposits  formed 
at,  62 

vein  zone,  minerals  of,  65 
Intersections,  dikes  and  fissures,  66, 
199,  200 

ore  shoots  along,  96 
Intrusion,  mechanism  of,  289 
Intrusive  rocks,  relation  to  ore  de- 
posits, 283 
Inverted  saddles,  207 
Inyo  County,  Calif.,  deposits,  563 
lodyrite,  439 
Iron,  13,  290 

bacteria,  294 

deposited  in  lakes  and  seas,  294 

hat,  138 

in  earth,  291 

migration    during    weathering, 
293 

minerals,  290 


Iron,  per  cent,  of,  in  earth,  290 

production  in  United  States,  297 

Iron  Age  deposit,  Calif.,  341 

Iron  ore,  deposits,  290 

formed  by  weathering,  125 
formed  by  weathering  of  ser- 
pentine, 128 
genesis  of,  291 

Iron    Mountain,    Mo.,    deposits   of, 
86,  341 

Iron  Mountain,  Wyo.,  genesis  of,  94, 
343 

Iron  oxide  in  outcrops,  139 

Iron  River,  Mich.,  315 

Iron  Springs,  Utah,  deposits,  61,  388 

Ironwood    iron-bearing    formation, 
311 

Irregular  fracture  patterns,  212,  213 

Ishpeming  deposits,  Mich.,  317 

Isodiametric  deposits,  199 


Jadeite,  33,  149 
Jamesonite,  517 

Jarilla,  New  Mex.,  garnet  vein  at,  34 
Jaspurite,  515 
Jerome,  Ariz.,  385 
Joplin,   Mo.,  region,  brecciated  ore, 
82 

cadmium  in  ore,  529 

caves,  80 

ore  shoots,  97 
Juneau,  Alaska,  deposits,  433 


K 


Kalgoorlite,  71,  149 
Kant  hypothesis,  287 
Kaolin,  71,  88,  149,  502 
Keweenawan  basic  effusives,  314 

copper  deposits,  395 
Keweenaw  Point  veins,  Mich.,  260 
Kennedy     mine,     Cuyuna     Range, 

Minn.,  90 
Kennicott  copper  deposits,  Alaska, 

394 

Kimberly  diamond  deposits,  553 
Krennerite,  401 


598 


INDEX 


Kunzite,  555 
Kyanite,  23,  33,  149 


La  Colorada,  Mexico,  graphite  de- 
posits, 556 

Ladder  veins,  187 

Lake  City,  Colorado,  deposits  of, 
457 

Lake    Superior    region,  copper  de- 
posits, 70,  395 
formations,  300 
iron  deposits,  297,  298 
synclinorium,  298 

Lake  Valley  district,  New  Mex.,  sec- 
tion of,  228 

La  Motte  mine,  Mo.,  491 

Lancaster  Gap,  Pa.,  nickel  deposits 
of,  515 

Land  sediments,  7 

Laplace  hypothesis,  287 

La  Plata  Mountains,  Colo.,  457 

Last  Chance  mine,  Bingham,  Utah, 
wall-rock  alteration,  247 

Laterites,  origin  of,  127 

Laurite,  516 

Lead,  149,  489 

Leadhillite,  149 

Leadville,  Colo.,  451 

Lead-zinc  deposits,  segregation  by 
weathering,  476 

Ledge,  186 

Lens,  190,  89 

Lepidolite,  23,  51,  149 

Leucite,  13,  149 

Level  of  ground  water,  131 

Lila  C.  borax  mine,  Calif.,  573 

Lime,  545 

Limestone,  analysis  of  before  and 
after  contact  metamor- 
phism,  39 

Limonite,  88,  149,  290,  293 

Linden,  Wis.,  deposits,  81 

Lithium  minerals,  542 

Lithophysae,  284 

Lithosphere,  124 

Lockhart  vein,  52 

Lode,  184,  186 


Long  slender  deposits,  199 
Ludwigite,  33 

M 

Magmatic  differentiation,  9 

of  rocks,  9,  10 
Magmatic  segregations,  4,  5 
general  features,  9 
occurrence,  11 
ore  shoots  of,  94 
rock  fragments  in,  9 
shape,  13 
size,  14 
texture,  9,  14 

Magnesite,  71,  149,  579,  580 
Magnesium  minerals,  579 
Magnet  Cove,  Ark.,  rutile  deposits, 

532 
Magnetite,  13,  23,  33,  57,  65,  88,  149, 

290 

-bearing  veins,  60 
bodies  of  contact  metamorphic 

origin,  38 

ore,  New  Jersey,  337 
New  York,  337 
Pennsylvania,  337 
Malachite,  149,  345,  355 
Mallardite,  508 
Manganese,  508 

Cuyuna  range,  Minn.,  510 
deposits  formed  by  cold  solu- 
tions, 79,  80 
in  Blue  Ridge,  510 
in  gold  deposits,  405 
in  outcrops,  139 
use  and  production,  509 
Manganite,  149,  508 
Manhattan,  Nev.,  deposits,  72 
Marcasite,  71,  149,  290 
Marquette  district,  Mich.,  317 

synclinorium,  319 
Mary  Mine,  Tenn.,  section  of,  202 
Marysville,  Mont.,  72 
Massicot,  489 

Mayari  ore  deposits,  Cuba,  128,  129 
Meadow  Lake  district,  Calif.,  236 
Mechanical       concentration,       by 
weathering,  85 


INDEX 


599 


Medicinal  waters,  588 
Meerschaum,  579,  580 
Melaconite,  149,  345,  355 
Melanterite,  290 
Melilite,  13,  149 

Mendota  mine,  Colo.,  ore  of,  191 
Menominee  district,  Mich.,  315 
Mercury,  149,  499 

in  hot  spring  deposits,  262 
Mesabi  range,  Minn.,  deposits  of,  301 
contact  metamorphism  in,  37 
length  of,  89 
Metallogenic  epochs,  269,  270 

provinces,  2,  269 

Metamorphism, of  Modoc  formation, 
Morenci,  Ariz.,  39 
of  ore  deposits,  114 
Metasomatic  processes,  218 
Metasomatism,   in  vein  formation, 

218 

in  sulphide  enrichment,  165 
Meteoric    waters,    deposits    formed 

by,  75 

Metropolitan  district,  Mich.,  315 
Miami,  Ariz.,  deposits  of,  371 
Miami,  Okla.,  deposits  of,  477 
Miarolitic  cavities,  9,  174 
Mica,  540 
Michigan,  copper  deposits,  70,  395 

veins,  wall  alteration,  260 
salt  deposits,  563 
Microcline,  23,  149 
Microscopic  particle,  minimum  size 

of,  220 

Middle  zone  deposits,  4 
Milan  mine,  New  Hampshire,  ores 

of,  116,  191 
Millerite,  149,  511,  512 
Millstones,  548 

Minasragra,    Peru,    vanadium    de- 
posits, 529 

Mine  Hill  deposits,  N.  J.,  487 
Mine  waters,  272 

analyses,  159,  160 
changes  in  depth,  162 
composition  of,  158,  160 
general  character,  161 
precipitates  from,  162 
Mineral  associations,  230 


Mineral  associations,  in  contact 
metamorphic  deposits,  32, 
44 

fertilizers,  575 

paints,  560 

waters,  587 
Mineralizers,  21,  289 

activity  in  deep  vein  zone,  57 

elements  of,  in  contact  meta- 
morphic ores,  43 

Minerals,  lists  of,  13,  23,  32,  33,  51, 
65,  71,  88,  119,  148 

cold  solution  deposits,  80 

in  magmatic  segregations,  13 

of  sedimentary  deposits,  87 

primary  and  secondary,  148 

stable,  during  weathering,  126 
Minium,  489 

Mississippi  Valley  zinc  and  lead  de- 
posits, 74,  76 
Missouri,  barite  deposits,  558 

iron  deposits,  342 

lead  deposits,  491 

zinc  deposits,  477 

Moderate  depths,  deposits  formed 
at,  4,  6,  62 

composition,  79 

general  features,  62,  74 

minerals  of,  65 

occurrence,  76 

shape,  80 

size,  81 

texture,  81 

Molecular  replacement,  220 
Molecule,  size  of,  220 
Molten  magmas,  284 
Molybdenite,  13,  23,  33,  51,  65,  71, 

119,  149,  530 
Molybdenum,  530 
Molybdic  ocher,  530 
Molybdite,  149,  530 
Monazite,  13,  23,  33,  88,  149,  584 
Monte  Cristo,  Wash.,  deposits  of,  519 
Montroydite,  501 
Morenci,  Ariz.,  376 

chalcocite  ore,  144 

contact  zones,  39 

magnetite  ore,  38 

wall-rock  alteration,  246 


600 


INDEX 


Mortar,  544 

Mother  Lode,  Calif.,  430 

Mount  Mica,  Paris,  Me.,  95 

Moyie,  B.  C.,  233 

Muscovite,  13,  23,  33,  51,  65,  71,  119, 

149,  540 
Mysore,  India,  deposits,  205 


X 


Nacimiento,    New    Mexico,    copper 

deposits,  78 
Native  copper,  354 

deposits  of,  70,  260,  395 
Native  gold,  401 
Nebular  hypothesis,  288 
Negaunee    iron-bearing    formation, 

317 
Nephelin,  13,  149,  502 

rocks,  weathering  of,  505 
Nevada  City,  Calif.,  deposits  of,  432 

wall-rock  alteration,  238 
Nevada  Consolidated  Copper  Com- 
pany, deposits,  379 
New  Caledonia,  515 
Niccolite,  65,  149,  511 
Nickel,  511 

uses  and  production,  512 
Nitrates,  574 
Nodular  ore,  145 
Nomenclature  of  faults,  102 
Nonesuch  copper  belt,  Mich.,  400 
Normal  faults,  106,  107 
Norway-Aragon    area,    Mich.,    317 
Noselite,  13,  149 
Nova  Scotia,  saddles  of,  206 


Oblique  fault,  106 
Ocean,  age  of,  287 

composition  of,  561 
Ocher,  561 
Octahedrite,  149 
Offset  of  faults,  106 
Oilstones,  549 
Ojo  Caliente,  New  Mexico,  deposits, 

263,  266 
Olivine,  13,  33,  149,  290,  579 


Oolitic  ores,  89,  145,  331 

Opal,  65,  71,  149 

Openings,  along  anticlines,  181 

in  rocks,  170,  184 
Ophir  district,  Calif.,  deposits  of,  237 

wall  rock,  alteration  of,  238 
Ore,  definition,  1 

deposits,  definition,  1 

deformation  of,  100 

faulting  and  folding,  102 

mineral,  2 

shoots,  93 
Organic     matter     as     precipitating 

agent  of  metals,  77 
Orientation,  of  fragments  in  replace- 
ment deposits,  223,  227 

of  ore  bodies  during  dynamic 

metamorphism,  117 
Origin  of  magmas,  287 
Orpiment,  65,  149,  519 
Orthoclase,  13,  23,  33,  51,  65,  149 
Ottrelite,  117,  149 
Ouray,  Colo.,  deposits  of,  456 
Outcrops,  above  sulphide  deposits, 
138 

of  copper  deposits,  351 

of  ore  deposits,  215 
Overlap,  106 

Overlapping  lenses,  7,  190 
Overthrust,  107 
Oxidized  zone,  depth  of,  140 
Oxidation,  of  pyrite,  154 

of  sulphides,  order  of,  163 


Pacific  coast  gold  deposits,  403 

Paigeite,  33 

Pandermite,  573 

Paragenesis,  3 

of  contact  metamorphic  ores,  37 
of  metamorphosed  deposits,  120 
of  secondary  ores,  166 

Parallel  fracture  systems,  211 

Park  City,  Utah,  deposits  of,  460 

Pay  streak,  93 

Pearcite,  439,  444 

Pegmatite  veins,  4,  5 


INDEX 


601 


Pegmatites,  5 

banding  in,  25,  26 

composition  of,  23 

general  features,  18 

gradation,  into  deep  zone  veins, 

57 

into  gold  veins,  282 
into  quartz  veins,  26 

occurrence,  22 

ore  shoots  of,  95 

outline  of  characteristics,  18 

shape  of,  24 

size  of,  24 

texture  of,  25 
Penokee-Gogebic  range,  311 

length  of,  89 

Pentlandite,  13,  65,  511,  512 
Peridotite,  556 
Permeability  of  rocks,  135 
Petzite,  65,  149,  401 
Philipsburg,  Montana,  silver-gold 
deposits,  464 

contact  metamorphism  at,  41 

manganese  deposits,  511 

similar  ores  in  different  rocks, 
280 

sketch  showing  veins,  63 

veins,  depth  of  formation,  54 

wall  rock  alteration,  248 
Phlogopite,  541 
Phosphate  rock,  576 
Picotite,  13,  33,  149 
Pilot  Knob,  Mo.,  deposits,  341 
Pinetucky,  Ala.,  gold  deposits,  425 
Pisolitic  ore,  145 
Pitchblend,  526 
Pitches,  81,  190 
Placers,  7,  87,  411 

gold,  408 

shape  of,  89 

Placerville,  Colo.,  uranium  deposits, 
527,  529 

vanadium  deposits,  527,  529 
Planetessimal  hypothesis,  288 
Platinum,  13,  33,  51,  65,  71,  88,  516 
Plattnerite,  489 
Plumas  County,  Calif.,  graphic  ore 

of,  147 
Pocket,  93 


Pod,  190 
Polianite,  508 

Polybasite,  65,  71,  149,  439,  444 
Porcupine,  Ontario,  422 
Porphyritic  minerals  formed  during 
dynamic      metamorphis- 
117 

Porphyry  copper  ores,  67,  350 
Porosity  of  altered  material,  223 
Potash  salts,  565 
Potential  of  minerals,  155 
Potsdam  ores,  S.  Dak.,  429 
Pre-Cambrian  gold  deposits,  403 
Precious  stones,  552 
Precipitation,    caused    by    organic 
materials,  76 

near  the  surface,  72 

of  metals,  76,  154 

of  sulphides  above  water  level, 

143 

Pressure  of  solutions,  177 
Primary  deposits,  4 

•diagram    showing    genesis    of, 

91 

Primary  openings,  171 
Primary  ore,  influence   of   on    sec- 
ondary zones,  168 

ore  shoots,  93 
Propylitic  alteration,  249 

and  sericitic  alteration  compared, 

250 

Protore,  1 

Proustite,  65,  71,  149,  439,  444 
Pseudomorphs,  224 

in  secondary  ore,  143,  146 
Psilomelane,  88,  149,  588 
Pulpstone,  548 

Pulsations  of  ground  water,  134 
Pumice,  174 
Puzzolan  cement,  545 
Pyrargyrite,  65,  71,  149,  439,  444 
Pyrite,  13,  23,  33,  51,  65,  71,  149, 
571 

oxidation  of,  145,  154 
Pyrolusite,  88,  149,  508 
Pyromorphite,  149,  490 
Pyrrhotite,  13,  23,  33,  51,  290 

gold  veins,  60 

lead  veins,  60 


602 


INDEX 


Quartz,  13,  23,  33,  51,  65,  71,  88,  149, 

551 

bodies    elongated     during    dy- 
namic metamorphism,   117 
Quicksilver  in  hot  spring  deposits, 

262 
production,  501 


R 


Radial  fracture  patterns,  212 

Radium,  526 

Rammelsberg    deposits,     Germany, 

121,  122 

Rand  gold  deposits,  420 
Ray,  Ariz.,  deposits  of,  375 
Realgar,  65,  71,  519 
Recrystallization  of  minerals  during 
dynamic      metamorphism, 
117 

Redemption  iron  mine,  near  Philips- 
burg,  Mont.,  38 
Reef,  186 

Regional  metamorphism  of  ore  de- 
posits, 114 
Relief,  influence  of,  on  enrichment, 

135,  136 

Reopened  veins,  190 
Replacement,  3 

during  sulphide  enrichment,  166 
in  Black  Hills,  S.  D.,  229 
mechanism  of,  218 
veins,  3,  223 

Replacement  deposits,  223 
boundaries  of,  226 
recognition  of,  223 
Republic  Trough,  Mich.,  317 
Residual  deposits,  7 

iron  ore,  292 
Residual  minerals,  of  weathering,  88 

in  replaced  rocks,  229 
Residuary  ores  of  Iron   Mountain, 

Mo.,  86 

Reticulated  vein,  188 
Reverse  faults,  106,  107 

deposition  in,  195 
Rhodochrosite,  23,  65,  71,  149,  508 


Rhodonite,  23,  33,  65,  71,  149,  508 
Ribbons  of  ore,  199 

faulting  of,  112 
Rico,  Colo,,  457 

ore  shoots  of,  97 
Ridd'e,  Ore.,  515 
Riebeckite,  13,  149 
Roberts  mine,  Linden,  Wis.,  81 
Rock  flowage,  101 

structure,   influence  of,  on  fis- 

suring,  196 

Rockwood  iron  ore,  87,  330 
Roscoelite,  528 
Rossland,  B.  C.,  232 
Rotation  during  faulting,  103 
Rotational  strains,  179 
Ruby,  23,  33,  149 
Ruby  Trust  mine,  Granby,  Mo.,  78 
Run,  190 

Rutile,  13,  23,  33,  51,  88,  149,  531 
Ryerson  mine,   Morenci,   Ariz.,  ore 
of,  144 


S 


Saddle  reef,  201 
Saddles,  203 

origin  of,  204,  205 
Saint  Eugene  mine,  B.  C.,  233 
Salt,  561,  562 

domes,  564 
Samarskite,  526,  532 
San  Francisco  region,  Utah,  236,  496 
San  Juan  region,  Colo.,  454 
San    Luis     Obispo     region,     Calif., 

chromite  of,  508 
Santa  Rita,  New  Mexico,  381 
Sapphire,  13,  23,  33,  149 
Scapolite,  23,  33,  51,  149 
Scheelite,  23,  33,  51,  524 
Schistose  sulphide  ores,  116 
Schistosity,  of   metamorphosed  de- 
posits, 120 

relation  of  fissures  to,  197 
Schlegelmilch,  South  Carolina,  veins, 

55 

Schlieren,  9,  14 
Schuermann's  series,  157 
Scour  and  fill,  413 


INDEX 


Scythestones,  549 
Sea,  age  of,  28 

water,  amount  of,  286 
composition  of,  561 
precipitation  of,  562 
Secondary  deposits,  4 

criteria,  150 

Secondary  enrichment,  124 
of  copper,  352 
of  gold,  405 
of  silver,  441 
Secondary  minerals,  148 
Secondary  openings,  175 
ore  shoots,  99 

textures,  143 

sulphide  zone,  position  and  ex- 
tent, 140 

relation  to  water  level,  142 
veinlets  in  sulphide  ore,  145 
zones,  influenced  by  primary 

ore,  168 

Sedimentary  deposits,  4,  6,  84,  87 
general  features  of,  84 
minerals  of,  88 
ore  shoots  of,  98 


size,  89 

texture,  90 

Sedimentation,  a  process  of  concen- 
tration, 84 

Segments,  search  for,  107 
Segregated  veins,  7,  118 
Selenite,  149 
Selenium,  531 
Senarmontite,  517 
Separation,  by  faults,  105 
Sericite,  33,  51,  65,  71,  149,  502 
Sericitic    alteration    and    propylitic 
alteration  compared,   250  ' 

calcitic  gold  veins,  237 

copper  veins,  244 

copper-silver  veins,  242 

silver-gold  veins,  248 

zinc-silver  veins,  242 
Serpentine,  149,  579,  581 
Sevier  County,  Ark.,  deposits, 

518 

Seward   Peninsula,   Alaska,  tin  de- 
posits, 524 


Shallow  depth,  deposits  formed  at, 

4,  6,  69 
minerals  of,  71 

Shasta  County,  Calif.,  deposits  of, 

386 

pegmatites     grade     into     gold 
bearing  quartz  veins,  27 

Shear  zone,  189 

Sheeted  zone,  66,  188,  189 

Shift  of  faults,  105 

Shoots  of  ore,  93 

Shrinkage,  by  cooling  176 
cavities,  175 
of  earth,  114,  178 

Sicilian  sulphur  deposits,  570 

Siderite,  51,  65,  71,  290 

Sideritic  lead  veins,  240 

Sienna,  560 

Silication  by  contact  metamorphism, 
38 

Sillimanite,  33,  149 

Silver,  13,  51,  71,  439,  443 

Silver  City,  Idaho,  wall  rock  alter- 
ation at,  248 

Silver  Peak,  Nev.,  deposits  of,  58,  59 
gold  deposits,  282 
pegmatites     grade     into     gold 
deposits,  27 

Silver    Plume,     Colo.,    deposits   of, 
450 

Silverton,  Colo.,  deposits  of,  455 

Slags,  18 

Slate,  535 

Slender  deposits,  199 

Slickensides,  108 

Slip  of  faults,  105 

Smaltite,  515 

Smithsonite,  149,  474 

Sodalite,  13,  149 

Sodium  sulphate,  560 

Solution  cavities,  175 

Solution,  of  gold  in  placers,  414 
of  metals,  154 

of  salt  and  water,  behavior  of, 
19 

Soret's  principle,  10 

Soudan  formation,  322 

Sources  of  ascending  thermal  metal- 
liferous waters,  271,  274 


604 


INDEX 


Southeast    Missouri    lead    deposits, 

491 
Southern  Appalachian  gold  deposits, 

424 
Southern  Cross,  Mont.,  deposits  of, 

466 

weathering  at,  219 
Spaces  along  fissures,  185 
Specularite,  13,  23,  33,  51,  149 
Sperrylite,  516 
Spessartite,  508 
Sphalerite,  474,  475,  476 
Spherulites,  284 
Spinel,  13,  23,  33,  51,  149,  555 
Spodumene,  13,  23,  149 
Spokane.  Wash.,  tin  deposits,  523 
Springs,  265,  266,  267 
Stable  minerals,  88 

residual  minerals,  126 
Stagnant  water  zone,  133 
Stalactites  in  oxidized  zone,  143 
Stassfurt  salt  deposits,  564,  566 
Stauffer  borax  deposits,  Calif.,  573 
Stannite,  521 
Staurolite,  33,  117,  149 
Steamboat  Springs,  Nev.,  262 
Steatite,  149 

Stephanite,  65,  71,  439  444 
Stibnite,  51,  65,  71,  517 
Stilbite,  71 

Stirling  Hill  deposits,  N.  J.,  487 
Stockwerk,  188 
Stratton's  Independence  mine,  Colo., 

435 

Strength  of  rocks,  115 
Stresses  in  earth,  289 

openings  formed  by,  177 
Stride,  107,  110 
Stromeyerite,  71,  149 
Structure  of  openings,  184 
Structures,     influence   on   fissuring, 

196 
Subcapillary  openings,  170 

sheet  openings,  size  of,  220 
Submicroscopic  spaces,   174 
Subsidiary  fractures,  ore  in,  195 
Sudbury  nickel  deposits,  genesis  of, 

94 
Sudbury,  Ontario,  12,  513 


Sulphates,  solubilities  of,  156 
Sulphide  enrichment,  130,  352 

of  copper,  352 

of  silver,  441 

Sulphides,  solubilities  of,  157 
Sulphur,  149,  569 

uses  of,  571 

Sulphur  Bank,  Calif.,  263 
Sulphuric  acid,  571 
Supercapillary  openings,  170 
Superficial  alteration,  124 

enrichment,  gold,  405 
Surface  connections,  during  vein  for- 
mation, 56 

Sweetwater,  Tenn.,  iron  ores,  336 
Syenite,  weathering  of,  505 
Sylvanite,  65,  71,  401 
Symmetrical  banding,  2,  3 
Synclinal  deposits,  207 
Syngenetic  deposits,  4 
Synthetic   experiments,    in    contact 
metamorphism,  44 


Tabular  body,  2 
Taconite,  307 
Talc,  149,  579,  580 
Tamaulipas,    Mex,    contact    meta- 
morphism at,  40 
Tantalite,  532 
Tantalum,  532 

Telluride,  Colo.,  deposits  of,  456 
Tellurides,  51,  65,  71,  149 

of  gold.  410 

of  silver,  439 
Tellurium,  531 

adularia  veins,  254 

gold  veins,  254 

Temperatures,  of  formation  of  con- 
tact metamorphic  deposits, 
45 

of  formation  of  pegmatites,  27 
Temporary  surface,  216 
Tenabo  Peak,  Nev.,  section  of,  228 
Tennantite,  65,   71,  345,  357,  439, 

444,  519 

Tennessee  copper  deposits,  390 
Tennessee  valley  iron  ore  deposits, 
328 


INDEX 


605 


Tenorite,  149,  345,  355 

Tensional  fractures,  180 

Tephroite,  508 

Terlingua  district,  Texas,  500 

Terlinguite,  501 

Tertiary  gold  deposits,  404 

Tertiary  iron  ores  of  Texas,  332 

Tetrahedrite,  65,  71,  345,  357,  439, 

444,  517,  520 
Texas  iron  ores,  332 
Texture    of    contact    metamorphic 

deposits,  36 

Textures  of  secondary  ores,  143 
Thermal  waters,  271 
Thetford,    Que.,   asbestos   deposits, 

582 

Throw  of  faults,  105 
Thrust  of  solutions  in  vein  forma- 
tion, 55 
Thuringite,  71 

Ticonderoga  graphite  deposits,  556 
Tidal  stresses  in  solid  earth,  289 
Tiger-Poorman  vein,  Idaho,  60 
Time  element  in  enrichment,  136 
Tin,  149,  521 

enrichment  of,  522 

production  of,  522 

uses  of,  522 
Tintic,  Utah,  deposits,  462 

zonal  arrangement  of  ore  de- 
posits, 280 

Titaniferous  iron  ores,  343 
Titanite,  13,  33,  149 
Titanium,  531 

Tombstone,  Ariz.,  deposits  at,  203 
Tonopah,  Nevada,  deposits,  70,  470 

faulting  at,  110 

sources  of  water  that  deposited 
ores  of,  279 

wall-rock  alteration  at,  250 
Topaz,  13,  23,  33,  51,  149 
Topographic  expression  of  deposits, 

215 

Torbernite,  526 
Torsion,  effect  on  glass,  183 
Torsional  fractures,  182 
Tourmaline,  13,  23,  33,  51,  555 

veins,  236 
Tower,  Minn.,  iron  deposits,  322 


Traders  iron-bearing  formation,  316 

Trail  Creek,  B.  C.,  232 

Tremolite,  33,  51,  579 

Trenton  iron  ore,  87 

Tridymite,  13,  149 

Tripoli,  549 

Troughs,  207 

True  fissure  veins,  81,  186 

Tungsten,  524 

Tungstite,  524 

Turgite,  149,  290 

Turquoise,  149,  555 

Tuscarora,  Nev.,  deposits,  70 

Tyuyamunite,  526 


U 


Ulexite,  573 
Umber,  560 
Underground  water,  movements  of, 

132 

United  Verde  Co.,  Ariz.,  385 
Uralite,  149 
Uraninite,  526 
Uranium,  526 

ores  in  sandstone,  78 
Utah  Copper  Company  deposits,  365 
Utah,  ore  deposits,  281 

relation    of    different    deposits 
to  different  intrusives,  281 
Uvanite,  526 


Vadose  circulation,  132 
Valencianite,  149 
Valentinite,  517 
Vanadium,  528 

ores    in    sandstone,    southwest 
Colo.,  78,  528 

uses  of,  529 

Variations  in  widths  of  veins,  227 
Varicite,  556 
Vein,  2,  186 

breccia,  188 

filling,  4 

gash,  189 

ladder,  187 

ore  shoots  in,  95 


606 


INDEX 


Vein,  reopened,  180 

reticulated,  188 

walls,  2 

Vesuvianite,  33,  149 
Virgilina  district,  147 
Virginia,  barite  deposits,  558 

iron  ores,  335 

Virginia  Minn,  deposits,  302 
Vermilion  Range,  Mich.,  321 
Vertical  separation,  by  faults,  105 
Verticals,  197 

Black  Hills,  S.  D.,  229 
Vesicular  spaces,  173 
Volcanic  ash,  549 
Volcanic  rocks,  water  in,  285 
Vug,  2,  72 
Vulcan  iron-bearing  formation,  316 

W 

Wad,  508 

Wagon  Wheel  Gap,  Colo.,  hot  waters 

of,  263,  265 
vein  deposits  of,  263 

Wall-rock  alteration,  230 

Wandering     Boy     vein,     Tonopah, 
Nev.,  110 

Water,  585 

in  igneous  rocks,  284 
in  molten  magmas,  284 
in  sea,  amount  of,  286 
in  volcanic  glasses,  284 

Water  level,  131 

relation  of  secondary  zone  to, 

142 
table,  131 

Weathering,  124  ,125 
concentration  by,  85 
increase  of  value  by,  126 
minerals  that  resist,  87 
slumping  during,  126 

Whetstones,  549 

Whiteoak  Mountain  syncline,  Ala., 
87 

Willemite,  33,  51,  474,  475 

Wisconsin,  iron  ores,  311,  332 
zinc  deposits,  76,  97,  481 


Witherite,  149,  559 
Witwatersrand  gold  deposits,  419 
Wolframite,  23,  51,  149,  524 
Wollastonite,  33,  149 
Wood   River   district,    Idaho,   frac- 
tures in,  212 

wall-rock  alteration,  240 
Wurtzite,  149,  474,  475,  476 
Wyoming,  copper  deposits,  389 

iron  deposits,  338,  343 


Xenotime,  13,  23,  584 
Y 

Yankee  Girl  Deposit,  Colo.,  455 
Yttrialite,  149 

Yukon,  Alaska,  gold  deposits,  282 
pegmatites    grade    into    quartz 

veins,  27 
Yukon  veins,  58 


Zeolites,  71,  149 

Zeolitic  native  copper  veins,  70,  260 

Zinc,  474 

Zink  blend,  23,  33,  51,  65,  71,  149 

Zinc-lead  deposits,  477 

ores,  weathering  of,  476 
Zincite,  33,  51,  149,  474 
Zinnwald,  Saxony,  tin  deposits,  52 
Zircon,  13,  88,  584 
Zoisite,  33,  51,  119,  149 
Zonal  arrangements  of  ore  deposits, 
at  Butte,  Mont.,  280 
at  Tintic,  Utah,  280 
Zone  of  faulting,  101 

of  flowage,  101 

of  fracture,  115 

Zones,  of  acid  and  alkaline  ground 
waters,  134 

in  sulphide  deposits,  130 
Zwitter,  235 


DATE  DUE 


